Paul,
A J Pole for any HF band like 10M, will be 3/4 wave tall! TWELVE FEET. A J
pole also needs a ground plane! This was recently covered on the L. B. Cebik
antenna web site, (W4RNL) [Just search his call and you will get the long URL
he has at Univ. of Knoxville, Tn.]
There are a couple of other limited space mobile antennas. In the Don Johnson
mobile books, he talks about the John Reinartz antenna. This was a pole with
a wire going up, doubling back in a series of little "folded dipoles" and on
up the pole, with those "dipoles" distributed around the pole. Would have to
be done with small gauge wire on a bike flag pole of fiberglass, but might
work. Start with a quarter wave for the lowest frequency band of interest.
This might make 20M fit on a bike pole as a guess. You still would need a
counter poise that might work if it were a horizontal ham stick helical whip,
and you would need a L network antenna transmatch, but, it should work better
than a wire straight up the flag mast, and be shorter than a J pole. Two ham
sticks could be done as an Ell antenna, one vertical, one on ground side,
horizontal to rear of bike. Of course, Ten Tec made shortened ham stick
antennas at one time, and still had a stock of ones for some WARC band.
Of course, if what you intended is VHF, 2M and above, a J pole is doable on a
bike. It is hard to match without an adequate ground plane under it, however.
You of course need something like an Autek or MFJ antenna analyzer to find the
tap point of a J. You might be able to with SWR alone, but they are a class
of ground dependent antennas, since the matching parallel line is just base
loading a whip, per L. B. The parallel line simply replaces a coil and cap
resonating network.
Good Luck, and be careful to not have too unwieldy a vertical, for it affects
your balance!
Stuart K5KVH
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