Oct 262014

Earlier this week I had an idea.  We’ve got an old clock radio that picks up interference from the fridge when it turns on and the buttons on it are starting to fail with age.

I thought: “Why not build a new one?”

So the requirements are simple.  We need a real-time clock, display driver, and of course, a receiver.  The unit we have spends most of its time tuned to 792kHz AM (4QG or “ABC Radio National”), so a simple direct conversion receiver was what I was thinking of.  But what about the LO?

Now I do have some clock radio ICs that implement the timing circuitry, alarm function and LED panel driver somewhere in a junk box.  You feed them with the 50Hz or 60Hz waveform that comes out of the transformer and they use that as the timing source.  Easy to use a 555 timer for the time source, and I’d make a traditional receiver.  Another option is to use a AVR microcontroller, I have a few ATMega8Ls in the junk box with a NXP I2C RTC chip which I also have a few of.

The ATMega8L has a couple of PWM channels one 16-bit and one 8-bit: could they be used as an LO?

So: after digging around and locating my bought-years-ago and not-yet-used AVR programmer, and dusting off a breadboard that had an ATMega8L on it from a previous experiment I set to work.

This page explains in good detail how the PWM channels work. I started with those examples as a guide and tweaked from there.

For the PWM channel to work as a receiver LO, I want it to cover 540kHz to ~2MHz, with reasonable granularity. Question is, how far can I crank this? I have a 4MHz crystal, not the fastest I can use with this chip, but the absolute top of the range for the ATMegas isn’t much higher: 16MHz or maybe 20MHz. So if you’ve got a 16MHz crystal, you can expect to quadruple what I do here.

I started off with some blink code. If you take out all the delays, you get the following code:

#include <avr/io.h>
int main(void)
        DDRB |= (1 << DDB1);
        while (1)
                PORTB ^= (1 << DDB1);

and the following waveform:

Waveform done in software with GPIOs

Waveform done in software with GPIOs

The yellow waveform there is off one of the crystal pins. The cyan one is the PWM pin output, which in this case is a software driven GPIO. Even if this one worked, you wouldn’t want to do it this way unless your chip was doing only this task, and who’d use a programmable chip like an ATMega8L for that?

So, after reading through the documentation and examples, I loaded in the following code:

#include <avr/io.h>

#define TCCR1_COM1A     1
#define TCCR1_COM1B     0
#define TCCR1_FOC1A     0
#define TCCR1_FOC1B     0
#define TCCR1_WGM1      0xf
#define TCCR1_ICNC1     0
#define TCCR1_ICES1     0
#define TCCR1_CS1       1
#define TCCR1A_VAL      (                       \
                        (TCCR1_COM1A    << 6)   \
                |       (TCCR1_COM1B    << 4)   \
                |       (TCCR1_FOC1A    << 3)   \
                |       (TCCR1_FOC1B    << 2)   \
                |       (TCCR1_WGM1 & 0x3)      )
#define TCCR1B_VAL      (                       \
                        (TCCR1_ICNC1    << 7)   \
                |       (TCCR1_ICES1    << 6)   \
                |       (((TCCR1_WGM1 & 0xc) >> 2) << 3) \
                |       TCCR1_CS1)

int main (void)
        DDRB |= (1 << DDB1);
        OCR1A = 0x001;
        TCCR1A = TCCR1A_VAL;
        TCCR1B = TCCR1B_VAL;

The frequency can be adjusted by playing with OCR1A. If I leave it at 1 (basically as fast as the PWM can go) I get the following:

Waveform from AVR PWM

Waveform from AVR PWM

Bump it up one, and it sinks to 600kHz. Way too coarse for what I want sadly. I guess I was hopeful, but maybe the above might serve as a useful spring-off point for experiments with PWM.

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:


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.

Aug 132011

This post may also be read here.

Well, today I did some more work on the 2m linear.  Earlier this week I ordered some SMA connectors and some 1N5711 diodes for the project.

Two 1N5711 diodes will be used to make a voltage peak detector, to detect when the amplifier is subjected to power above 60mW.  The SMA connectors will be the interconnects between the modules.  This afternoon’s effort was spent soldering the SMA connectors onto two of the boards, and mounting the 2m amplifier module onto the heatsink.

