Mar 262017
 

Yesterday’s post was rather long, but was intended for mostly technical audiences outside of amateur radio.  This post serves as a brain dump of volatile memory before I go to sleep for the night.  (Human conscious memory is more like D-RAM than one might realise.)

Radio interface

So, many in our group use packet radio TNCs already, with a good number using the venerable Kantronics KPC3.  These have a DB9 port that connects to the radio and a second DB25 RS-323 port that connects to the computer.

My proposal: we make an audio interface that either plugs into that DB9 port and re-uses the interface cables we already have, or directly into the radio’s data port.

This should connect to an audio interface on the computer.

For EMI’s sake, I’d recommend a USB sound dongle like this, or these, or this as that audio interface.  I looked on Jaycar and did see this one, which would also work (and burn a hole in your wallet!).

If you walk in and the asking price is more than $30, I’d seriously consider these other options.  Of those options, U-Mart are here in Brisbane; go to their site, order a dongle then tell the site you’ll come and pick it up.  They’ll send you an email with an order number when it’s ready, you just need to roll up to the store, punch that number into a terminal in the shop, then they’ll call your name out for you to collect and pay for it.

Scorptec are in Melbourne, so you’ll have to have items shipped, but are also worth talking to.  (They helped me source some bits for my server cluster when U-Mart wouldn’t.)

USB works over two copper pairs; one delivers +5V and 0V, the other is a differential pair for data.  In short, the USB link should be pretty immune from EMI issues.

At worst, you should be able to deal with it with judicious application of ferrite beads to knock down the common mode current and using a combination of low-ESR electrolytic and ceramic capacitors across the power rails.

If you then keep the analogue cables as short as absolutely possible, you should have little opportunity for RF to get in.

I don’t recommend the TigerTronics Signalink interfaces, they use cheap and nasty isolation transformers that lead to serious performance issues.

Receive audio

For the receive audio, we feed the audio from the radio and we feed that via potentiometer to a 3.5mm TRS (“phono”) plug tip, with sleeve going to common.  This plugs into the Line-In or Microphone input on the sound device.

Push to Talk and Transmit audio

I’ve bundled these together for a good reason.  The conventional way for computers to drive PTT is via an RS-232 serial port.

We can do that, but we won’t unless we have to.

Unless you’re running an original SoundBLASTER card, your audio interface is likely stereo.  We can get PTT control via an envelope detector forming a minimal-latency VOX control.

Another 3.5mm TRS plug connects to the “headphone” or “line-out” jack on our sound device and breaks out the left and right channels.

The left and right channels from the sound device should be fed into the “throw” contacts on two single-pole double-throw toggle switches.

The select pin (mechanically operated by the toggle handle) on each switch thus is used to select the left or right channel.

One switch’s select pin feeds into a potentiometer, then to the radio’s input.  We will call that the “modulator” switch; it selects which channel “modulates” our audio.  We can again adjust the gain with the potentiometer.

The other switch first feeds through a small Schottky diode then across a small electrolytic capacitor (to 0V) then through a small resistor before finally into the base of a small NPN signal transistor (e.g. BC547).  The emitter goes to 0V, the collector is our PTT signal.

This is the envelope detector we all know and love from our old experiments with crystal sets.  In theory, we could hook a speaker to the collector up to a power source and listen to AM radio stations, but in this case, we’ll be sending a tone down this channel to turn the transistor, and thus or PTT, on.

The switch feeding this arrangement we’ll call the “PTT” switch.

By using this arrangement, we can use either audio channel for modulation or PTT control, or we can use one channel for both.  1200-baud AFSK, FreeDV, etc, should work fine with both on the one channel.

If we just want to pass through analogue audio, then we probably want modulation separate, so we can hold the PTT open during speech breaks without having an annoying tone superimposed on our signal.

It may be prudent to feed a second resistor into the base of that NPN, running off to the RTS pin on an RS-232 interface.  This will let us use software that relies on RS-232 PTT control, which can be added by way of a USB-RS232 dongle.

The cheap Prolific PL-2303 ones sold by a few places (including Jaycar) will work for this.  (If your software expects a 16550 UART interface on port 0x3f8 or similar, consider running it in a virtual machine.)

Ideally though, this should not be needed, and if added, can be left disconnected without harm.

Software

There are a few “off-the-shelf” packages that should work fine with this arrangement.

AX.25 software

AGWPE on Windows provides a software TNC.  On Linux, there’s soundmodem (which I have used, and presently mirror) and Direwolf.

Shouldn’t need a separate PTT channel, it should be sufficient to make the pre-amble long enough to engage PTT and rely on the envelope detector recognising the packet.

Digital Voice

FreeDV provides an open-source digital voice platform system for Windows, Linux and MacOS X.

This tool also lets us send analogue voice.  Digital voice should be fine, the first frame might get lost but as a frame is 40ms, we just wait before we start talking, like we would for regular analogue radio.

For the analogue side of things, we would want tone-driven PTT.  Not sure if that’s supported, but hey, we’ve got the source code, and yours truly has worked with it, it shouldn’t be hard to add.

Slow-scan television

The two to watch here would be QSSTV (Linux) and EasyPal (Windows).  QSSTV is open-source, so if we need to make modifications, we can.

Not sure who maintains EasyPal these days, not Eric VK4AES as he’s no longer with us (RIP and thank-you).  Here, we might need an RS-232 PTT interface, which as discussed, is not a hard modification.

Radioteletype

Most is covered by FLDigi.  Modes with a fairly consistent duty cycle will work fine with the VOX PTT, and once again, we have the source, we can make others work.

Custom software ideas

So we can use a few off-the-shelf packages to do basic comms.

