Message-ID: <393A6092.1E13426E@netscapeonline.co.uk>
Date: Sun, 04 Jun 2000 13:58:43 +0000
From: "g3kev" <g3kev@netscapeonline.co.uk>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Re: Aerial tests
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James Moritz wrote:

> 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 this makes the whole comparison tests between the small inv
L and the large vertical
FLAWED. Next time compare the large vertical against a small 9 metre
vertical without any horizontal wires. Resonate each vertical in turn and
check the reports. I will put my money on the large vertical.
G3KEV

> 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