WU2X wrote: He had purposefully chosen 78 feet over even higher heights
because HFTA showed that higher heights actually raised his angle of
radiation towards Europe. .....
Take off angle from irregular terrain has even less meaning than it does
from flat ground. The only thing of significance is the gain at a
particular angle. If you are saying the gain toward Europe (at a
particular angle) is less at one particular height than at another, then
I would buy that. So that depends on where you are located (to
determine the best angle), your terrain, and the height of the antenna.
It is very possible (over any terrain) to raise an antenna height to a
point where it will show less gain at the angle you calculate as best
for Europe.
It's very difficult to determine how accurate HFTA is. You would need
to do a lot of A/B testing with two identical antennas located at
different places, and those places would likely be separated by a
significant distance to have one of them be on flat ground and the other
to have sloping ground. Then how do you know that the separation of the
places is not a contributor. When you show a difference, how do you
determine the cause?
In general when you have sloping ground and you have just one antenna
for general purpose use, the answer you are most likely looking for is:
what is the minimum height I have to put this antenna before I reach a
point of diminishing returns on low angle gain, and at the same time
keep the first null from getting to low. But that is also the same
question when you have flat ground, however, the answer is usually a lot
different for uneven terrain. In general with good sloping ground,
there will be an increase in low angle gain (under about 10 degrees) and
at the same time the first null will only move a very small amount lower
in angle, compared to flat ground. Over good sloping ground that makes
a single antenna look more like a stack over flat ground. It is also
very easy to get the antenna too high and cause the first null to creep
down very low.
If you go by HFTA data, it shows that my antenna on 20 meters at 55 ft
high meets the criteria I stated above. There is no significant
difference in low angle gain between antenna heights of 55 to 100 ft,
however the higher angle gain changes a lot. The lower antenna is the
winner at higher angles. To be fair, there is a significant low angle
gain that happens when the antenna is placed between 140 and 170 ft, but
that is out of my budget, because I would also need a lower antenna to
cover the higher angles.
Jerry, K4SAV
Scott McClements wrote:
> Hi All,
>
> I've been in several discussions with some local contesters/DXers over
> the High Frequency Terrain Assessment (HFTA) software included with
> the ARRL antenna book. I have used the program myself several times. I
> was a bit surprised when a local had chosen his new tower height
> solely based on the HTFA assessment. He had purposefully chosen 78
> feet over even higher heights because HFTA showed that higher heights
> actually raised his angle of radiation towards Europe. There would
> have been a limit to his tower height at some point, for aesthetic and
> neighbor relation reasons, but I don't think momentary reasons.
>
> This brings me to my main point. I went back and read the HFTA
> documentation looking for statements about its accuracy. The author
> doesn't try to oversell it accuracy (+/-3dB), but its clear not much
> has done to validate its results. So the question - Is there any real
> value to this tool?
>
> My opinion was that I would go as high as I could despite what HFTA
> said (esp. since it will include a 40m yagi). Perhaps I am wrong - I
> was curious what the consensus was. I am asking this with all due
> respect - I know there was much effort put into this tool and it uses
> the technology and information we have available to us today to the
> fullest.
>
> -Scott, WU2X
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