  • We need auditability of our messaging system.  Analogue FM, we can just use a VOX-like function on the computer to record individual received messages, and to record outgoing traffic.  Text messages and files can be logged.
  • Ideally, we should have some digital signing of logs to make them tamper-resistant.  Then we can mathematically prove what was sent.
  • In a true  emergency, it may be necessary to encrypt what we transmit.  This is fine, we’re allowed to do this in such cases, and we can always turn over our audited logs for authorities anyway.
  • Files will be sent as blocks which are forward-error corrected (or forward-erasure coded).  We can use a block cipher such as AES-256 to encrypt these blocks before FEC.  OpenPGP would work well here rather doing it from scratch; just send the OpenPGP output using FEC blocks.  It should be possible to pick out symmetric key used at the receiving end for auditing, this would be done if asked for by Government.  DIY not necessary, the building blocks are there.
  • Digital voice is a stream, we can use block ciphers but this introduces latency and there’s always the issue of bit errors.  Stream ciphers on the other hand, work by generating a key stream, then XOR-ing that with the data.  So long as we can keep sync in the face of bit errors, use of a stream cipher should not impair noise immunity.
  • Signal fade is a worse problem, I suggest a cleartext (3-bit, 4-bit?) gray-code sync field for synchronisation.  Receiver can time the length of a fade, estimate the number of lost frames, then use the field to re-sync.
  • There’s more than a dozen stream ciphers to choose from.  Some promising ones are ACHTERBAHN-128, Grain 128a, HC-256, Phelix, Py, the Salsa20 family, SNOW 2/3G, SOBER-128, Scream, Turing, MUGI, Panama, ISAAC and Pike.
  • Most (all?) stream ciphers are symmetric.  We would have to negotiate/distribute a key somehow, either use Diffie-Hellman or send a generated key as an encrypted file transfer (see above).  The key and both encrypted + decrypted streams could be made available to Government if needed.
  • The software should be capable of:
    • Real-time digital voice (encrypted and clear; the latter being compatible with FreeDV)
    • File transfer (again, clear and encrypted using OpenPGP, and using good FEC, files will be cryptographically signed by sender)
    • Voice mail and SSTV, implemented using file transfer.
    • Radioteletype modes (perhaps PSK31, Olivia, etc), with logs made.
    • Analogue voice pass-through, with recordings made.
    • All messages logged and time-stamped, received messages/files hashed, hashes cryptographically signed (OpenPGP signature)
    • Operation over packet networks (AX.25, TCP/IP)
    • Standard message forms with some basic input validation.
    • Ad-hoc routing between interfaces (e.g. SSB to AX.25, AX.25 to TCP/IP, etc) should be possible.
  • The above stack should ideally work on low-cost single-board computers that are readily available and are low-power.  Linux support will be highest priority, Windows/MacOS X/BSD is a nice-to-have.
  • GNU Radio has building blocks that should let us do most of the above.
Mar 252017
 

So, there’s been a bit of discussion lately about our communications infrastructure. I’ve been doing quite a bit of thinking about the topic.

The situation today

Here in Australia, a lot of people are being moved over to the National Broadband Network… with the analogue fixed line phone (if it hasn’t disappeared already) being replaced with a digital service.

For many, their cellular “mobile” phone is their only means of contact. More than the over-glorified two-way radios that was pre-cellular car phones used by the social elites in the early 70s, or the slightly more sophisticated and tennis-elbow inducing AMPS hand-held mobile phones that we saw in the 80s, mobile phones today are truly versatile and powerful hand-held computers.

In fact, they are more powerful than the teen-aged computer I am typing this on. (And yes, I have upgraded it; 1GB RAM, 250GB mSATA SSD, Linux kernel 4.0… this 2GHz P4 still runs, and yes I’ll update that kernel in a moment. Now, how’s that iPhone 3G going, still running well?)

All of these devices are able to provide data communications throughput in the order of millions of bits per second, and outside of emergencies, are generally, very reliable.

It is easy to forget just how much needs to work properly in order for you to receive that funny cat picture.

Mobile networks

One thing that is not clear about the NBN, is what happens when the power is lost. The electricity grid is not infallible, and requires regular maintenance, so while reliability is good, it is not guaranteed.

For FTTP users, battery backup is an optional extra. If you haven’t opted in, then your “land line” goes down when the power goes out.

This is not a fact that people think about. Most will say, “that’s fine, I’ve got my mobile” … but do you? The typical mobile phone cell tower has several hours battery back-up, and can be overwhelmed by traffic even in non-emergencies.They are fundamentally engineered to a cost, thus compromises are made on how long they can run without back-up power, and how much call capacity they carry.

In the 2008 storms that hit The Gap, I had no mobile telephone coverage for 2 days. My Nokia 3310 would occasionally pick up a signal from a tower in a neighbouring suburb such as Keperra, Red Hill or Bardon, and would thus occasionally receive the odd text message… but rarely could muster the effective radiated power to be able to reply back or make calls. (Yes, and Nokia did tell me that internal antennas surpassed the need for external ones. A 850MHz yagi might’ve worked!)

Emergency Services

Now, you tell yourself, “Well, the emergency services have their own radios…”, and this is correct. They do have their own radio networks. They too are generally quite reliable. They have their problems. The Emergency Alerting System employed in Victoria was having capacity problems as far back as 2006 (emphasis mine):

A high-priority project under the Statewide Integrated Public Safety Communications Strategy was establishing a reliable statewide paging system; the emergency alerting system. The EAS became operational in 2006 at a cost of $212 million. It provides coverage to about 96 per cent of Victoria through more than 220 remote transmitter sites. The system is managed by the Emergency Services Telecommunications Agency on behalf of the State and is used by the CFA, VICSES and Ambulance Victoria (rural) to alert approximately 37,400 personnel, mostly volunteers, to an incident. It has recently been extended to a small number of DSE and MFB staff.

Under the EAS there are three levels of message priority: emergency, non-emergency, and administrative. Within each category the system sends messages on a first-in, first-out basis. This means queued emergency messages are sent before any other message type and non-emergency messages have priority over administrative messages.

A problem with the transmission speed and coverage of messages was identified in 2006. The CFA expressed concern that areas already experiencing marginal coverage would suffer additional message loss when the system reached its limits during peak events.

To ensure statewide coverage for all pagers, in November 2006 EAS users decided to restrict transmission speed and respond to the capacity problems by upgrading the system. An additional problem with the EAS was caused by linking. The EAS can be configured to link messages by automatically sending a copy of a message to another pager address. If multiple copies of a message are sent the overall load on the system increases.

By February 2008 linking had increased by 25 per cent.

During the 2008 windstorm in Victoria the EAS was significantly short of delivery targets for non-emergency and administrative messages. The Emergency Services Telecommunications Agency subsequently reviewed how different agencies were using the system, including their message type selection and message linking. It recommended that the agencies establish business rules about the use of linking and processes for authorising and monitoring de-linking.

The planned upgrade was designed to ensure the EAS could cope better with more messages without the use of linking.

The upgrade was delayed several times and rescheduled for February 2009; it had not been rolled out by the time of Black Saturday. Unfortunately this affected the system on that day, after which the upgrade was postponed indefinitely.

I can find mention of this upgrade taking place around 2013. From what I gather, it did eventually happen, but it took a roasting from mother nature to make it happen. The lesson here is that even purpose built networks can fall over, and thus particularly in major incidents, it is prudent to have a back-up plan.

Alternatives

For the lay person, CB radio can be a useful tool for short-range (longer-than-yelling-range) voice communications. UHF CB will cover a few kilometres in urban environments and can achieve quite long distances if good line-of-sight is maintained. They require no apparatus license, and are relatively inexpensive.

It is worth having a couple of cheap ones, a small torch and a packet of AAA batteries (stored separately) in the car or in a bag you take with you. You can’t use them if they’re in a cupboard at home and you’re not there.

The downside with the hand-helds, particularly the low end ones, is effective radiated power. They will have small “rubber ducky” antennas, optimised for size, and will typically have limited transmit power, some can do the 5W limit, but most will be 1W or less.

If you need a bit more grunt, a mobile UHF CB set and magnetic mount antenna could be assembled and fitted to most cars, and will provide 5W transmit power, capable of about 5-10km in good conditions.

HF (27MHz) CB can go further, and with 12W peak envelope power, it is possible to get across town with one, even interstate or overseas when conditions permit. These too, are worth looking at, and many can be had cheaply second-hand. They require a larger antenna however to be effective, and are less common today.

