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.