The EME157B2 preamplifier kit was originally intended to be mounted in a masthead box, with BNC connectors soldered to the PCB, and stuck up a pole near the antenna.  In my application, I wanted it to be in the same enclosure (with suitable shielding) as the power amp, so that I could use its RF detection to automatically switch the power amplifier on.  I will also be using different relays, mounted on a separate board.

Instead of mounting the SPDT relays for the kit on the EME157B2 board, I’ve instead left these off.  I also omitted the 2N7000 MOSFET which turns on the relays, and L4, an RF-blocking choke which permits the preamp to run from a 12V source supplied up the coax.  I instead will power the preamp directly.

Since the relays will be on a separate board, the plan is to run wires from the gate and source connections where the 2N7000 belongs, and run those out to a controller board.  With the relays gone, the RF detection and the preamp are essentially two distinct circuits.  So 3 SMA connectors will be needed.  Here is the completed board with the SMA connectors fitted, and suitable jumpers installed, ready for tuning.

Completed 2m preamplifier

Completed 2m preamplifier. Connectors going left-to-right: Antenna input, Amplifier output, RF detector input.

Next, I finished off the power amplifier board, mounting it to the heatsink.  I have left one EMC filter disconnected for now, as the instructions say to power it up first with it disconnected to set the trim pot for 4.5V bias.  Rather than mounting the board flat on the heatsink, I have instead opted to mount the PCB at 90? to the module.  I had to make the supplied eyelets a fraction longer to accommodate this.  I also mounted SMA connectors on this board.

Completed 2m power amplifier

Completed 2m power amplifier. RF input is on the left.

The plan is, I’ll route RG195 coax on the left side to a small module which will contain the overload detection circuit and two SMAs for an external attenuator module.  On the right, a low-pass filter will be connected.  I also had a stab at tapping holes into the sides of the heatsink for mounting a bracket.  This bracket would hold the fans on top, and would bolt onto the main enclosure.  In doing this, I managed to bugger up two of the 8 holes, and thus what I’ll probably do, is buy a M4 tap tomorrow, drill them all out to 3mm and tap them to 4mm.  These are structural holes, so bigger is probably better anyway.

Much of today though was spent designing the controller.  I’m still finalising the design, but a rough schematic is below.

2m amplfier controller

2m amplfier controller

So, not yet going, but big parts are built now.

Aug 042011

This build log is also viewable here.


A few months back, I grabbed the trusty FT-290R II ready to do my weekly run from The Gap to Tarragindi.  Quick test to check everything’s okay… the power meter swings to full scale, but strange, I’m not hitting any repeaters.

Okay, grabbed the FT-897D instead, and I just did my weekly radio duties with that instead.  When I came home that evening, I had a closer look.  The FT-290R II was emitting a signal, the hand-held was picking that up.  It was also receiving just fine.  On a hunch I took off the FL-2025 linear, and hooked the antenna up directly to the radio.  Bingo… the radio works, the linear does not.

So, the linear had died, and thus I was in need of a new one.  Hand helds really don’t have much punch for mobile use, in fact, the FT-290 has been brilliant on the bike.  Not menu driven, so it’s real easy to drive while riding, simple, no frills, and sufficient grunt to get out of a bad area.  It also does SSB (and CW, but I’ll leave that to LY2KW).

I could buy a new set, in fact, I may get a FT-857D, as the 897D is a heavy lump of a radio to lug around, and there are times when HF capability is useful.  It is less than ideal on the bike however due to its size and weight.  There was nothing wrong with the FT-290, just its linear was dead, thus I was limited to its barefoot transmit power of 2.5W, even less than most handhelds.

So, I decided I’d try my hand at a semi-homebrew linear amplifier.

The concept

I wanted an amplifier that could achieve at least 25W of transmit power using SSB.  As I’d likely use it for things like WIA broadcasts, I wanted one that would also handle transmitting for a long period of time.

Designing a full blown amplifier on 2m is a bit beyond me with my limited homebrew experience.  It is also an issue sourcing the PCB material needed for VHF projects.  A lot out there call for FR4 grade fibreglass PCBs.  I have no idea what Jaycar sell.  So this was going to be a potential minefield.  Thus, I opted for a kit.

Minikits sell one based on the Mitsubishi RA30H1317M.  The same kit, can also take the 60W module, which sounded good to me.  Most of the time I’d be running it at 30W, but having 60W capability sounded good.  I purchased this, along with the 30W module as well just in case.

