Hello Jon,
My apology for a slow response, waging war with the grass all day!
To terminate the radiating part of the coax I use a high impedance trap
whose inductance is a length of the incoming feeder wound as a solenoid,
with the turns spaced slightly. To determine the capacitance required to
resonate the coil formed by the outer of the coiled coax I use an air
variable capacitor, then replace it with a length of open ended RG-213 as
the capacitor. The important thing to note here is that the capacitor made
using coax must not be connected as an open ended stub, which could upset
the trap. I would also suggest that relying on stray capacitance to resonate
a trap is a questionable approach.
During my first attempt I used a large quantity of cores over the coax which
did not work well because the choking impedance was too low, in fact I blew
some!
With this method of feed it also becomes a relatively simple task to feed a
top loaded T antennas from ground level without the use of a high impedance
matching network, placing the current maximum at the top.
For multiband use I had considered using Close Coupled Resonators in a fixed
installation, but not for something to be rolled up.
Thank you for the link to LA1IC's antenna, much appreciated.
73,
Geoff
GM4ESD
Jon K Hellan LA4RT wrote on Friday, May 09, 2008 1:28 PM
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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