Hello Brian,
thanks for the description of your antenna system.
all constructed in 2.5mm aluminium wire.
I think you wrote a little note on alumium wire.
Alumium can be a valid alternative for copper. The specific reststance is
abt. 1.75 times that of copper, but the skindepth (at 136kHz) is 0.24mm for
Al while only 0.17mm for Cu. So if the diamter of your wire much more than
twice the skindepht (as it is for a 2.5mm wire), Alumium will be only 25%
worse than Cu while its weight is only 30% compared to copper.
Under these conditions I measured the feed point resistance in the region
of 140 to 150 ohms using a noise bridge and managed about 0.8-0.9A aerial
current using the appropriate tapping on the matching transformer. The
output of the tx was 200V peak to peak or about 100W. Ignoring cable
losses R=100/(0.85)^2=138ohms and there is rough confirmation between
the measurements.
This seems acceptable if you have many trees close to the antenna. I have
several 100 trees in a range of 30m arround my antenna, loss resistance
varies from 100 Ohm in winter (strong frost) to 140 Ohm in summer.
The new measurement for feedpoint resistance is around 70ohms with a very
wide flat minima on the noise bridge. The tx is now one module of the
Decca with 54v ht and 8A or 432W input when transmitting. I measured
400W into a 50ohm dummy load so efficiency is 92.6%! The best match
on the matching transformer is between 40 and 50ohms. Aerial current
is now 3A so R=400/(3)^2=44ohm and there is a big difference between
noise bridge and power measurement.
I have some doubts about measuring a big antenna with a (low power) noise
bridge. The antenna voltage (just picking up QRN and strong broadcast) can
be so high that it 'overrules' the noise from the bridge.
I tried low power measurments in the past (with a signal generator, not a
noise source) and found that I needed at least 5V generator voltage (+/-
0.5 Watt) to get good results.
Based on the height (23m) and topload (40m) the theoretical radiation
resistance of your antenna is +/- 0.115 Ohm, so with 3A antennacurrent you
will be close to 1 Watt ERP. But measurements have shown that in many cases
the real ERP is 3 to 6dB below the calculated, so an ERP in the range of
250 to 500mW is more likely.
73, Rik ON7YD
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