Beware of fakes though… A CB radio must meet “type approval”, just being technically able to transmit in that band doesn’t automatically make it a CB, it must meet all aspects of the Citizens Band Radio Service Class License to be classified a CB.

If it does more than 5W on UHF, it is not a UHF CB. If it mentions a transmit range outside of 476-478MHz, it is not a UHF CB.  Programming it to do UHF channels doesn’t change this.

Similarly, if your HF CB radio can do 26MHz (NZ CB, not Australia), uses FM instead of SSB/AM (UK CB, again not Australia), does more than 12W, or can do 28-30MHz (10m amateur), it doesn’t qualify as being a CB under the class license.

Amateur radio licensing

If you’ve got a good understanding of high-school mathematics and physics, then a Foundation amateur radio license is well within reach.  In fact, I’d strongly recommend it for anyone doing first year Electrical Engineering … as it will give you a good practical grounding in electrical theories.

Doing so, you get to use up to 10W of power (double what UHF CB gives you; 6dB can matter!) and access to four HF, one VHF and one UHF band using analogue voice or hand-keyed Morse code.

You can then use those “CB radios” that sell on eBay/DealExtreme/BangGood/AliExpress…etc, without issue, as being un-modified “commercial off-the-shelf”, they are acceptable for use under the Foundation license.

Beyond Voice: amateur radio digital modes

Now, all good and well being able to get voice traffic across a couple of suburban blocks. In a large-scale disaster, it is often necessary to co-ordinate recovery efforts, which often means listings of inventory and requirements, welfare information, etc, needs to be broadcast.

You can broadcast this by voice over radio… very slowly!

You can put a spreadsheet on a USB stick and drive it there. You can deliver photos that way too. During an emergency event, roads may be in-passable, or they may be congested. If the regular communications channels are down, how does one get such files across town quickly?

Amateur radio requires operators who have undergone training and hold current apparatus licenses, but this service does permit the transmission of digital data (for standard and advanced licensees), with encryption if needed (“intercommunications when participating in emergency services operations or related training exercises”).

Amateur radio is by its nature, experimental. Lots of different mechanisms have been developed through experiment for intercommunication over amateur radio bands using digital techniques.

Morse code

The oldest by far is commonly known as “Morse code”, and while it is slower than voice, it requires simpler transmitting and receiving equipment, and concentrates the transmitted power over a very narrow bandwidth, meaning it can be heard reliably at times when more sophisticated modes cannot. However, not everybody can send or receive it (yours truly included).

I won’t dwell on it here, as there are more practical mechanisms for transmitting lots of data, but have included it here for completeness. I will point out though, due to its simplicity, it has practically no latency, thus it can be faster than SMS.

Radio Teletype

Okay, there are actually quite a few modes that can be described in this manner, and I’ll use this term to refer to the family of modes. Basically, you can think of it as two dumb terminals linked via a radio channel. When you type text into one, that text appears on the other in near real-time. The latency is thus very low, on par with Morse code.

The earliest of these is the RTTY mode, but more modern incarnations of the same idea include PSK31.

These are normally used as-is. With some manual copying and pasting pieces of text at each end, it is possible to encode other forms of data as short runs of text and send files in short hand-crafted “packets”, which are then hand-deconstructed and decoded at the far end.

This can be automated to remove the human error component.

The method is slow, but these radioteletype modes are known for being able to “punch through” poor signal conditions.

When I was studying web design back in 2001, we were instructed to keep all photos below 30kB in size. At the time, dial-up Internet was common, and loading times were a prime consideration.

Thus instead of posting photos like this, we had to shrink them down, like this. Yes, some detail is lost, but it is good enough to get an “idea” of the situation.

The former photo is 2.8MB, the latter is 28kB. Via the above contrived transmission system, it would take about 20 minutes to transmit.

The method would work well for anything that is text, particularly simple spread sheets, which could be converted to Comma Separated Values to strip all but the most essential information, bringing file sizes down into realms that would allow transmission times in the order of 5 minutes. Text also compresses well, thus in some cases, transmission time can be reduced.

To put this into perspective, a drive from The Gap where that photo was taken, into the Brisbane CBD, takes about 20 minutes in non-peak-hour normal circumstances. It can take an hour at peak times. In cases of natural disaster, the roads available to you may be more congested than usual, thus you can expect peak-hour-like trip times.

Radio Faximile and Slow Scan Television

This covers a wide variety of modes, ranging from the ancient like Hellschreiber which has its origins in the German Military back in World War II, various analogue slow-scan television modes through to the modern digital slow-scan television.

This allows the transmission of photos and visual information over radio. Some systems like EasyPAL and its elk (based on HamDRM, a variant of Digital Radio Mondiale) are in fact, general purpose modems for transmitting files, and thus can transmit non-graphical data too.

Transmit times can vary, but the analogue modes take between 30 seconds and two minutes depending on quality. For the HamDRM-based systems, transmit speeds vary between 86Bps up to 795kBps depending on the settings used.

Packet Radio

Packet radio is the concept of implementing packet-switched networks over radio links. There are various forms of this, the most common in amateur radio being PACTOR, WINMOR, the 1200-baud AFSK and 9600-baud FSK and 300-baud AFSK packet modes.

300-baud AFSK is normally used on HF links, and hails from experiments using surplus Bell 103 modems modified to work with radio. Similarly, on VHF and UHF FM radio, experiments were done with surplus Bell 202 modems, giving rise to the 1200-baud AFSK mode.

The 9600-baud FSK mode was the invention of James Miller G3RUH, and was one of the first packet radio modes actually developed by radio amateur operators for use on radio.

These are all general-purpose data modems, and while they can be used for radioteletype applications, they are designed with computer networking in mind.

The feature facilities like automatic repeating of lost messages, and in some cases support forward error correction. PACTOR/WINMOR is actually used with the Winlink radio network which provides email services.

The 300-baud, 1200-baud and 9600-baud versions generally use a networking protocol called AX.25, and by configuring stations with multiple such “terminal node controllers” (modems) connected and appropriate software, a station can operate as a router, relaying traffic received via one radio channel to a station that’s connected via another, or to non-AX.25 stations on Winlink or the Internet.

It is well suited to automatic stations, operating without human intervention.

AX.25 packet and PACTOR I are open standards, the later PACTOR modems are proprietary devices produced by SCS in Germany.

AX.25 packet is capable of transmit speeds between 15Bps (300 baud) and 1kBps (9600 baud). PACTOR varies between 5Bps and 650Bps.

In theory, it is possible to develop new modems for transmitting AX.25, the HamDRM modem used for slow-scan television and the FDMDV modem used in FreeDV being good starting points as both are proven modems with good performance.

These simply require an analogue interface between the computer sound card and radio, and appropriate software.  Such an interface made to link a 1200-baud TNC to a radio could be converted to link to a low-cost USB audio dongle for connection to a computer.

If someone is set up for 1200-baud packet, setting up for these other modes is not difficult.

