June 20, 2010

VK4MSL/BM Part 2: Upgrade of bike… upgrade of set… HF here I come

Well… just this afternoon, I made some steps towards getting HF going on the bicycle mobile station.

I’ve had the station going a while now… and with the new workplace being so close to home, I’m often heard on 2m, usually on three repeaters:

  • Bayside VK4RBS: 146.875MHz FM
  • Mt. Cotton VK4RAX: 147.075MHz FM
  • Mt. Glorious VK4RBN 147.000MHz FM

Usually, it’s VK4RBS in the centre of town unless there’s activity on VK4RAX.  VK4RAX as I get closer to home.  I’d listen to the latter all the way, but pagers clobber the receiver on the poor FT-290R II.  Hearing pagers is bad enough … try it through a headset!  Made worse by the fact that the headset is a semi-homebrew design, embedded inside a motorcycle helmet… so I can’t easily take it off.  Once I’m out of the city however, the pagers aren’t an issue.

Range is pretty good… I use a quarter-wave ground-plane antenna on 2m… actually, the antenna itself is the same as on my previous bicycle-mobile station… a tunable whip antenna.  The antenna is intended for mobile use on a car, so to give it a reasonable counterpoise, I cut a 500mm long piece of aluminium angle, and bolted the antenna mount to that.  I found that alone, wasn’t good enough, and since added a 500mm long piece of copper wire that hangs out the back.  That brings the SWR down nicely into the range where the FT-290R II is happy to work with it.

I have been able to open the squelch of the Toowoomba (VK4RDD) repeater, once waiting at lights at West Ashgrove, and another time, underneath the Goodwill Bridge at the Queensland Maritime Museum near South Bank in Brisbane.  I also can work the Ipswich repeater, VK4RAI while walking up the hill along Cooper’s Camp Road at Bardon… about a distance of 80km out to the repeater’s location (near Marburg).

This is using the FT-290RII with the 25W linear option, and the aforementioned antenna.

This afternoon, I figured out how to interface the electret microphone in the headset to the FT-897D.  The wiring standard I use for my headsets is a customised one… using DB15HD connectors (VGA-like).  A female DB15HD exists on the headset side, this is to prevent some goose trying to plug a headset into a VGA card on a computer.  The following is a rough schematic of a typical headset using this wiring scheme.

Typical headset wiring schematic... looking into female DB15HD connector.

There are three pins that are normally unused… On a couple of my interfaces, +5V and 0V are wired up… it was initally thought I’d use these for power rails … one supplied by the headset (one of my planned “headsets” was a former in-car hands-free kit for a Nokia 3310, and so you’d be able to charge the phone this way), the other supplied by the device (many radios supply a 3.3V or 5V rail).

For the FT-897D, the microphone used is normally the dynamic type… that is, uses a balanced (differential) audio feed.  On the FT-290R II, I tie Mic – to 0V, and just use it single-ended, which works fine… but a better way is to actually convert the single-ended microphone signal to differential.  How does one do this?  Well, the answer came out of the TI TLV320AIC3204 datasheet which I’ve been reading quite a bit lately.

TLV320AIC3204 Typical Circuit Configuration, showing microphone wiring (Source & Copyright: Texas Instruments)

Typical electret microphone configuration (Source: Wikipedia)

I noticed something odd about the way they wired up an electret microphone.  Rather than wiring it up as shown on the left… they instead mirror the positive side; feeding through a resistor to 0V, but tapping off via a series capacitor to the CODEC input (see right; click image to enlarge).  Why were they doing this I wondered?  Then I found it.  Inside the electret capsule is a J-FET which amplifies the weak signal from the microphone itself.  By hooking a resistor on both sides, and using two capacitors, they were creating a phase splitter.  I stumbled across that article on Wikipedia, and it was then I knew what they were doing.

So I’ve done the same thing here… Rather than a single-ended design, I have interfaced the electret microphone to the radio using the phase-splitter technique.  The schematic I use (with DB15HD pinouts) is below:

FT-897D Headset interface

I’m yet to take the whole shebang for a ride… I have a 6′ long CB (27MHz) whip that, last time I tried, tuned up nicely on 10m and 6m… might work somewhat down on 20m. I have had a VK2 station come roaring in at S7 when listening on 80m via this antenna on the back of my fold-up bicycle, but unsurprisingly it’s pretty deaf there… I plan to get a second antenna mount and suitable spring (so the antenna doesn’t get snapped by a low branch), make up a new bracket, and mount that some time in the coming weeks… then we shall see what the bands are like around Brisbane.