If youre building a Beverage/BOG as the thread title indicates, resonance
does not matter, it is a nonresonant slow wave antenna.
For a resonant antenna up in the air and used for transmitting the
insulation adds 3-5% to the electrical length.
Carl
KM1H
----- Original Message -----
From: "Bruce" <k1fz@myfairpoint.net>
To: <topband@contesting.com>
Sent: Sunday, December 30, 2012 1:15 PM
Subject: Topband: Fw: Short Bogs
.>> If we want to make an antenna electrically longer through cable
velocity
factor, it can't be done by the insulation slowing the wave inside the
shield.
73 Tom
In my experience, external insulation (through synthetic covering or
frost) lowers the resonant frequency of wire antennas of a given length
and height above ground.
For example, I typically use 453/F as a starter length for insulated wire
dipoles...it's higher for bare wire of the same AWG diameter.
It may not be due to "velocity factor" (not sure of the reason), but it
is observable and repeatable.
73, Gary NL7Y
I also have had insulated wires of the same AWG diameter resonate lower in
frequency than bare ones of the same length.
Think we need multiple people experiment:
Put up an antenna with covered wire. Measure the frequency, then cut the
insulation away (of this same piece), and re-measure the frequency.
If we get different results from different people, then we can start
looking
at the insulation. Is it economy grade made for 60 cycles (600 volts), or
is
it the good stuff ???
1. If it is low grade insulation with resistance at 1.8, 3.5 MHZ, then it
may be making the wire appear to be "fatter". If true, then that would be
valid reason.
2. If the insulation is high quality at HF radio frequencies, and IF the
frequency changes, then we need to look further
73
Bruce-K1FZ
www.qsl.net/k1fz/
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Stew Perry Topband Distance Challenge coming on December 29th.
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Stew Perry Topband Distance Challenge coming on December 29th.
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