I also thought a pre-amp would be nice.  The same supplier sells this preamp kit.  The kit also offers RF sensing, which would allow the amplifier to auto-detect the radio transmitting, and switch into transmit mode automatically.  This also allows for filtering, to prevent reception of pagers (not fun copping an earful of one of those when you’re wearing a helmet-embedded headset riding a bicycle).


Minikits recommends using a Pentium 4 heatsink for 30W modules, however it wasn’t clear if this would be sufficient for 60W modules.

I wanted the amplifier module to stay below 100?C while operating with ambient temperatures at 40?C.  Pretty sure I don’t want to operate a radio under such conditions, so it should work fine in all conditions that I’m likely to encounter.

The amplifier module is about 45% efficient, thus about 135W is dissipated when operating at full power.  By my calculations, I was looking for cooling that can provide 0.22?C/W.

A quick search revealed that I could get one via Conrad which in the open air achieves 0.84?C/W.  Combined with a fan, it can achieve 0.24?C/W.  Jaycar sell this fan, which is quite capable.  In fact, two of them will fit across the back of the heatsink, so with dual fans, I should be well and truly within limits.

I placed the order for the heatsink a fortnight ago.  Due to a mix-up, I didn’t get it until Wednesday, but that’s fine, I wasn’t in any hurry.  With the heatsink now in my possession, I today headded to Jaycar to pick up some of the bits and pieces I’d need for this project, starting with the enclosure.

One thing I did neglect to procure today, were the fans… but no biggie, I’ll get those later.

Prior work

Well, technically day one was some time ago.  I had already mostly built the amplifier kit, and the preamp.  The preamp got built way back when I first obtained the kits.

The power amp was built later, however the instructions suggested that I wait until I have the amplifier module mounted on the heatsink before I go soldering it to the PCB.

Amplifier kit and 60W module

Amplifier kit and 60W module

Day 1

Having got the heatsink, enclosure and tools, I set to work.  Initially I positioned, drilled and tapped the two M3 holes for mounting the amplifier module.  I haven’t tried putting the amplifier in place yet, but it looks like the holes are positioned pretty well.

Amplifier module on heatsink

Amplifier module on heatsink

My plan, is to bend the pins on the module at 90? and mount the PCB horizontally.  Both module and PCB would be passed through the side wall of the enclosure, with the heatsink outside.  I originally wanted the heatsink inside the case (with vent holes), but of course, Jaycar are not good at providing internal dimensions, and I soon discovered it’d be awkward to fit.

It took a bit of experimentation to cut the hole in the side.  No, I won’t be winning any prizes for my metal work, in fact, it never was one of my best subjects.

Heatsink and empty enclosureHeatsink and empty enclosure (side view)

Heatsink and empty enclosure

Next steps:

My immediate next step will be to mount the amplifier module, solder it to the PCB, and mount the PCB inside the case.  Then I mount the heatsink and fans to the case.

I have a controller that I have designed at digital logic level, however I’ll need to do some further design work to make sure it’ll do what I intend, before procuring the parts and building it.

May 012011

Well, this afternoon I decided to fix a couple of problems with the bicycle mobile… firstly, the mounting of one noisy headlamp.  I’ve re-done the mount using a more solid piece of plastic this time, so we shall see how it goes.  No noise on HF so things are looking up there.  I also fixed the headset connection which was causing speakers to drop out… the problem turned out to be in the headset connection on the bike, rather than in the helmet.

It was approaching 2PM and thus nearly time for the daily Travellers net on 21.185MHz USB.  I’ve never made a contact on 15m before, but knew the antenna did tune up there, so I gave it a shot.  Ross VK5KMH popped up with a 58 signal out of Adelaide listening… after a prolonged silence, I decided to throw a call out.  Ross responded, reporting my signal into his station was also a 58 signal.

This was from the driveway at my home location, using 100W transmit power (not far from where I reached VK100WIA on 20m with a CB whip).  So evidently this home brew whip works quite well on 15m.  I have since brought the FT-897D inside and plugged it into my G5RV-like antenna, and after moving off frequency to tune, I notice Ross is still a 58 signal, so evidently my HF antenna doesn’t do much better than the whip does.