High speed data

Going beyond standard radios, amateur radio also has some very high-speed data links available. D-Star Digital Data operates on the 23cm microwave band and can potentially transmit files at up to 16KBps, which approaches ADSL-lite speeds. Transceivers such as the Icom ID-1 provide this via an Ethernet interface for direct connection to a computer.

General Electric have a similar offering for industrial applications that operates on various commercial bands, some of which can reach amateur frequencies, thus would be usable on amateur bands. These devices offer transmit speeds up to 8KBps.

A recent experiment by amateurs using off-the-shelf 50mW 433MHz FSK modules and Realtek-based digital TV tuner receivers produced a high-speed speed data link capable of delivering data at up to 14KBps using a wideband (~230kHz) radio channel on the 70cm band.  They used it to send high definition photos from a high-altitude balloon.

The point?

We’ve got a lot of tools at our disposal for getting a message through, and collectively, 140 years of experience at our disposal. In an emergency situation, that means we have a lot of different options, if one doesn’t work, we can try another.

No, a 1200-baud VHF packet link won’t stream 4k HD video, but it has minimal latency and will take less than 20 minutes to transmit a 100kB file over distances of 10km or more.

A 1kB email will be at the other end before you can reach for your car keys.  Further experimentation and development means we can only improve.  Amateur radio is far from obsolete.

Nov 202016
 

The Yaesu FT-897D has the de-facto standard 6-pin Mini-DIN data jack on the back to which you can plug a digital modem.  Amongst the pins it provides is a squelch status pin, and in the past I’ve tried using that to drive (via transistors) the carrier detect pin on various computer interfaces to enable the modem to detect when a signal is incoming.

The FT-897D is fussy however.  Any load at all pulling this pin down, and you get no audio.  Any load.  One really must be careful about that.

Last week when I tried the UDRC-II, I hit the same problem.  I was able to prove it was the UDRC-II by construction of a crude adapter cable that hooked up to the DB15-HD connector, converting that to Mini-DIN6: by avoiding the squelch status pin, I avoided the problem.

One possible solution was to cut the supplied Mini-DIN6 cable open, locate the offending wire and cut it.  Not a solution I relish doing.  The other was to try and fix the UDRC-II.

Discussing this on the list, it was suggested by Bryan Hoyer that I use a 4.7k pull-up resistor on the offending pin to 3.3V.  He provided a diagram that indicated where to find the needed signals to tap into.

With that information, I performed the following modification.  A 1206 4.7k resistor is tacked onto the squelch status pin, and a small wire run from there to the 3.3V pin on a spare header.

UDRC-II modification for Yaesu FT-897D

UDRC-II modification for Yaesu FT-897D

I’m at two minds whether this should be a diode instead, just in case a radio asserts +12V on this line, I don’t want +12V frying the SoC in the Raspberry Pi.  On the other hand, this is working, it isn’t “broke”.

Doing the above fixed the squelch drive issue and now I’m able to transmit and receive using the UDRC-II.  Many thanks to Bryan Hoyer for pointing this modification out.

Nov 122016
 

So, recently, the North West Digital Radio group generously donated a UDRC II radio control board in thanks for my initial work on an audio driver for the Texas Instruments TLV320AIC3204 (yes, a mouthful).

This board looks like it might support the older Pi model B I had, but I thought I’d play it safe and buy the later revision, so I bought version 3 of the Pi and the associated 7″ touch screen.  Thus, an order went to RS for a whole pile of parts, including one Raspberry Pi3 computer, a blank 8GB MicroSD card, a power supply, the touch screen kit and a case.

Fitting the UDRC

To fit the UDRC, the case will need some of the plastic cut away,  rectangular section out of the main body and a similarly sized portion out of the back cover.

Modifications to the case

Modifications to the case

When assembled, the cut-away section will allow the DB15-HD and Mini-DIN6 connectors to protrude out slightly.

Case assembled with modifications

The UDRC needs some minor modifications too for the touch screen.  Probe around, and you’ll find a source of 5V on one of the unpopulated headers.  You’ll want to solder a two-pin header to here and hook that to the LCD control board using the supplied jumper leads.  If you’ve got one, use a right-angled header, otherwise just bend a regular one like I did.

5V supply for the LCD on the UDRC

5V supply for the LCD on the UDRC

You’ll note I’ve made a note on the DB15-HD, a monitor does NOT plug in here.

From here, you should be ready to load up a SD card.  NWDR recommend the use of Compass Linux, which is a Raspbian fork configured for use with the UDRC.  I used the lite version, since it was smaller and I’m comfortable with command lines.

Configuring screen rotation

If you try to boot your freshly prepared SD card, the first thing you’ll notice is that the screen is up-side-down.  Clearly a few people didn’t communicate with each-other about which way was up on this thing.

Before you pull the SD card out, it is worth mounting the first partition on the SD card and editing config.txt on the root directory of that partition. If doing this on a Windows computer ensure your text editor respects Unix line endings! (Blame Microsoft. If you’re doing this on a Mac, Linux, BSD or other Unix-ish computer, you have nothing to worry about.)

Add the following to the end of the file (or anywhere really):

# Rotate the screen the "right way up"
lcd_rotate=2

Now save the file, unmount the SD card, and put it in the Pi before assembling the case proper.

Setting up your environment

Now, if you chose the lite option like I did, there’ll be no GUI, and the touch aspect of the touchscreen is useless.  You’ll need a USB keyboard.

Log in as pi (password raspberry), run passwd to change your password, then run sudo -s to gain a root shell.

You might choose like I did to run passwd again here to set root‘s password too.

After that, you’ll want to install some software.  Your choice of desktop environment is entirely up to you, I prefer something lightweight, and have been using FVWM for years, but there are plenty of choices in Debian as well as the usual suspects (KDE, Gnome, XFCE…).

For the display manager, I’ll choose lightdm. We also need an on-screen keyboard. I tried a couple, including matchbox-keyboard and the rather ancient xvkbd. Despite its age, I found xvkbd to be the most usable.

Once you’ve decided what you want, run apt-get install with your list of packages, making sure to include xvkbd and lightdm in your list.  Other applications I included here were network-manager-gnome, qasmixer, pasystray, stalonetray and gkrellm.

Enabling the on-screen keyboard in lightdm

Having installed lightdm and xvkbd, you can now configure lightdm to enable the accessibility options.

Open up /etc/lightdm/lightdm-gtk-greeter.conf, look for the line show-indicators and tack ;~a11y on the end.

Now down further, look for the commented out keyboard setting and change that to keyboard=xvkbd. Save and close the file, then run /etc/init.d/lightdm restart.

You should find yourself staring at the log-in screen, and lo and behold, there should be a new icon up the top-right. Tapping it should bring up a 3 line menu, the bottom of which is the on-screen keyboard.

On-screen keyboard in lightdm

On-screen keyboard in lightdm

The button marked Focus is what you hit to tell the keyboard which application is to receive the keyboard events.  Tap that, then the application you want.  To log in, tap Focus then the password field.  You should be able to tap your password in followed by either the Return button on the virtual keyboard or the Log In button on the form.

