Message-ID: <5.1.0.14.2.20020125183918.00a2b890@mail.pncl.co.uk>
Date: Fri, 25 Jan 2002 21:17:48 +0000
To: rsgb_lf_group@blacksheep.org
From: "Walter Blanchard" <blanch@pncl.co.uk>
Subject: LF: Gain
MIME-Version: 1.0
Content-Type: text/html; charset=windows-1252
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>
Content-transfer-encoding: 8bit

<html>
Thanks for erudite replies Jim and Rik, very interesting but I don't
think they change my proposition.<br><br>
Now, here's a follow-on. This is an extract from the two-volume 1400-page
&quot;Handbook of Antenna Design&quot; (pub. by IEE in 1982)<br>
Chapter 15 - VLF, LF and MF Antennas edited by J. S. Belrose.&nbsp; My
italics.<br><br>
Quote:<br>
15.2.1<x-tab>&nbsp;&nbsp;</x-tab>Characteristics of vertical
antennas<br>
If the vertical dipole is some distance above the earth, its impedance
and radiation characteristics are identical with a free-space dipole.
However, vertical antennas at VLF, LF and MF are usually not far removed
from the ground, and generally the antenna is fed against the ground
plane as a monopole radiator. Such an antenna is said to be base-fed or
series-fed, or base-loaded in the case of electrically short antennas.
For the purposes of analysis, <i>neglecting losses in the ground</i>, the
monopole can be considered as a dipole whose lower half has been cut off
and replaced by its image in the ground. For some purposes, the concept
of the monopole radiator and its image is useful. The currents on the
vertical parts of the radiator and on its image flow in the same
direction i.e. the currents are in phase. The currents on the horizontal
parts of the radiator and its image are out of phase. <i>If the height of
the antenna is small compared with the wavelength, radiation due to
current flowing on the horizontal part of the antenna is almost exactly
cancelled by that due to the oppositely phased current on its image, and
the antenna is described as having a 'non-radiating' top load.</i> The
equivalent length of the monopole, and its input impedance, is one-half
that of the corresponding dipole antenna. The power gain of the
'grounded' monopole antenna is, however, 3 dB greater that a dipole
antenna in free space,this is because, for a grounded antenna, all the
power is radiated into a hemisphere, rather than a sphere.<br><br>
end quote.<br><br>
So, another effect of the very lossy ground is that radiation from the
horizontal top loading wires is NOT cancelled out and they will radiate.
Unfortunately, in all the eight different volumes I have on LF antenna
design, there is not one mention of what effect this horizontal radiation
might have. The assumption by all the authors seems to be that no LF
antenna designer worth his salt would dream of NOT installing the most
efficient ground he could and therefore this situation does not occur in
practice.<br><br>
Walter G3JKV<br><br>
</html>