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Date: Fri, 25 Feb 2000 15:43:12
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: 73kHz on Friday? / Field strength
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At 12:59 25/02/00 +0000, G0MBU wrote:
>There is therefore a discrepancy of -9.6dB between expected and 
>actual ERP - even greater than the -6.4dB difference that was 
>found for the M0BMU measurements a few weeks ago (calculated 
>E = 408uV/m, measured E = 195uV/m). It was suggested at that 
>time that part of the difference might be due to the short distance 
>between transmitter and receiver not being enough to achieve 'far 
>field' conditions, however, Mike's QTH is nearly twice as far as 
>mine, so would be expected to show less loss from this cause.

To find out where the 'far field' begins following experiment would be
interesting :
Put an electric and magnetic 'field probe' on a car. The electric 'field
probe' could just be a short (50 cm) vertical (eg. 144MHz lambda/4) matched
to 136kHz (by LC circuit), for this small antenna the car will be
sufficient 'stable' ground. The magnetic 'field probe' can be a small loop
or ferite rod antenna.
Then measure the signal strength with both antennas at different distances
of the transmitter. As long as you are in the 'far field' the ratio between
the 2 signalstrengths should be constant, when the 'electric' becomes
relatively stronger then you are entering the 'near field'.
As you are measuring ratios the antennas do not have to be calibrated, so
they are easy to make. But you will need a calibrated receiver
(signalstrength) or adjustable attenuator between the RX and the antennas.

73, Rik  ON7YD