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
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