Homebrew

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

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

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

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.

Well, I did some experiments this weekend regarding the bicycle mobile station using an autotransformer to match the shortened end-fed whip (6′ 27MHz helically-wound) to the 50ohm impedance my set demands.

I haven’t tried this on the bike itself… these were done in my bedro^W(ahhem) “radio shack”… with the antenna and its bracket sitting on a wooden chair connected to a longer bit of coax than I’ll probably use on the bike.  I will have to try this on the bicycle (ideally with an antenna analyser) to get a better idea of performance, and the up-coming Remembrance Day contest may provide a good opportunity to gauge performance, as the bands will be sufficiently “busy”.

The autotransformer is a simple design… about 50 turns of cheap speaker wire (~1.5mm diameter; ~0.2mm insulation thickness) on a 42mm OD PVC pipe section about 160mm long.  The pipe has small holes down its length, drilled at a spacing approximately identical to the wire centre-to-centre spacing (2mm).  The two extremes of the coil connect to the antenna and the metal antenna bracket respectively, the latter also connecting to the braid on the coax going back to the radio (about 30cm RG58C/U coax).  The centre conductor of the coax connects to a short piece of wire which is soldered to a small map pin, which is simply pushed through the wire’s insulation, piercing it and poking through the hole in the PVC pipe… I can select any turns ratio from 1:1 to 1:50, achieving a very wide matching ratio.  (On paper, able to match >10kOhm impedances to 50ohm.)

Early testing seems to indicate the ability to obtain good SWR figures on 6m, 10m, 20m and 40m.  80m (the holy grail for me) eludes me… and I suspect the actual radiated signal will be very low down to most stations… but if I can cover parts of metropolitan Brisbane with it, I guess I’m doing okay.  The fact that this antenna got down to VK5 with 100W up it, is a promising sign… perhaps with the autotransformer, it may do better.  Then again, it may do a lot worse… We’ll see.  Plan is to try and get it up and running before the ILLW… so that I can stir up some activity on the bands on my way to the Bulwer Island lighthouse (AU0003) before becoming VK4MM and operating a station there.

Well, I’ve been riding a lot between West End and The Gap, and I get a lot of questions from people on the band about my setup. I was doing some repairs… one of the wires to the PTT had disconnected, luckily there was a 0V return via other connections… and so while I had the bike outside fixing that (it was too dark in the garage) I took the opportunity to snap some photos.

The light was fading at the time, so the pictures aren’t particularly great… I’ve touched them up to make them brighter, hence there’s a bit of noise in the photo… The last two showing the HF setup, required a flash (which I was trying to avoid due to the aluminum and reflectors)… and of course I didn’t spend time putting the FT-897D in the back… maybe later when I get everything tuned up and actually do make a true bicycle-mobile contact on HF (this one was not made mobile).

VK4MSL/BM on 2m: side view

Above, is the station in its entirety… fairly simple. The antenna is a plain 2m ground-plane, formed using a tunable mobile whip cut for 145.700MHz, the aluminum angle bracket makes up one counterpoise, and an additional counterpoise hangs out the back. Adjusting the angle has an influence on the SWR… in this arrangement, it works nicely.

VK4MSL/BM: Closeup of rear basket

Shown here is the rear basket where the FT-290R II (or FT-897D for HF) lurks… along with a 9Ah gel cell battery, which also powers the tail light. I haven’t been very neat about the cables. Two leads run from the front controls, the grey one (shielded) carries transmit/receive audio and the PTT, the blue one (Cat5e UTP) carries the four directional buttons — with spare wires connected to 0V. A DB15HD (“VGA”) connector terminates the cable at each end.

VK4MSL/BM: Radio controls

Remember how I mentioned the hand-mic in the last station was going to be temporary? Well… this is the arrangement here. Shown here is the PTT switch (red) and four directional buttons. Not all radios make use of all buttons … the FT-290R II uses only the up/down buttons, the FT-897D uses the right-hand button in addition for the “fast” button. Homebrew microcontroller-based radios I build will probably use all five shown for a menu interface. There’s no display in front of me, so don’t ask for an accurate signal report, I can tell you whether it’s a Q3 or a Q5, but any S-meter reading will be a wild guess. Future expansion of this may include a small potentiometer for a local volume control, and a small microcontroller-driven LCD that could be used to interface to the FT-897D’s CAT interface… but this is just early days.

Now… I did say I managed to mount a HF antenna on here and make a contact with it. The contact into VK5 that yielded this QSL card was made using a 6′ long CB whip… the station looked a lot like this:

VK4MSL/BM: HF Antenna

The flash was needed here, took me a while to figure out where I had put the bracket (I don’t plan to ride with the HF antenna or bracket mounted very often). This is fortuitous in a way since you can now more visibly see the 2m antenna. The CB antenna mounts on a nearly identical bracket. I don’t bother with the radial out the back, as there’s no way I’ll make one long enough that would be practical. The antenna will need some work, in particular, either addition of a base-load coil, or modification, to make it resonant on the amateur bands.

VK4MSL/BM: Closeup of basket with HF mount

With a small base-load coil, I should be able to make it a half-wave end-fed on 6m… some more and I should be able to make an end-fed quarter-wave on 10m. This will be the subject of future experiments. This tuning section will probably mount on top of the mounting itself, underneath the spring shown on the right.

And to answer the number one question I get from non-radio amateurs… no… I do not have a camera on my helmet (this is not a camera)… I am not broadcasting video. Kindly don’ t act like someone excited to be on television… you’re not. 😉