Dear LF Group,
To complement the 136kHz field strength measurements done a few days ago, I
have now made some measurements on 503.8kHz. The average of 57 measurements
with an antenna current of 400mA came to 43mW (-13.7dBW), and the antenna
efficiency 0.59%. This is for the 10m high, 40m long inverted L, which was
one of the antenna configurations used for the 136k measurements. The
antenna efficiency on 500k is 59 times greater than on 136k - illustrating
how much easier it is to "get out" on 500kHz, in spite of the poor
efficiency on both bands.
Qualitatively, the field strength results were rather similar to previous
experience at LF - a few dB variation in ERP occured between measuring
sites, with most of the results being within 2dB of the mean value. I
rejected 4 measurements that were made along the road where the overhead
phone line to my QTH runs - as was the case on 136k, some of these showed
several dB variation from the average, and it seems the phone line acts as
an extension of my antenna over quite a wide frequency range (unfortunately,
not a practically useful one, though - it clearly does not do much for the
overall ERP, I suspect it is also responsible for coupling some of the local
QRM into the TX antenna). I made measurements at distances between about
230m and 6km according to the GPS receiver, and a wide range of different
directions - plotting ERP against distance did not reveal any particular
varying trend, indicating a F.S. proportional to 1/distance relationship
holds over this range. Plotting ERP against bearing indicated no detectable
directional effects. This is what you would expect for an electrically small
500kHz vertical antenna.
I also re-measured the dimensions of the antenna, since various changes have
been made over the years. I calculated a new value for the effective height
of 8.3m (previously, I had used 9m). Working backwards from the ERP
measurements, the effective height at 503.8kHz is 5.8m, and at 136kHz it is
4.5m. Whatever is reducing the radiation from the antenna is frequency
dependent, by about -3dB at 500k and about -5dB at 136k.
Cheers, Jim Moritz
73 de M0BMU
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