Try using only two yagis and more separation. I think that the gain will be
about the same and it will have better F/B.
John KK9A aka P40A
To: "towertalk@contesting.com" towertalk@contesting.com
From: "David J. Sourdis - HK1A"
Date: Wed, 21 Apr 2010 19:42:33 -0500
We have been running HFTA for a 3 stack of 5 L for 28 MHz. The tower is on
the
side slope of a small hill. The tower base is about 15 meters above the
relatively flat terrain in an azimuth of interest. The horizontal distance
from
the tower to the hill foot where the "flat" terrain starts is about 60 - 70
meters.
Testing with HFTA at different heights and stack separation, we found that
the
best heights are 18/21/24 meters, in order to cover with the best gain and
angles the wave angle of arrival stats.
Obviously, that is quite a short separation compared with the range of stack
separations that is normally used.
This short stack separation has its toll on the antennas' feed point
impedance.
MMANA shows that F/B ratio goes down from 20 to 10 dB and gain losses almost
2
dB. Of course, MMANA calculates this on flat terrain.
We just tested one antenna today at 0,5 WL and the SWR curve is as expected.
Let's say terrain profiles are OK. Keeping in mind that HFTA can be tricky
with
10 and 15 meters. What should we do?:
-Follow HFTA results, short stacking distance and readjust the antennas to
recover some FB and gain and fix feed point values?
-Try to find another combination with similar results and more spacing?
-Look for lower heights? Why this?
Because one thing I believe is that on short slopes or small hills, if your
antennas are too high the "ray" that goes down and is reflected by the
ground
is going to hit on the flat terrain away from the hill instead of taking
advantage of the terrain profile by hitting on the downward slope and
getting a
lower angle on ray reflection compared to the flat terrain reflection
geometry.
Has anyone had a similar experience?
73
David
HK1A
EC5KXA
ex-HK1KXA
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