HF Bicycle Mobile Experiment - Part 1

Today I decided to make an HF antenna for my bike. I did an experiment with this once last year but it was not terribly successful. That experiment used the ATX antenna. No contacts were made but I learned something about grounding.

I decided to use the frame as a ground. G4AKC says that this can be improved by tuning the frame. I found that with a variable capacitor in series with the frame I could adjust for low SWR with the ATX set up as in the instructions. I can’t remember what bands I tried it on.

The primary aim this time was to improve the ground. Last time, I had a very temporary setup with croc clips to the frame so I decided to improve it with fixed wires. I cleaned and sanded around a bolt on the carrier and two brazons on the frame and connected the points with some mains wire. I also attached a flexible wire to connect to the antenna system.

While I was pondering how best to connect the aerial and earth with the very limited selection of BNC parts I had I decided I might as well make a proper antenna, which would make this easier. I took some of the sections of the fishing pole I use for SOTA and duct taped them to the carrier. The longest I thought I could reasonably get away with was 4m which is what I used. I decided to use the lowest HF band I could with the given antenna space, as the higher bands seem to be open far less often, so I decided on a 1/4 wave for the 21MHz band.

I started with 3.5m of wire and found the tuning was way off. I decided to try and tune it against a good earth so I drove in an earth stake. This actually made it slightly worse. I then laid out three radials and tried various combinations before giving up on finding an improvement. I then started trying to tune to what I had. I had to cut away quite a lot of wire but got the SWR down to 1.1. This was with the capacitor in series with the frame.

I tied a SLAB to the carrier and put the FT-817 in the bar bag and set off for a maiden voyage. The SWR remained constant despite shaking, moving of antenna and presumably different ground, although I did only go around the block. I could hear several American stations and one in Africa but my CQs were not answered. Eventually though, some unexpected success! I replied to N8RA in Connecticut and received a (contest) 59. He had no trouble copying my callsign first time so it must have been a reasonable signal. I tried for a while longer but with no further contacts so stopped for the day.

Future Work

Some ideas:

  • Other bands - I think I can fit in 17m and 10m may be useful now too.

  • Replace top section of fibreglass and wire with bendy steel wire. This should reduce wind resistance, weight and effect of crashing into trees.

  • Try other earth tuning arrangements. Measure earth current - possibly with a clip on ferrite transformer I got from the electricity company.

  • Physical improvement - replace duct tape. I require a system for easy removal, connection and disconnection. Must also be weather, oil and dirt proof. Should not reduce load carrying capacity of rack - needs to stay out the way of panniers.


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