Apr 292011

Well, the antenna I tuned up in my last post, I can say, while it doesn’t work that great on 80m, it did get a contact into Victoria this evening on the AWNOI net.  Terry VK2TEZ near Coffs Harbour gave me a 4-3 signal report, so still lots of room for improvement… part of that was due to static crashes from storms in NSW, but I think with a better tuned antenna, we should be able to get towards having a workable antenna.  At the moment the autotransformer I use has ~95 turns, with output taps at 0, 25, 50 and 75 turns.  I think one somewhere between 0 and 25, and/or some extra turns might help… so I might wind a new one and see where that gets us.

The headlight still continues to give me grief.  An interesting discovery though this evening.  Since the battery is no good, I’ve permanently mounted it to the bicycle frame.  This was achieved by removing the plastic bracket which is used to mount the headlight on the handlebars or on the helmet mount (using a rubber O-ring), and replacing this with a bracket bent out of a short piece of aluminium.  It fastens to the bicycle frame at the front right above the front wheel, using a bolt hole normally used for mounting rim brakes (my bike has disc brakes).

The upshot is that the headlight’s casing has a pretty good electrical connection to the bicycle frame.  Turns out this is a big no no with these lights.  Kiss goodbye HF if you do… you’ll get crap everywhere from 400kHz right up into the VHF.  I’ll have to do some further investigation, but I found that if I insulated the case from the frame, it helped on the 400kHz and HF emissions.  I think something parasitic is causing the 2m grief as this continues (that, or it’s less critical on the case being earthed).

For a while I thought it might’ve been something lurking around 415kHz… the standard IF frequency of most superhetrodyne receivers, but alas, can’t see anything there.  Otherwise it’d explain why it appears to be everywhere.  I definitely suspect it’s not supposed to be oscillating there though, so I think parasitic oscillations are the cause here.  I’m slowly researching my own power supply for the LED in this headlamp, so its days are numbered.

The insulation was achieved by breaking a cheap plastic picnic knife, drilling a couple of mounting holes, and mounting the headlight on that.  That quelled the HF interference quite a bit, and I was able to listen to the HF bands on my way into Brisbane.  At least it was nice to listen to something other than that sodding wedding in the UK.  (C’mon fellas, yes, great and all but can’t we just confine it to one station?)

I was concerned about the longevity of this arrangement however.  And as it turned out, I was right to be concerned.  It broke as I approached the Normanby Fiveways.  I went over a bump, heard a crack, and noticed the headlight dangling by the power lead.  I pulled over, threw it in the basket and grabbed the backup headlight.  At least there was one on the helmet, a 1W LED, so I still complied with local laws for night riding.  I didn’t have a mounting for the backup light, I just pointed it forward sitting in the bottom of the front basket, with it on flash as a warning to drivers.

Once at the destination, I reverted the headlight back to being directly mounted on the bicycle frame.  Interference was intermittent, but when it was acting up, it did wipe out 80m with S6 noise.  Not good when most stations are barely making S6 as it is.  I wound up turning off the main headlamp as for the most part I could see where I was going, and I knew the route.  As I got out of town this was less of an issue due to the lack of traffic, and of course I was on bicycle paths or the footpath for 90% of it.  That at least allowed me to hear what was going on with the net.

The other flaw I had was that the helmet’s speaker connections were acting up… wound up unplugging the earpiece side of the headset adaptor and using the internal speaker.  Thankfully I could still use the helmet’s microphone and the rest of the wiring harness… just not the speakers in the helmet.  I noticed this as I pulled out of my street, in fact I was aware there was a problem, but now I know where the problem is now.  I’ll get onto it tomorrow.  And I’ll look at a better way to mount this headlamp in an insulated fashion as an interim solution to a power supply replacement.

Apr 172011

Well, I figured I better post up pics and notes on the improved antenna design for my HF bicycle mobile station.  I spent some time tuning it up today, and without resorting to the autotuner, I’ve successfully managed to tune up all bands available to me from 40m through to 6m.  80m still remains ellusive however.

The new design incorporates a version of the autotransformer used in the earlier attempt, using more turns of wire on the same size former, and multiple output tap points.  This allows me to accomodate a very wide turns ratio to match the antenna to various bands.

