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From: "Ken" <ken.h.wright@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Thu, 25 Feb 2010 10:05:56 -0000
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Subject: LF: RE: Re: VLF_8.79 kHz
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Hi Jim.
I think I could knock up a quick 9kHz RX here in Luton over the weekend if
that would help. I have been following this thread with interest, could you
put up some diagrams etc of your parallel capacitor and other antenna
configurations and formulae.

73.
Ken
M0KHW    IO91TV

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of James Moritz
Sent: 24 February 2010 21:14
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: VLF_8.79 kHz

Dear LF Group,

A couple of years ago I did some measurements of Rloss of the 10m high, 40m 
long inv-L antenna at my home QTH, and also a near-identical antenna set up 
in the middle of a field away from trees and buildings. The attached graph 
shows Rloss plotted against frequency over the range 10kHz to 600kHz for 
both antennas. At all frequencies, the loss resistance of the open-field 
antenna is much lower than the home QTH antenna, which is surrounded by 
numerous small trees. The ground in both cases was 4 x 1m ground rods, close

to the feed point of the antenna. The actual ground around both antennas was

very similar - a waterlogged clay soil.

At 10kHz, the open  field antenna has Rloss of 50R, against 380R for the 
home QTH antenna. Both antennas show a decreasing Rloss with frequency - 
this suggests dielectric losses are dominant (the antenna voltage increases 
at lower frequencies for a given current) in both cases. The text books say,

for electrically small antennas, that dielectric losses will dominate at low

frequencies, while at high frequencies the skin effect will eventually cause

resistance to start increasing - in the case of the open field antenna, a 
turn-over point might have been reached at a few hundred kHz.

The 50R figure suggests that antenna efficiency might actually be higher at 
9kHz than people are expecting - at least in an open field site. A suitable 
loading coil would be a problem. The antenna capacitance was around 350pF - 
in these experiments I used a ferrite-cored coil of around 0.7H with a Q of 
about 150, but this had a loss resistance of about 300R. If you tolerated 
loosing half the TX power in the loading coil, a similar inductance with a Q

of around 1000 would be needed. Increasing the top-loading capacitance of 
the antenna would definitely be useful...

Cheers, Jim Moritz
73 de M0BMU