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
| 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
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.
VK4MSL/BM HF: Mark II
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.
well done stewart