Dear Rik, Johan, LF Group,
I should explain that I re-measured my "10m high x 40m" inverted L, which is
actually 9.5m and 10.5m at the ends, and reaches a lowest point of about
8.7m near the middle, partly due to the wire sagging and partly due to the
ground not being perfectly flat. Based on this, I calculated a somewhat
lower radiation resistance, 0.022ohms at 136k and 0.31ohms at 503.8k. This
should give about 88mW ERP at 503.8k with 0.4A antenna current, and at
136.0k, with 3.9A antenna current, ERP should be 610mW.
This compares with the values derived from the field strength measurements
of 43mW at 503.8k, and 180mW at 136.0k. The efficiency calculation must also
take into account the resistance at the antenna feed point (essentially the
loss resistance). At 503.8k this was 25ohms, making the TX power 4.0W.
Assuming the antenna has 2.6dB directive gain over a dipole, 43mW ERP
corresponds to radiated power of 24mW, making the efficiency 24mW/4W =
0.59%. At 136.0k, Rloss was 63ohmsand TX power was therefore 958W. Radiated
power was 100mW, making efficiency 0.01% at 136k. Hence the factor of 59.
The difference in efficiency could be divided into 3 areas:
i) Increase in radiation resistance at higher frequency - assuming the
antenna remains electrically "small", Rrad should increase by a factor of
13.7 (11.4dB) from 136.0k to 503.8k.
ii) Decrease in loss resistance at higher frequency - Rloss decreases by a
factor of 2.5 (4.0dB) from 136.0k to 503.8k, for this particular antenna.
iii) Changes in an additional "site loss" that represents the difference
between the ERP calculated from antenna geometry and antenna current, and
the ERP determined from the field strength measurements. This site loss at
136k is 180mW/610mW = 5.3dB, while at 503.8k it is 43mW/88mW = 3.1dB, making
the difference in site loss between the two frequencies 2.2dB
This adds up to 17.6dB difference in efficiency in total, which is a factor
of 58... there is a bit of rounding off involved!
The "site loss" is substantial in both cases, although less at the higher
frequency. This was also true in comparing the results of FS measurements I
did on 73kHz and 136kHz some years ago.
I don't think ground losses have a significant effect on the propagating
signal level over these relatively short distances; if they did, I would
expect the field strength to decrease more rapidly than 1/distance, and so
give an ERP that decreases with distance, which it does not. For the same
reason, I think that the "site loss", whatever it is, must occur close to
the antenna, certainly within a couple of hundred metres, otherwise this
would also cause the ERP calculated from the field strength measurements to
vary with distance - this is why I think the term "site loss" is justified.
I guess that for this argument to be convincing, what would be desirable
would be to do experiments on an antenna on a site that gave field strengths
corresponding exactly to the calculated values, i.e. site loss of 0dB, which
would probably need a big flat field somewhere. Of course, it would be even
more interesting if there were an antenna with significant "site gain"...
Cheers, Jim Moritz
73 de M0BMU
|
