David,
HFTA will give some false information with close spacing for stacked
antennas. I don't think you would want to stack them that close -
less than a half wavelength.
Not that you can do it but it would be interesting to see what the
relative figure of merit was if the tower was instead located 60
meters closer to the drop off. In that location you may find that one
antenna at a certain height is as good as it gets in the direction of
the drop off.
Stan, K5GO
Sent from Stan's IPhone
On Apr 21, 2010, at 7:42 PM, "David J. Sourdis - HK1A" <hk1kxa@hotmail.com
> wrote:
>
> 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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