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LF: Gain

To: [email protected]
Subject: LF: Gain
From: "Walter Blanchard" <[email protected]>
Date: Fri, 25 Jan 2002 21:17:48 +0000
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Thanks for erudite replies Jim and Rik, very interesting but I don't think they change my proposition.

Now, here's a follow-on. This is an extract from the two-volume 1400-page "Handbook of Antenna Design" (pub. by IEE in 1982)
Chapter 15 - VLF, LF and MF Antennas edited by J. S. Belrose.  My italics.

Quote:
15.2.1  Characteristics of vertical antennas
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, neglecting losses in the ground, 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. 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. 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.

end quote.

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.

Walter G3JKV

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