Making FVWM touch-friendly

I have a pretty old configuration that has evolved over the last 10 years using FVWM that was built around keyboard-centric operation and screen real-estate preservation.  This configuration mainly needed two changes:

  • Menus and title bar text enlarged to make the corresponding UI elements finger-friendly
  • Adjusting the size of the FVWM BarButtons to suit the 800×480 display

Rather than showing how to do it from scratch, I’ll just link to the configuration tarball which you are welcome to play with.  It uses xcalendar which isn’t in the Debian repositories any more, but is available on Gentoo mirrors and can be built from source (you’ll want to install xutils-dev for xmake), stalonetray and gkrellm are both in the standard Debian repositories.

FVWM on the Raspberry Pi

FVWM on the Raspberry Pi

Enabling the right-click

This took a bit of hunting to figure out.  There is a method that works with Debian Wheezy which allows right-clicks by way of long presses, but this broke in Jessie, and the 2016-05-23 release of Compass Linux is built on the latter.  So another solution is needed.

Philipp Merkel however, wrote a little daemon called twofing.  Once installed, doing a right click is simply a two-fingered tap on the screen, there’s support for other two-fingered gestures such as pinching and rotation as well.  It is available on Github, and I have forked this, adding some udev rules and scripts to integrate it into the Raspberry Pi.

The resulting Debian package is here.  Download the .deb, run dpkg -i on it, and then re-start the Raspberry Pi (or you can try running udevadm trigger and re-starting X).  The udev rules should create a /dev/twofingtouch symbolic link and the installed Xsession.d/Xreset.d scripts should take care of starting it with X and shutting it down afterwards.

Having done this, when you log in you should find that twofing is running, and that right clicks can be performed using a two-fingered prod.

Finishing up

Having done the configuration, you should now have a usable workhorse for numerous applications.  The UDRC shows up as a second sound card and is accessible via ALSA.  I haven’t tried it out yet, but it at least shows up in the mixer application, so the signs are there.  I’ll be looking to add LinBPQ and FreeDV into the mix yet, to round the software stack off to make this a general purpose voice/data radio station for emergency communications.

Oct 132016
 

Well, today’s mail had a surprise.  Back about 6 years ago, I was sub-contracted to Jacques Electronics to help them develop some device drivers for their video intercom system.  At the time, they were using TI’s TLV320AIC3204 and system-on-modules based on the Freescale i.MX27 SoC.

No driver existed in the ALSA tree for this particular audio CODEC, and while TI did have one available under NDA, the driver was only licensed for use with a TI OMAP SoC.  I did what just about any developer would do, grabbed the closest-looking existing ALSA SoC driver, ripped it apart and started hacking.  Thus I wound up getting to grips with the I²S infrastructure within the i.MX27 and taming the little beast that is the TLV320AIC3204, producing this patch.

As the code was a derivative work, the code was automatically going to be under the GPLv2 and thus was posted on the ALSA SoC mailing list for others to use.  This would help protect Jacques from any possible GPL infringement regarding the use of that driver.  I was able to do this as it was a clean-room implementation using only material in TI’s data sheet, thus did not contain any intellectual property of my then-employer.

About that time I recall one company using the driver in their IP camera product, the driver itself never made it into the mainline kernel.  About 6 months later, another driver for the TLV320AIC3204 and 3254 did get accepted there, I suspect this too was a clean-room implementation.

Fast forward to late August, I receive an email from Jeremy McDermond on behalf of the Northwest Digital Radio.  They had developed the Universal Digital Radio Controller board for the Raspberry Pi series of computers based around this same CODEC chip.  Interestingly, it was the ‘AIC3204 driver that I developed all that time before that proved to be the code they needed to get the chip working.  The chip in question can be seen up the top-right corner of the board.

Universal Digital Radio Controller

Timely, as there’s a push at the moment within Brisbane Area WICEN Group to investigate possible alternatives to our aging packet radio system and software stack.  These boards, essentially being radio-optimised sound cards, have been used successfully for implementing various digital modes including AX.25 packet, D-Star and could potentially do FreeDV and other digital modes.

So, looks like I’ll be chasing up a supplier for a newer Raspberry Pi board, and seeing what I can do about getting this device talking to the world.

Many thanks to the Northwest Digital Radio company for their generous donation! 🙂

May 032015
 

The Problem

I’ve been running a station from the bicycle for some time now and I suppose I’ve tried a few different battery types on the station.

Originally I ran 9Ah 12V gel cells, which work fine for about 6 months, then the load of the radio gets a bit much and I find myself taking two with me on a journey to work because one no longer lasts the day.  I replaced this with a 40Ah Thundersky LiFePO4 pack which I bought from EVWorks, which while good, weighed 8kg!  This is a lot lighter than an equivalent lead acid, gel cell or AGM battery, but it’s still a hefty load for a bicycle.

At the time that was the smallest I could get.  Eventually I found a mob that sold 10Ah packs. These particular cells were made by LiFeBatt, and while pricey, I’ve pretty much recouped my costs. (I’d have bought and disposed of about 16 gel cell batteries in this time at $50 each, versus $400 for one of these.)   These are what I’ve been running now since about mid 2011, and they’ve been pretty good for my needs.  They handle the load of the FT-857 okay on 2m FM which is what I use most of the time.

A week or two back though, I was using one of these packs outside with the home base in a “portable” set-up with my FT-897D.  Tuned up on the 40m WICEN net on 7075kHz, a few stations reported that I had scratchy audio.  Odd, the radio was known to be good, I’ve operated from the back deck before and not had problems, what changed?

The one and only thing different is I was using one of these 10Ah packs.  I’ve had fun with RF problems on the bicycle too.  On transmit, the battery was hovering around the 10.2V mark, perhaps a bit low.  Could it be the radio is distorting on voice peaks due to input current starvation?  I tried after the net swapping it for my 40Ah pack, which improved things.  Not totally cleared up, but it was better, and the pack hadn’t been charged in a while so it was probably a little low too.

The idea

I thought about the problem for a bit.  SSB requires full power on voice peaks.  For a 100W radio, that’s a 20A load right now.  Batteries don’t like this.  Perhaps there was a bit of internal resistance from age and the nature of the cells?  Could I do something to give it a little hand?

Supercapacitors are basically very high capacity electrolytic capacitors with a low breakdown voltage, normally in the order of a few volts and capacitances of over a farad.  They are good for temporarily storing charge that needs to be dumped into a load in a hurry.  Could this help?

My cells are in a series bank of 4: ~3.3V/cell with 4 cells gives me 13.2V.  There’s a battery balancer already present.  If a cell gets above 4V, that cell is toast, so the balancer is present to try to prevent that from happening.  I could buy these 1F 5.5V capacitors for only a few dollars each, so I thought, “what the hell, give it a try”.  I don’t have much information on them other that Elna Japan made them.  The plan was to make some capacitor “modules” that would hook in parallel to each cell.

