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Dear LF Group,
Didn't work first time, so try
again...
----- Original Message -----
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. The field strength
measurements generally give lower values for ERP than those calculated from the
dimensions of the antenna and the antenna current, which could be interpreted as
reduced effective height of the antenna due to the screening effect of the
environment around the antenna. I did several sets of measurements at 136kHz and
503.8kHz with the antenna in 4 different configurations - inverted L about 40m
long at nominally 8m and 10m high, and the same wire supported in the middle
with a telescopic fibreglass mast to make an inverted V with the apex at about
13m and 15m high. The radiation resistance of each configuration was
calculated using NEC-based antenna simulator software, and from this the
effective height was calculated. This was compared with a "measured" Heff
determined by working backwards from the ERP values and the antenna current
measurement
The graph in the attachment compares the calculated and
measured Heff for the four different antennas ("Winter 2007"). For comparison,
the graph also includes results of similar measurements on the 10m inv L and 15m
inv V made earlier this year ("Summer 2007"), and the 10m inv L "Open field
site" measurements done recently. In the current measurements, the measured Heff
for the home QTH antenna is roughly 2m less than the calculated value, with Heff
being slightly lower for 136kHz than 503kHz. The summertime measurements show a
somewhat larger reduction in Heff, and a greater difference between 136k (about
4m reduction in Heff) and 500k (about 3m reduction). As mentioned before, the
measured and calculated open-field site antenna results match quite closely. For
the 10m inv L, for which all 3 sets of measurements were available, the apparent
effective height for essentially the same antenna varies over a range of about
2:1.
So it looks like a simple approximate way to model the effects of
the environment on the radiated signal from the antenna is to subtract a fixed
amount from the effective height of the antenna. The amount to be subtracted
appears to depend on the time of year. Perhaps this is due to the trees
shedding their leaves. The cold weather last week when the measurements
were made also has the effect of reducing the loss resistance of the antenna -
perhaps the freezing conditions also affect the effective height. Also, the
reduction in Heff seems to be somewhat frequency dependent, with a greater
reduction at lower frequencies. The same was true for 136k/73k measurements I
did a few years back.
I also measured the loss resistance of the 4
antenna configurations - Rloss reduces with increasing antenna height. At 503kHz
it was 24ohms for the 8m inv L, decreasing to 19ohms for the 15m inv V. At
136kHz, it was 66ohms for the 8m inv L, down to 43ohms for the 15m inv V. These
figures are much larger than the "open field site" antenna, which was about
8.5ohms at both 136k and 503k.
Happy Christmas to all on LF, and
all the best for 2008,
Cheers, Jim Moritz
73 de
M0BMU
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