Having just had to make an active antenna for HF (for gainful
employment-type work, not Am. radio purposes. The commercially made
one we've ordered is on five weeks delivery and it was needed before
yesterday) I was wondering about a helical element.
As the thing had to be resonably lightweight, I made the antenna
element from copper tape on 15mm plastic water pipe rather than use a
solid copper tube. Just for a bit of novelty I wound the tape in a
helix, but then started wondering if doing that would make any
difference to performance. Normally, helically winding an antenna
(rubber duck type at V/UHF) only serves to distribute loading
inductance into a short antenna to make it resonate - unlikely to
change improve the loss terms at all. But I did wonder if the added
extra inductance, or increased conductor length (not element length -
that is 1.2m) would change the performance significantly from a
straight tube.
Does anyone have any ideas - food for thought if nothing else? The
base amplifier is one I've used several times before for V/LF up to HF
based around a J310 source follower running at 20mA with bootstrapped
input followed by a 2N5109 emiter follower at Ic = 80mA. I may have
published it way-back-when in the amateur press in the 73kHz days.
Not the best design judging by some of those published in more recent
years, but did mean all the components were to hand for an instant
job.
Andy G4JNT
www.scrbg.org/g4jnt
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