Bicycle Mobile with KX3 July 31st, 2014
The picture above shows my latest set up for bicycle mobile. I hasten to add that I do not operate on the move as I don’t think it is practical or indeed safe. I use an Elecraft KX3 transceiver powered by AA cells. That way I operate in the lightest possible way and use the Elecraft dedicated paddle key. My bike is a Tern Link7 that folds up really small, yet is very rugged. That way I can easily put it in the boot of even a small car.
For the antenna I have chosen the Buddistick with their clamp p[tion arrangement to attach it to my cycle carrier. The complete antenna set up can be purchased as the Buddistick Deluxe kit. I run a short earth wire from the bike frame to the base of the antenna feed socket on the Buddistick clamp. The whole antenna system together with the KX3, easily fit into my pannier bag which in turn easily detaches from the bike. For more information on the bike, check Evans Cycles on the web.
There are a number of considerations to observe when using the setup. The most important is the earth counterpoise. I try to keep everything simple and compact. A separate earth counterpoise does the job but is a bit of a nuisance in laying it out, particularly in public places. Setting up the resonance, even with the counterpose, can be unstable. The whole system VSWR can vary depending on whether you are touching the transceiver case or not. At first I failed to resolve this issue, but some thought and some tests resolved the problem.
Firstly try and find the best tappings for each band on the Buddistick. You can fine tune with the whip and then make a note of the settings. The coil taps can be fixed so it is only the whip length that needs to be recorded. Do all this without an antenna tuner. This is very important. On the KX3 you can programme one of the short cut keys to bypass the internal ATU for you. Now once this is done you really do need that antenna tuner to stabilise the VSWR and make the system less “touchy.” I love the internal ATU for the KX3 as this is just amazing in the way that it matches in a fraction of a second, even wide impedance ranges.
The bike metalwork does not provide enough earth for bands below 21MHz. I found that the easy and most effective way is to simply use a fairly long feeder to the transceiver which also acts as a counterpoise. Mine is about 15ft long. A choke formed by a few turns of coax on a ferrite former at the transceiver end will make things even more stable.
The beauty of the system is that it is very simple, very efficient and very compact. It is possible to consider using a radial stretched above the ground as a resonant counterpoise, but it does mean a little more work in setting up. I prefer to use just the bike frame and the coax feeder. The KX3 has super low battery consumption. If you really wanted to raise the power you could use an external 12v supply and run up to 12W. The KX3 still works at battery levels down to 8V so there is no sudden death. The radio also has very comprehensive system monitoring so you can watch the battery volts very easily.
Give it a try.