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.
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.