VK4MSL/BM HF: The autotransformer

VK4MSL/BM HF: The autotransformer


Band Test Frequency Primary turns Secondary turns Approximate SWR Comments
80m 3.590MHz 1 90 Too high to measure This seems to get the strongest signals. Autotuner is able to tune from here.
40m 7.120MHz 26 48 ~2:1 +/- a turn on the secondary to cover the entire 40m band.
20m 14.210MHz 26 1 ~1.2:1
15m 21.200MHz 26 27 ~1.4:1 Slightly out, there is probably a better one.
10m 29.200MHz 26 27 ~1.1:1
6m 53.000MHz 26 52 ~1.2:1 +/- a turn on the secondary, able to hit VK4RBX with 10W from the driveway


On the top of the autotransformer are for selecting the secondary tap; one of 0 turns, 25 turns, 50 turns or 75 turns (caveat; I might be slightly out with my counts here).  Having done this I think in hindsight I’d have been better off moving the 0t one down to maybe 10t instead, as there aren’t too many bands that seem to work on the 0t setting.  The primary side is selected by means of a wire soldered on to a thumbtac.  The wire wraps around the tube with a piece of balsa wood for the pin to stick into.  You select the turn by piercing the insulation as you push the thumbtac through the wire and into the balsa wood behind.  Crude, but it works.

VK4MSL/BM HF: Primary tap

VK4MSL/BM HF: Primary tap

In place of the CB whip, I have taken a fibreglass whip and cut it down, stripped the winding, and used it as a support with a base-load spring to take any shock loads.  In place of the original antenna winding, is two sections of brass tubing which telescope out.  This allows for an antenna that can be partially dismantled and reassembled on the run, unlike the other antenna which was permanently fixed at 6′ length.



I have a third solid section I can insert in there too, which would further extend the antenna to 2.5m, but it becomes very top heavy when I do this.  The antenna can extend to 1.6m length, or for portable use I can throw a wire up into a tree, or support it using a squid pole and connect that wire to the autotransformer output taps.

I didn’t make any contacts while tuning the thing up, although I was hearing New Zealand on 20m quite strongly, and on 10m I could hear the VK8 (Northern Territory) beacon going quite well.  I tried a few calls on 28.390MHz, but had no contacts.


I’ve also re-inforced the antenna bracket.  Prior to doing this the antenna would sway wildly from side to side.  Yes, it meant the cars gave me a wide birth (something I greatly appreciate) but I fear had adverse affects on the signal, and probably was asking for trouble in the long run.  Putting a brace between the two brackets seems to steady things up just a little bit, and now I can rock the bike side-to-side quite violently without the antenna swinging too far.

I’m yet to go mobile with the new improved station.  Weather permitting, I shall give it a try Monday evening.  I have a meeting with Brisbane area WICEN.  Due to headlight QRM I may or may not be active while mobile, we’ll give it a shot, but I should be able to work portable once I get there.

Mar 092011

For a little bit I’ve been struggling with poor performance on my bicycle mobile station.  It was an intermittent fault.  Sometimes it’d work great, other days the FT-290R II would complain bitterly about a SWR issue, and receive performance would be abysmal.  But then I’d set off anyway, get a block away, and the problems just disappeared.  Or the thing would be working perfect, and I’d get down the road and it’d stop working.

Damn frustrating.  Intermittent faults such as these are the worst kind to try and locate.  I thought of all kinds of possibilities, but the one thing I hadn’t considered was the antenna.

Performance had been pretty patchy ever since the weekend before LCA.  It was on the Saturday that somewhere between Annerley and Milton, I lost the ¼ wavelength stainless-steel whip that I had been using.  So I spent that evening rigging up a SO-239 socket so that I could use the commercial antenna I had; a Nagoya NL-77BH that I bought at BARCfest in 2008.

I rigged that up, and on the Monday I did successfully make a contact from the bicycle on my way to LCA, but it was patchy.  I did find a few glitches, so fixed those, and Friday I made a contact in the afternoon, but it was still pretty hit-and-miss.  Not the consistent behaviour I got out of my ¼ wave at all.  Okay, maybe the coax is damaged.  Tried different leads, no dice.  Recently I bought a front basket for the bicycle, and so I could put the FT-290RII in there.  Ran coax back to the antenna, last Wednesday afternoon and Thursday morning it worked beautiful.  However Monday it gave me no end of grief.

