Mike, G3XDV wrote
I look forward to further discussion on exactly what theory fits this
practical result.
I don't think I can add a good portion of theory why it works, however, I
can through my two cents into the discussion supporting what Mike (and
other's) have found out when we elevated our loading coils.
When I started with my 18m umbrella antenna (based on a fiberglas-mast, for
details see http://www.qru.de/station.htm) I used a 3.8 mH coilt that was
inside my shack, basically at the base of my antenna. When I decided to
bring a 3.8 mH coil 1/3 up the mast height (= 6m above ground) I could
measure DCF39 coming in about 2 dB better. Reports from other stations as
well as actual field strength measurements that I performed indicated that
I gained even more dBs on my transmit way. As the actual mast that carries
the antenna is a fiberglas telescope mast, the loading coil is not
disturbed by and metal around it other than the antenna wire itself.
My explanation for the un-symetrical gain on RX and TX: with the coil being
in my shack and a voltage of around 10 kV at the top of the coil the losses
from the antenna wire along the wall and especially through the wall might
have eaten up some of the power I tried to radiate simply by shortcutting
the voltage to ground. After I now have only a 900 uH variometer in my
shack for fine-tuning, the voltage fell probaly to less than 1 kV and,
accordingly the losses inside my shack are much lower, helping to increase
the overall ERP.
As said in an earlier mail where we discussed how to isolate the coil
against rain, snow, etc. I described that my coil is not protected at all
against humidity or snow. With 350 Watt output I have no flashover
problems. From field strength measurment I estimate my ERP (not EMRP) to be
around 350 mW, giving an overall 'gain' of this antenna configuration of
around -30 dB.
Best 73
Geri, DK8KW (W1KW)
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