Geoffrey Mackenzie-Kennedy wrote:
A type of antenna that I keep packed for use at short notice is a
halfwave centre fed dipole for use as a vertical, where part of the coax
feeder forms the bottom half of the dipole.
How do you terminate the radiating part of the coax? I've tried antennas
based on this principle both on HF and 2m. On HF, I tried the "no
counterpoise antenna" by M3KXZ with very disappointing results. On VHF,
I've had good results with the coax antenna by LA1IC described (in
Norwegian!) at http://la2t.org/teknikk/vertikal2m.html.
Choking off the radiating part of the coax is challenging, because we're
trying to choke a voltage node. I was told that the coil which
terminates the radiating part of the coax on the 2m antenna is chosen to
resonate as a parallell trap at 2m with its stray capcitance. On the HF
antenna, I just wrapped the feeder around a large toroid 10-15 times,
and that did not work well.
My 2m antenna needs are modest, so take this with a grain of salt.
73
LA4RT Jon
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