Suspecting that the weight of the radio pressing down on the BNC terminations may have damaged that section of coax, I grabbed a length of RG195 and terminated it with BNC connectors.  Still no good.  Using the SWR meter in the FT-897D, the impedance match was out by miles.

Today I had another look.  I took the antenna off the bicycle and placed it on a mag-mount antenna base, and placed the base in the centre of an open garage door.  So big ground plane, not much different to most cars.  Checked SWR, still through the roof.  Tuned to the Mt. Cotton repeater on the FT-897D, no signal.  Pulled out a hand-held, perfectly clear 5/8 signal on its original rubber-ducky antenna.  As I was unplugging the antenna base, I watched the signal strength suddenly shoot up and the radio crackle to life when the shield was disconnected (leaving just the centre pin).  I had noticed a dead short before, but thought it was the antenna mount on the bike… something was up.

So, I grabbed a bit of solid copper wire, a PL-259 plug, and some offcut insulation.  I made a new ¼ wavelength antenna, cutting it initially at 60cm.  Swapped it for the NL-77BH and the performance was beautiful.  Check SWR, and yes, it’s high, but then again, 60cm is waay too long.  I estimated about 51cm and folded the wire over at that point, twisting the excess around the body of the antenna.  Signal strength immediately went up two S points, and on checking SWR, it was significantly reduced.  I moved it back to the bicycle where I tweaked it further.

Once happy, I cut off the excess, used pliers to fold the end sharply and soldered the folded end to the body of the antenna to prevent it hooking anything.  Then used some heat-shrink tubing to finish it off so there were no sharp ends to poke eyes out with.  The antenna provides a good match from 144 right through until 148 MHz at 30W using FM.

I haven’t tried a contact on the bike yet, nor have I got any pics to share, but the radios seem happy with it, and it appears to be hitting repeaters in the area once again, including Ipswich.  Given it’s a good 30km between The Gap and Marburg (as the crow flies) with some decent hills to boot, that’s not bad going.

It would appear the additional complexity of these high-gain commercial antennas comes at a significant cost, they don’t like getting shaken to bits on the back of a bicycle.  I’m not sure how repairable the commercial antenna I have is, it may be a case of throw the thing out, at which case I think any love affair I had with commercial mobile whips might be over.  At least my ¼ wave antennas can be made for <$20 in about 10 minutes from parts I can buy in town, versus spending >$50 and having to wait for it to arrive in the post.

Dec 122010

I picked up a few new toys recently.  I’ve been looking around for a small microcontroller based device to act as a combined remote face / DTMF generator for my FT897D.  The idea is that this device could interface with the FT897D via its CAT port, and allow me to adjust the frequency and mode, recalling the information from internal flash or an SD card.

The remote face would then be mounted on the front of the bicycle, and connect to the radio at the rear to allow easy bicycle mobile operation.  An extension of this would be control of a separate 2m radio, and a GPS to allow APRS from the bicycle.

The idea was to have the memory work like a relational database.  Rather than just recalling memory channels, and having a big long list, I could scroll through the repeaters by callsign, location (service area), or combined with GPS, proximity.  Modern flash technology would make this easily doable.

Likewise, for DTMF, rather than having to carry around a cheat sheet or remember IRLP node numbers, wouldn’t it be nice to just be able to scroll through a node list by country/region/callsign, select one, hit the “Call” button, put your callsign across and have it automatically dial the moment you raised the PTT?

I don’t have the ability to manufacture PCBs of the standard required for ICs such as most 32-bit microcontrollers.  SOIC is about as fine as I can muster, and prototyping services are expensive.  Thus I was looking for a stamp module or premade board.

Luminary Micro (now TI) make a few nice ones, and during my work at Laidley, I got to use the LM3S8962 Ethernet/CAN evaluation board.  One nice feature was that it had the JTAG built-in via a FTDI USB-serial chip.  However, the licensing for the board support package irks me — despite their code being useless on anything other than one of their chips, they still see it necessary to modify the BSD license adding a clause that prohibits its use on non-TI microcontrollers.  I had a crack at writing my own “free-software” Stellaris library, but haven’t gotten that far with it.