My 13.2V battery pack, out of case

My 13.2V battery pack, out of its case

Supercapacitors

Supercapacitors

For my modules, the construction was simple, two reasonably heavy gauge wires tacked onto the terminals, the whole capacitor then encased in heatshrink tubing and ring lugs crimped to the leads. I was wondering whether I should solder a resistor and diode in parallel and put that in series with the supercap to prevent high in-rush current, but so far that hasn’t been necessary.

The re-assembled pack

I’ve put the pack back together and so far, it has charged up and is ready to face its first post-retrofit challenge.  I guess I’ll be trying out the HF station tomorrow to see how it goes.

Assembled pack

Assembled pack

The Verdict

Not a complete solution to the RF feedback, it seems to help in other ways. I did a quick test on the drive way first with the standard Yaesu handmic and with the headset. Headset still faces interference problems on HF, but I can wind it up to about 30W~40W now instead of 20.

More pondering to come but we’ll see what the other impacts are.

Mar 192015
 
Tropical Cyclone Nathan, Forecast map as of 2:50PM

Tropical Cyclone Nathan, Forecast map as of 2:50PM

This cyclone has harassed the far north once already, wobbled out in the Pacific like a drunken cyclist as a tropical low, has gained strength again and is now making a bee-line for Cape Flattery.

As seen, it also looks like doing the same stunt headed for Gove once it’s finished touching up far north Queensland.  Whoever up there is doing this rain dancing, you can stop now, it’s seriously pissing off the weather gods.

National and IARU REGION III Emergency Frequencies (Please keep clear and listen for emergency traffic)

  • 80m
    • 3.600MHz LSB (IARU III+WICEN)
  • 40m
    • 7.075MHz LSB (WICEN)
    • 7.110MHz LSB (IARU III)
  • 20m
    • 14.125MHz USB (WICEN)
    • 14.300MHz USB (IARU III)
    • 14.183MHz USB: NOT an emergency frequency, but Queensland State WICEN hold a net on this frequency every Sunday morning at around 08:00+10:00 (22:00Z Saturday).
  • 15m
    • 21.190MHz USB (WICEN)
    • 21.360MHz USB (IARU III)
  • 10m
    • 28.450MHz USB (WICEN)

I’ll be keeping an ear out on 14.125MHz in the mornings.

Update 20 March 4:31am: It has made landfall between Cape Melville and Cape Flattery as a category 4 cyclone.

Jan 032015
 

I’ve been riding on the road now for some years, and while I normally try to avoid it, I do sometimes find myself riding on the road itself rather than on the footpath or bicycle path.

Most of the time, the traffic is fine.  I’m mindful of where everyone is, and there aren’t any problems, but I have had a couple of close calls from time to time.  Close calls that have me saying “ode for a horn”.

By law we’re required to have a bell on our bikes.  No problem there, I have a mechanical one which is there purely for legal purposes.  If I get pulled over by police, and they ask, I can point it out and demonstrate it.  Requirement met?  Tick to that.

It’s of minimal use with pedestrians, and utterly useless in traffic.

Early on with my riding I developed a lighting system which included indicators.  Initially this was silent, I figured I’d see the lights flashing, but after a few occasions forgetting to turn indicators off, I fitted a piezo buzzer.  This was an idea inspired by the motorcycles ridden by Australia Post contractors, which have a very audible buzzer.  Jaycar sell a 85dB buzzer that’s waterproof, overkill in the audio department but fit for purpose.  It lets me know I have indicators on and alerts people to my presence.

That is, if they equate the loud beep to a bicycle.  Some do not.  And of course, it’s still utterly useless on the road.

I figured a louder alert system was in order.  Something that I could adjust the volume on, but loud enough to give a pedestrian a good 30 seconds warning.  That way they’ve got plenty of time to take evasive action while I also start reducing speed.  It’s not that I’m impatient, I’ll happily give way, but I don’t want to surprise people either.  Drivers on the other hand, if they do something stupid it’d be nice to let them know you’re there!

My workplace looks after a number of defence bases in South-East Queensland, one of which has a railway crossing for driver training.  This particular boom gate assembly copped a whack from a lightning strike, which damaged several items of equipment, including the electronic “bells” on the boom gate itself.  These “bells” consisted of a horn speaker with a small potted PCB mounted to the back which contained an amplifier and bell sound generator.  Apply +12V and the units would make a very loud dinging noise.  That’s in theory; in practise, all that happened was a TO-220 transistor got hot.  Either the board or the speaker (or both) was faulty.

It was decided these were a write-off, and after disassembly I soon discovered why: the voice coils in the horn speakers had been burnt out.  A little investigation, and I figured I could replace the blown out compression drivers and get the speakers themselves working again, building my own horn.

A concept formed: the horn would have two modes, a “bell” mode with a sound similar to a bicycle bell, and a “horn” mode for use in traffic.  I’d build the circuit in parts, the first being the power amplifier then interface to it the sound effect generator.

To make life easier testing, I also decided to add a line-in/microphone-in feature which would serve to debug construction issues in the power amplifier and add a megaphone function.  (Who knows, might be handy with WICEN events.)

Replacing the compression drivers

Obviously it’d be ideal to replace it with the correct part, but looking around, I couldn’t see anything that would fit the housing.  That, and what I did see, was more expensive than buying a whole new horn speaker.

There was a small aperture in the back about 40mm in diameter.  The original drivers were 8ohms, and probably rated at 30W and had a convex diaphragm which matched the concave geometry in the back of the horn assembly.

Looking around, I saw these 2W mylar cone speakers.  Not as good as a compression driver, but maybe good enough?  It was cheap enough to experiment.  I bought two to try it out.

I got them home, tacked some wires onto one of them and plugged it into a radio.  On its own, not very loud, but when I held it against the back of a horn assembly, the amplification was quite apparent.  Good enough to go further.  I did some experiments with how to mount the speakers to the assembly, which required some modifications to be made.

I soon settled on mounting the assembly to an aluminium case with some tapped holes for clamping the speaker in place.  There was ample room for a small amplifier which would be housed inside the new case, which would also serve as a means of mounting the whole lot to the bike.

Bell generator

I wasn’t sure what to use for this, I had two options: build an analogue circuit to make the effect, or program a microcontroller.  I did some experiments with an ATMega8L, did manage to get some sound out of it using the PWM output, but 8kB of flash just wasn’t enough for decent audio.

A Freetronics LeoStick proved to be the ticket.  32kB flash, USB device support, small form factor, what’s not to like?  I ignored the Arduino-compatible aspect and programmed the device directly.  Behind the novice-friendly pin names, they’re an ATMega32U4 with a 16MHz crystal.  I knocked up a quick prototype that just played a sound repeatedly.  It sounded a bit like a crowbar being dropped, but who cares, it was sufficient.

Experimenting with low-pass filters I soon discovered that a buffer-amp would be needed, as any significant load on the filter would render it useless.

