Hi, Tony.
The optimum angle (takeoff or reception) for signals is a function of
your latitude/longitude, the particular path you are trying to work, and
the magnitude of solar flux. N6BV ran tons of VOACAP analyses for every
month over an entire sunspot cycle to determine statistics on the
optimum angle for various great circle paths and included them in chart
form on the CD that comes with the ARRL Antenna Book. That same data is
incorporated into HFTA, the terrain analysis program he authored that
is also included on that same CD. I don't know if he ran that analysis
for QTH's in South America, but I'll check later tonight and see.
In general, angles less than 5 degrees (sometimes down to 2 degrees)
show up as being important, although we may need to take that with just
a small grain of salt. I've used VOACAP quite a bit myself and the
author of the program states that of all the various parameters it is
capable of predicting, takeoff angle has the least supporting data and
the greatest probable error. I can't guarantee that angles down to 2
degrees are relevant, but the benefits of tall towers for opening and
closing a band suggest that those angles may indeed be important.
As far as your potential QTH goes, you may not need to worry about that
hill anyway, though. HFTA shows that refraction can bend a signal over
some obstructions to a surprising degree. I've even run dummy terrain
profiles in HFTA that predict a lower taekoff angle with a small hill in
the distance than if the hill wasn't there. It all depends upon the
shape of the obstruction and the incident angle of a ray hitting it.
Any chance that you can get topographic data for the terrain between
your QTH and that hill? If so, I'll work up a terrain file for HFTA and
check it out for you. HFTA can, within it's accuracy limits as noted
above, even tell you what percent of the time your setup would be unable
to achieve the preferred angle.
Now for some pure conjecture on my part. The shape of the terrain
between you and that hill (and immediately beyond it) influences the
strength of your transmitted signal as a function of takeoff angle, but
I strongly suspect that the profile of the terrain on the far side of
that hill influences the strength of your received signal as a function
of incident angle. I can't prove it since HFTA doesn't have the
capability of looking at angles that are negative with respect to the
horizon, and I haven't yet figured out any other practical way to
investigate it. But it seems reasonable that if the terrain profile
affects signals going in one direction that it would affect them in the
reverse direction as well, and if the terrain features were not
symmetric (most aren't) that they could have a different impact in one
direction versus the other, especially for really low angles. If you
buy into any of this last paragraph, the real answer to your question on
long haul reception would be "indeterminate".
73,
Dave AB7E
Tony Rogozinski wrote:
> I am trying to determine what would be the lowest angle from which one could
> expect
> to receive DX signals? It seems that at one time I read 5 degrees but I've
> done some
> research and can't find the answer so far.
> I am looking at a QTH here in Colombia that in one important direction has a
> hill about
> 330' higher than would be my antenna. That hill is 1KM or 3,250' away from
> the antenna.
> Using right triangle calculations the angle between my antenna and the top
> of the hill
> would be about 5.8 degrees. I'm concerned that the hill would have some
> affect on
> long haul signal reception. Should I be concerned???? Otherwise the
> location is
> fabulous.
>
> Tony W4OI/HK7AR
>
>
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