Hi Jim, that is a very interesting comparison. I estimated the effective
height of my inverted L by extrapolating Dick's (PA0SE) field strength
measurements for DCF39 and got a figure of 6m for my 9m high aerial. The
vertical is pulled away from the gable end of the house by about 6 to 8
feet, and the top wire goes over several 12foot hight fruit trees and
alongside some 50 foot Eucalyptus for a short distance. The deficit is
amazingly similar to your findings. One thing that is nice to note is that
the extra loss for increasing the height is not proportional, so that there
is a good amount to be gained (as we found) by increasing the height of a
aerial, even in a lossy location. The effective height did not depend on the
amount of top-load capacitance (the minimum length was 30m) but the measured
loss resistance (RF Bridge) reduced considerably as I went from 250pF (30m
top-wire) to 850pF.(150m)
One question that may be pertinent ....how do the heights of the
trees/environment compare with the physical aerial height? I am wondering
whether the loss, which might be considered as fraction of the power,
capacitively coupled to a loss resistor, is mostly from the lower part of
the vertical. This would make sense because any current flowing in the top
wire does not materially add to the radiated signal and current diverted
from "just above the loading coil" cannot flow throught the rest of the
distributed (along the vertical leg) radiation resistance. This might help
explain the effective-height deficit only increasing slowly with increase in
physical height
Great Work
Happy Christmas
Alan G3NYK
----- Original Message -----
From: James Moritz
To: [email protected]
Sent: Monday, December 24, 2007 4:13 PM
Subject: LF Antenna Effective height comparison
Dear LF Group,
After doing the "open field" antenna tests a couple of weeks ago, I have
been repeating field strength measurements on my home QTH antenna for
comparison.
-------------snip
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