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[CQ-Contest] Local Terrain effect

Subject: [CQ-Contest] Local Terrain effect
From: W1HIJCW@aol.com (W1HIJCW@aol.com)
Date: Fri Jan 12 01:10:16 2001
Local Terrain -- the most valuable asset your station and antenna can have!!

Seriously, I was led to some analysis of local terrain (out from 6 to 15 Km
from the antenna) by the fortunate circumstance of having a QTH which was
located on a ridge top which afforded me a very strong signal from southern
California into Europe using 100 W from a TS940 and a classic coax fed dipole
on 20 meters. Experimentation demonstrated that antenna height above local
terrain was critical.

In my case the antenna was sited on a roof top (about 1 meter above the roof
and about 8 meters above the ground level immediately below the antenna) and
broadside to approximately 020 degrees true, the great circle bearing to
northern and central EU. Experiment showed that I could often open and close
20CW to EU from the location even though 100W and a dipole would make that
extremely unlikely.

I did some analysis with several programs and topographic maps which
indicated that the height produced approximately 9 dBd gain over a dipole
located at the same height above flat ground. MORE IMPORTANTLY, the elevation
pattern of the antenna showed that the peak gain in the vertical plane
occurred at an angle of between 1 and 2 degrees above the nominal horizon. To
achieve the same result over flat ground the antenna would have had to have
been at a height of more than 30 meters.

On the bearing in question the average down slope was approximately 12 to 15%
for a distance of about 1/2 Km, and continued at an average of 4 to 5% down
for another 3 to 4 Km before flattening out.

In the reverse situation, I was able to use a club station here in southern
California for the 1999 CW sweepstakes. Power was 1500W and the antenna was a
seven element log periodic at a height of just about 40 meters (130 feet)
above level ground. My experience (about which I wrote to the reflector back
then) was that such a height above average terrain was way too high for a
domestic contest. My hypothesis was (and is) that the resulting low angle of
radiation placed the first return point of the refracted energy somewhere in
Ohio (which was OK) but put the second return point somewhere near Greenland
(not OK for a domestic contest). The result was that I seemed to work every
possible station in Ohio, but I had a hard time hearing (and being heard by)
stations in W1, W2 and W3.

So the bottom line is that "higher or lower?" isn't a complete question. One
has to ask higher or lower for what purpose. And always keep in mind that
higher or lower should be measured using what's often called HAAT (height
above average terrain) which is usually considered to be that out 5 to 10 Km
from the antenna, but in any case a minimum of 20 to 30 wavelengths.

The best treatment of the whole subject is by N6BV in recent editions of the
ARRL Antenna Book and some editions of the Antenna Compendiums(ia)

de Bill, W1HIJ/6, FO0SCH

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