I happened to stumble on this board based around the STM32F103VET.  They were being sold on eBay for about $60 at the time, so I decided at that price I’d buy three.  ST’s driver library appears to be very liberal in its licensing (in fact they claim there is “no license”, I don’t know if this means “public domain”, or whether I treat it like BSD).

The LCD panel uses the Ilitek ILI9320 display controller with internal graphics RAM, and is capable of 18-bit colour.  The board also features a RTC backup battery, Texas Instruments TSC2046 touchscreen controller and on-board RS232 level converter.  The STM32 also functions as a USB peripheral, and can be programmed using the stm32loader bootloader script via RS232.

Interestingly, the LCD controller documentation states that no part of that documentation may be reproduced without written permission.  I’m not sure if writing an open-source driver classes as “reproducing” the documentation (as I’d be making documented #define statements in C).

The devices come with example source and have a pre-loaded µC-GUI demonstration on them.  So far I’ve managed to distill enough out of these sources to get working touchscreen, LCD and UART.  I’ll probably start looking at FreeRTOS next and seeing if I can get a workable device going.

STM32F103 board

STM32F103 board running a simple "Hello World" app

Hello world application for STM32

Hello world application for STM32

Guess now I had better start planning my application. 🙂

Aug 152010

Well… after borrowing an antenna analyser and tweaking a few things… I made my second stationary contact using the bicycle mobile station on 20m.  This time, using 20W transmit power.  I now know where to place at least one of the taps on this autotransformer for 20m use. 😉  The station borrows heavily on the “Wonder Whip” style concept, where an autotransformer provides a means of matching the wild variances in impedance of the antenna, to something reasonable for the radio to cope with.

Shown here, is the station, exactly as it was during this contact.  The fibreglass 6′ CB whip has been spray painted yellow to make it more obvious, I plan to put a flag on there so that it resembles a bicycle safety flag (a big one) so it arouses less suspicion.  Click on any of the photos for a closer view.

VK4MSL/BM HF: as set up during the contact

VK4MSL/BM HF: as set up during the contact

This weekend was the day of the Remembrance Day contest, which is one of the major contests ran by the WIA.  Tuning around on 20m, I heard Kirby VK7KC booming in a S8 from the apple isle.  At first I tried contact with 5W, no dice… then 10W, then 20W… no luck.

I tuned off, and tried a different tap on the autotransformer… bingo, that sounded a bit noisier… I hit the button on the autotuner to clean up any last issues with the SWR, then tuned back and had another go.  Eventually after some perseverance, contact was made.

Below is a shot of the FT897D showing the frequency and S-meter reading shortly after the contact was made…. I was weak into Tassie, but that didn’t matter to me… as far as I was concerned, if I got outside metropolitan Brisbane, I was happy.

FT-897D frequency and S-meter during contact with VK7KC

FT-897D frequency and S-meter during contact with VK7KC

I haven’t yet tried other bands, although I’ve figured out some tap points for 6m, 10m and 15m… and some possible maybe points for 40m although I think the antenna will be very deaf down there.

On 80m it’ll probably be a mostly receive-only antenna, with maybe a Tx range of under 10km… if it’s enough for me to know what’s going on with the AWNOI net before I get home … and to maybe get a message relayed to VK4SD so I don’t get hassled about a late note, it’ll be great. 😉

The transformer still uses a map pin pushed though to select the tap… I’m not sure how well this will go long-term, and I think moving towards using banana plugs (or at the very least, alligator clips) will be a better solution.  Switches are another possibility.  Something that will be a more reliable connection than a pin pushed through a wire.  Shown here, is a close-up of the rear basket, the autotransformer is shown underneath the antenna bracket… which helps provide a bit of capacitance.  I find having it close up against the bracket helps, although I made provisions to be able to hang it vertically too (thereby reducing the coupling).

Rear basket showing homebrew autotransformer

Rear basket showing homebrew autotransformer

For now I’ll probably solder the centre conductor of the coax in place of the map pin, so that it’ll stay put until I can find a more convenient solution.  At least I have something on HF that works to a moderate degree.  I’ll probably give it a try next weekend on my way to the Queensland Maritime Museum, where I’ll be operating the Bulwer Island lighthouse as VK4MM in the International Lighthouse Lightship Weekend.  Hopefully I can stir up 20m sufficiently so that there’ll be some activity when 00:00 UTC rolls around.