A 2W power amplifier

Initially I was thinking along the lines of a LM386, but after reading the datasheet I soon learned that this would not cut it.  They are okay for 500mW, but not 2W.  I didn’t have any transistors on hand that would do it and still fit in the case, then I stumbled on the TDA 1905.  These ICs are actually capable of 5W into 4 ohms if you feed them with a 14V supply.  With 9V they produce 2.5W, which is about what I’m after.

I bought a couple then set to work with the breadboard.  A little tinkering and I soon had one of the horn speakers working with this new amplifier.  Plugged into my laptop, I found the audio output to be quite acceptable, in fact turned up half-way, it was uncomfortable to sit in front of.

I re-built the circuit to try and make use of the muting feature.  For whatever reason, I couldn’t get this to work, but the alternate circuit provided a volume control which was useful in itself.

The pre-amplifier

For the line-level audio, there’s no need for anything more fancy than a couple of resistors to act as a passive summation of the left and right channels, however for a microphone and for the LeoStick, I’d need a preamp.  I grabbed a LM358 and plugged that into my breadboard alongside the TDA1905.

Before long, I had a working microphone preamp working using one half of the LM358, based on a circuit I found.  I experimented with some resistor values and found I got reasonable amplification if I upped some of the resistor values to dial the gain back a little.  Otherwise I got feedback.

For the LeoStick, it already puts out 5V TTL, so a unity-gain voltage follower was all that was needed.  The second half of the LM358 provided this.  A passive summation network consisting of two resistors and DC-blocking capacitor allowed me to combine these outputs for the TDA1905.

One thing I found necessary, the TDA1905 and LM358 misbehave badly unless there’s a decent size capacitor on the 9V rail.  I found a 330uF electrolytic helped in addition to the datasheet-prescribed 100nF ceramics.

Power supply

Since I’m running on batteries with no means of generating power, it’s important that the circuit does not draw power when idle.  Ideally, the circuit should power on when either I:

  • plug the USB cable in (for firmware update/USB audio)
  • toggle the external source switch
  • press the bell button

We also need two power rails: a 9V one for the analogue electronics, and a 5V one for the LeoStick.  A LM7809 and LM7805 proved to be the easiest way to achieve this.

To allow software control of the power, a IRF9540N MOSFET was connected to the 12V input and supplies the LM7809.  The gate pin is connected to a wired-OR bus.  The bell button and external source switch connect to this bus with signal diodes that pull down on the gate.

Two BC547s also have collectors wired up to this bus, one driven from the USB +5V supply, and the other from a pin on the LeoStick.  Pressing the Bell button would power the entire circuit up, at which point the LeoStick would assert its power on signal (turning on one of the BC547s) then sample the state of the bell button and start playing sound.  When it detects the button has been released, it finishes its playback and turns itself off by releasing the power on signal.

Sound effect generator

Earlier I had prototyped a bell generator, however it wasn’t much use as it just repeatedly made a bell noise regardless of the inputs.  To add insult to injury, I had lost the source code I used.  I had a closer look at the MCU datasheet, deciding to start from a clean slate.

The LeoStick provides its audio on pin D11, which is wired up to Port B Pin 7.  Within the chip, two possible timers hook up: Timer 0, which is an 8-bit timer, and Timer 1, which is 16-bits.  Both are fed from the 16MHz system clock.  The bit depth affects the PWM carrier frequency we can generate, the higher the resolution, the slower the PWM runs.  You want the PWM frequency as high as possible, ideally well above 20kHz so that it’s not audible in the audio output, and obviously well above the audio sampling rate.

At 16MHz, a 16-bit timer would barely exceed 240Hz, which is utterly useless for audio.  A 10-bit timer fares better, with 15kHz, older people may not hear it but I certainly can hear 15kHz.  That leaves us with 8-bits which gets us up to 62kHz.  So no point in using Timer 1 if we’re only going to be using 8-bits of it, we might as well use Timer 0.

Some of you familiar with this chip may know of Timer 4, which is a high-speed 10-bit timer fed by a separate 64MHz PLL.  It’s possible to do better quality audio from here, either running at 10-bits with a 62kHz carrier, or dropping to 8-bits and ramping the frequency to 250kHz.  Obviously it’d have been nice, but I had already wired things up by this stage, so it was too late to choose another pin.

Producing the output voltage is only half the equation though: once started, the PWM pin will just output a steady stream of pulses, which when low-passed, produces a DC offset.  In order to play sound, we need to continually update the timer’s Capture Compare register with each new sample at a steady rate.

The most accurate way to do this, is to use another timer.  Timer 3 is another 16-bit timer unit, with just one capture compare output available on Port C pin 3.  It is an ideal candidate for a timer that has no external influence, so it gets the job of updating the PWM capture compare value with new samples.

Timer 1 is connected to pins that drive two of the three LEDs on the LeoStick, with Timer 4 driving the remaining one, so if I wanted, I could have LEDs fade in and out with it instead of just blinking.  However, my needs are basic, and I need something to debounce switches and visibly blink LEDs.  So I use that with a nice long period to give me a 10Hz timer.

Here is the source code.  I’ll add schematics and other notes to it with time, but the particular bits of interest for those wanting to incorporate PWM-generated sound in their AVR projects are the interrupt routine and the sound control functions.

To permit gapless playback, I define two buffers which I alternate between, so while one is being played back, the other can be filled up with samples.  I define these on line 139 with the functions starting at line 190.  The interrupt routine that orchestrates the playback is at line 469.

When sound is to be played, the first thing that needs to happen is for the initial buffer to be loaded with samples using the write_audio function.  This can either read from a separate buffer in RAM (e.g. from USB) or from program memory.  One of the options permits looping of audio.  Having loaded some initial data in, we can then call start_audio to set up the two timers and get audio playback rolling.  start_audio needs the sample rate to configure the sample rate timer, and can accept any sample rate that is a factor of 16MHz (so 8kHz, 16kHz up to 32kHz).

The audio in this application is statically compiled in, taking the form of an array of uint8_t‘s in PROGMEM.

Creating the sounds

I initially had a look around to see if I could get a suitable sound effect.  This proved futile, I was ideally looking around for a simple openly-licensed audio file.  Lots of places offered something, but then wanted you to sign up or pay money.  Fine, I can understand the need to make a quid, and if I were doing this a lot, I’d pay up, but this is a once-off.

Eventually, I found some recordings which were sort of what I was after, but not quite.  So I downloaded these then fired up Audacity to have a closer look.

The bicycle bell

Bicycle bells have a very distinctive sound to them, and are surprisingly complicated.  I initially tried to model it as an exponentially decaying sinusoid of different frequencies, but nothing sounded quite right.

The recording I had told me that the fundamental frequency was just over 2kHz.  Moreover though, the envelope was amplitude-modulated by a second sinusoid: this one about 15Hz.  Soon as I plugged this second term in, things sounded better.  This script, was the end result.  The resulting bell sounds like this:

So somewhat bell-like.  To reduce the space, I use a sample rate of 6.4kHz.  I did try a 4kHz sample rate but was momentarily miffed at the result until I realised what was going on: the bell was above the Nyquist frequency at 4kHz, 6.4kHz is the minimum practical rate that reproduces the audio.

