Dear J, LF Group,
Experience with "back garden" wire antennas considerably smaller than yours
yields loss resistance of typically several 10s of ohms. Allthough it is
rather unpredictable, Rloss tends to get smaller as the size of the antenna
increases, so guessing at 15 - 30 ohms Rloss should not be too far out. With
Rrad around 0.2ohms, this would give efficiency of the order of 1%, which is
certainly rather good for an amateur LF antenna. The amount of TX power you
need depends on how the 1W effective radiated power is defined in your
licence - assuming 1% efficiency, 1W EMRP (relative to a monopole reference
antenna) would require 100W, 1W ERP (half-wave dipole reference antenna)
requires 55W, or 1W EIRP (isotropic reference antenna) about 33W. So a TX
giving a couple of hundred watts should have ample reserves of power.
I estimate you would need a total loading inductance around 1mH. It is
fairly easy to get a Q of around 200 with quite ordinary, physically large
coils, resulting in additional loss resistance of around 4ohms, which will
increase the TX power requirement by about 20%. The required antenna current
would need to be no more than a few amps, leading to an antenna voltage of
only a few kV, which should not cause breakdown problems. I don't think
having seperate loading coils for each top wire would bring any advantage -
since the loading coil effectively cancels the reactance of the antenna,
quite small errors in value of L would lead to a large imbalance in the
current between the two top loading wires, which is not desirable. Just
connecting both wires together and using a single loading coil would ensure
an even division of current and be easier to set up. Using a single downlead
would also reduce unwanted capacitance to the mast. It might be easier to
have downleads connected to the Vee wires several metres away from the mast
connected to a loading coil near ground level - this would cause a minor
reduction in effective height, but would be mechanically much easier.
As far as matching goes, adding a "fine tuning" coil as you suggest enables
tuning the reactance to exactly zero, and then a small ferrite-cored
transformer can be used to match the antenna resistance to the PA output.
Since this matching network effectively has only one reactive component, it
should result in the lowest losses.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: J. Allen <[email protected]>
To: <[email protected]>
Sent: Friday, January 07, 2005 7:35 AM
Subject: LF: Antenna plans de J.
Scott,
The main 137 antenna will be an inverted-L with two, 392 foot long #10
copper clad horizontal wires fanning out from the tower in a V.
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