Andy Talbot wrote:
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.
The voltage V to the fet will be a function of L
the antenna length.
The attenuation in the input circuit will be
Cfet/(Cant + Cfet)
So shortening a given strip of copper by wrapping
it in a helix will reduce V whilst keeping the
attenuation the same.
You would do better to either run the tape straight
to the same linear length which will increase the
voltage in the ratio StraightLenght/WrappedLength,
or fill in the gap in the helix which will increase
the voltage to the fet in the ratio
(GapArea + CopperArea)/CopperArea.
One problem with wrapping the copper around the outside
is that you make the antenna more sensitive to rain noise at
LF - or so I recall from the Decca folks.
Stewart/G3YSX
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