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Message-ID: <5.1.0.14.0.20020722115457.00aca6e0@gemini.herts.ac.uk>
Date: Mon, 22 Jul 2002 12:34:47 +0100
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: Re: LF: Calculating ERP from measured field strength
In-reply-to: <17Vx1m-26WSIKC@fwd06.sul.t-online.com>
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At 18:24 20/07/2002 +0200, you wrote:
>Hi Jim,
>
>you measured field strength at distances between about 1.5 and 3km.
>did you ever measure at distances >3xlambda, p.e. abt 7km?
>if you did, did you notice different results caculating the ERP?
>
>regards
>Uwe/dj8wx
>
Dear Uwe, LF Group,

Yes, last year I measured field strength at distances between a few 100 
metres and about 10km - The ERP calculated was reasonably constant provided 
the distance was greater than about 600m. Dick's measurements seem to show 
that this is also true at 329km, provided you take ground loss into 
account. Distances < 600m gave higher apparent ERP, which is consistent 
with being in the near field of the antenna.

The text books say, with various qualifications, that the far-field region 
with FS proportional to 1/distance starts at about lambda/2pi, ie 350m at 
136k. The limiting factor for long distances would seem to be the presence 
of skywave, especially at night. I used distances between 1.5 and 3km 
partly because I knew these would give consistent measurements with good 
SNR, and partly to reduce the amount of petrol I needed to put in my car!

Cheers, Jim Moritz
73 de M0BMU