Message-ID: <14546.200102201715@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Tue, 20 Feb 2001 17:19:56 +0000
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Subject: Re: LF: ERP
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Dear Stewart, LF Group,

> G3YSX> I thought that the Puckeridge test showed this, ie that your
> smaller antenna had a higher field strength than you expected.

Yes it did, but only by about 1.5dB as I recall - this would be within 
the limits of experimental error. The thing about Puckeridge was 
that it was a near ideal site - moist soil, open fields for miles and 
so on - where you might expect good results. Indeed, when I put 
the same antenna up at home, and repeated the experiment, the 
field strength was down on the calculated value by about 4dB, so 
there was 5dB or so lost just by moving the antenna to a less ideal 
location. A loss of several dB compared to calculated values is 
typical for field strength measurements on "back garden" antennas, 
such as mine and G3XDV's, and some bigger ones too, like the 
MB2HFC antenna at Windsor. 

There are some plausible explanations as to why the field strength 
of the small antenna at Puckeridge could have "higher than 
theoretical" efficiency, such as increased effective height due to 
the skin depth of the ground being significant compared to the 
physical height of the antenna at LF. There are also plausible 
reasons why antennas in built up areas give lower field strength, 
such as absorbtion by surroundings. High ground wave losses in 
urban areas are well known to MF broadcast people. Unfortunately, 
the losses seem to win over the gains for most of us. However, 
there is not really enough experimental data of high accuracy to 
say much more than that.

Cheers, Jim Moritz
73 de M0BMU