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Date: Fri, 11 Feb 2000 18:57:37
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: ERP and field strength again
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At 15:54 11/02/00 +0000, M0BMU wrote:
<...
>I have revised my estimates of the radiation resistance of my 
>antenna, using EZNEC. If zero losses and a perfect ground are 
>assumed, EZNEC gives a value for the radiation resistance of the 
>antenna as 19 milliohms, as against the original estimate of 
>21milliohms, for my 8.1m high antenna. I also tried estimating the 
>
>...
>Using this new value, and 1.4 A antenna current, radiated power 
>comes to 33mW, and ERP is 60mW. The expected field strength at 
>4.2km is 408uV/m, against the measured value of 195uV/m. The 
>discrepancy is reduced a bit, from 7.8 to 6.4dB.
>...

Hello Jim,

Whatever your ERP, is you have a good signal in the centre of Belgium. It
proves that with a modest TX and antenna you might not be 'the king of the
mountain' but you can work a lot of stations.
Regarding the ERP maesurements : you are using a magnetic antenna to
measure the ERP of an electric antenna, so as long as you are not 100% in
the far field it is quite normal that what you measure is lower than
expected. At 136kHz a distance of 4.2km is less than 2 wavelengths, maybe a
bit too less to have a proper far field. I remind that in an article about
medium wave antennas (600kHz range) the minimum distance to measure ERP was
set at 1 mile from the antenna (so about 3.2 wavelengths), converted to
136kHz this would mean 7km.
It would be very interesting to find a reliable (and not too difficult way)
to measure ERP as this is the only way to know wether we stay within the
legal 1W ERP limit.

73, Rik  ON7YD