Message-ID: <19213.200006021741@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Fri, 2 Jun 2000 18:48:27 +0000
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Subject: LF: Re: Aerial tests
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Dear LF Group,
	Some comments on G3JKV's recent E-mail -

I calculated the ERP on the basis of the measured antenna current 
and estimated radiation resistance of the two antennas. Part of the 
reasoning behind this was that the antenna efficiency would not be 
an issue; the radiated power is just I squared times the radiation 
resistance, and the calculated radiation resistance is a function 
only of the antenna geometry, and not it's losses. In principle at 
least, the only result of improved efficiency due to the Decca earth 
mat would be to reduce the amount of transmitter power required to 
produce the measured value of antenna current. So antenna 
efficiency did not actually enter into the calculation of ERP, and the 
presence or absence of the Decca earth would not have affected 
the result of the calculation. 

Having said that, clearly the ERP calculations are wrong, because 
there was a significant difference in signal strength, and so by 
definition ERP, between the two antennas, where the calculations 
said they should be the same. Unfortunately, my field strength 
measuring equipment is not accurate enough to say for certain that 
this was because the small antenna was producing more ERP than 
it should, or that the Decca antenna was producing less than it 
should. However, it is probably easier to believe the former, since 
the assumptions on which the calculations are based are more 
nearly met by the Decca antenna than the small inverted L.

I don't know if this experiment has much to tell us about the 
presence or absence of horizontally polarised signals; however, 
the ferrite rod antenna I used to measure field strengths showed 
the difference in field strength of roughly 4dB observed by nearly 
everyone else, and should not have been sensitive to horizontally 
polarised signals (ie. vertical H field).

Cheers, Jim Moritz
73 de M0BMU