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.
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