I used Audacity to pick a point in the waveform for looping purposes, to make it sound like a bell being repeatedly struck.

The horn

I wanted something that sounded a little gutsy.  Like an air-horn on a truck.  Once again, I hit the web, and found a recording of a train horn.  Close enough, but not long enough, and a bit noisy.  However opening it up in Audacity and doing a spectrum analysis, I saw there were about 5 tones involved.  I plugged these straight into a Python script and decided to generate those directly.  Using a raised cosine filter to shape the envelope at the start and end, and I soon had my horn effect.  This script generates the horn.  The audio sounds like this:

Using other sound files

If you really wanted, you could use your own sound recordings.  Just keep in mind the constraints of the ATMega32U4, namely, 32kB of flash has to hold both code and recordings.  An ATMega64 would do better.  The audio should be mono, 8-bits and unsigned with as lower sample rate as you can get away with.  (6.4kHz proved to be sufficient for my needs.)

Your easiest bet would be to either figure out how to read WAV files (in Python: wave module), or alternatively, convert to raw headerless audio files, then code up a script that reads the file one byte at a time. The Python scripts I’ve provided might be a useful starting point for generating the C files.

Alternatively, you can try interfacing a SDCard and embedding a filesystem driver and audio file parser (not sure about WAVE but Sun Audio is easily parsed), this is left as an exercise for the adventurous.

Finishing up

I’ll put schematics and pictures up soonish.  I’m yet to try mounting the whole set up, but so far the amplifier is performing fine on the bench.

Jun 152014
 

This is a simple vertical groundplane antenna intended for mounting atop a 10m Squid Pole. These can be made to nearly any frequency you desire, and can be self-supporting if needed. The main limitation is the stiffness of the wire used.

The antenna gets its name as the original was one I quickly knocked up just prior to a horse endurence ride event that took place at Donnybrook in 2011. I was assisting Brisbane Area WICEN with the emergency communications at this event, and this antenna, worked very well. 10W was more than sufficient to get back to base on 2m FM.

The design is very simple. You’ll need some stiff copper wire, and a panel-mount BNC connector. I used some strands from a thick mains cable: this was being tossed out at a ham radio meeting some years back. The cable had a black plastic coating and inside were 7 strands of solid copper, each about 2mm thick. Perfect for small antennas.

Similar wire can be found in non-stranded house mains cable.

First step is to work out what length to cut the elements. They should all be roughly the same length. This can be calculated by the simple formula:

v=f\lambda

which if you take v as being the velocity of light in a vacuum (~3\times10^8 m/s; radio will travel a little slower through air, but who’s counting?) and f as being 147.050\times10^6 and solve for \lambda you get 2.04m as the wavelength.  We want ¼ of this, so I’ve aimed for 51cm long elements.

Don’t worry about them being perfectly straight when measuring, extra length is good at this point, you’ll want a good 2cm extra.  You can make a wire shorter, you can’t make it longer.

Measuring the elements

Measuring the elements

Measure and cut the 4 elements. 3 will become your groundplane, and the 4th the radiating element. Also cut off about 10cm or so, give or take, which will be the ground wire used to hook the groundplane elements to the BNC connector. Also add to your parts list, some small velcro strips: you’ll find these handy to strap the coax to the squid pole.

Procured parts

Procured parts

Start with the short piece of wire. You’ll want to bend it into a rough triangle shape, with loops of wire at the corners. The groundplane radials will loop through these holes. The excess wire should be coiled up to one side: this is the loop the squid pole will pass through. The BNC connector will be fitted in between the 3 small loops.

Ground wire

donnybrook-03-ground-wire Ground wire

Be sure you can still put the nut back on.

Take 3 of the four elements, and make a hook at one end. Pass this hook through each of the small loops in the triangle. Try to make them sit roughly straight out from the centre of the triangle, then solder each hook into the loop.

Hooking the radials

Soldering the radialsAttaching the radials

Having done this, put the BNC connector in and do the nut up tight. You can do away with the eyelet with the solder tag. To finish off, take your remaining element, make a hook just big enough to go around the centre pin of the BNC connector, then solder into place.

Preparing radiating element

Hooking radiating elementSoldering the radiating element Attaching the radiating element

To finish off, bend this until it is vertical. The antenna is now ready for tuning.

Completed untuned antenna

Completed untuned antenna

Double check the length is about right. It should be around the 51~52cm mark.

Checking length

Checking length (close) Checking length

To check the tuning, use a SWR meter or antenna analyser if you have one. Here, I used the built-in SWR meter on my Yaesu FT-857D. When using a SWR meter, ensure you’re running minimum power. The following are some results from my set.  It is at this point, you do any trimming of your antenna.  The following are without trimming the antenna, you’ll note that in most examples, the SWR is very low, just a point or so showing up on the left side of the screen.

On 2m:

donnybrook-13-test-144500 donnybrook-13-test-145500 donnybrook-13-test-146500 donnybrook-13-test-147500

On 70cm:

donnybrook-13-test-432100 donnybrook-13-test-433100 donnybrook-13-test-434100 donnybrook-13-test-438100 donnybrook-13-test-439100 donnybrook-13-test-440100

To mount the antenna on your squid pole, feed the tip of the squid pole through the remaining loop.  Bend the tip of the antenna around the tip of the squid pole.  Hook your coaxial cable to the BNC connector and use velcro straps at regular points to hold the coax to the side of the squid pole.

Mounted antenna

Mounted antenna

Recommended coax for this purpose is RG-195.  RG-58 will work, but is lossy, RG-213 and LMR400 are too heavy to use on a squid pole and will cause it to bend or collapse.

Update: This antenna performed quite well.  Saturday, we used it for 2m packet, providing a digipeater for the stations in our area in case they couldn’t reach the main node (at “the pineapple farm” just outside Imbil).  We had stable packet communications all day.  Since the stations around us found they could work the main node directly, we swapped antennas around and used it instead for a VHF/UHF cross-band voice repeater.  Signal reports were good through the Imbil state forest.

Jul 152013
 

Well, this year’s International Rally of Queensland didn’t go the way everyone expected. We were there with Brisbane Area WICEN, providing the backup communications for the event. Our primary role was to relay the scores given to us by the post chief in the timekeeper’s tent. They looked after scheduling the cars, getting times, and sending the cars through. We just passed on scores (start/finish times) and other traffic.

Saturday went well. My father and I were set up at Kandanga North running the WICEN checkpoint for stages 6 and 12 of the rally. After some early hiccups getting the packet radio network going, we had the scores being sent out on time and everything running smoothly. Apart from some cows holding up traffic, there were no delays.

Sunday however… just about everyone would have heard about the fatality. My father and I ran the WICEN checkpoint at the start of the fateful Michell Creek Special Stage 14.

Having now seen the ABC website footage, looking at the competitor lists and my own logs, I can say with 90% certainty which car (and therefore 45% certainty who the deceased is) the unfortunate car was and when they left the stage.

My condolences go out to both driver and co driver at this difficult time.

Update: The names have been released.