[aprssig] bad PHG, or lots of big towers?

Robert Bruninga bruninga at usna.edu
Tue Dec 14 12:38:29 EST 2004

For Very High PHG Sites:

It is very easy.
And it is not scientific.  Just look all around that digi
as far as you can see.  The horizon that you see
is probably not at sea-level, but is also at some altitude.  
Estimate the the average altitude of that surrounding
terrain and subtract it from the altitude of the digi
and the difference is the HAAT and  is what
should be used in all radio calculations.

There is an exact definition using a 10 mile radius
but that only makes sense for digis that are only
a hundred feet or so high.  When you get higher
than that, then it makes sense to go out as far
as you can "see" and use that ground level
as the average.

Also, some directions will be very different from
others.  But APRS PHG does include the 
directivity term, so use that to indicate the
best direction (and it offsets the PHG circle
in that direction)...


>>> "Neville A. Cross" <nacross at gmail.com> 12/14/04 10:50:15 AM >>>
I found my self confused with he use of PGH... specially with that
thing about altitude and height. I operate a digi that is at a volcano
at 4800 feet (more o less)above see level and the ground around is 220
feet (more o less). If I use the height on the tower that will be 30
feet. The tower actually is 300 feet tall, but I was not sure which
are the advantages of running coax to the top of the tower. With the
antenna literally in the clouds, what difference make 100 feet, beside
all the other antennas are at upper half. (I also has to confess that
I suffer from vertigo, and 30 feet is the limit of my fear. Lacking of
some one else to do install the antenna, I settle for that height.)

I want to remark that I am a little bit confused, and I don't have and
opinion about what will be the correct answer.

I feel unfair to said that the digi operates at 30 feet. Saying so, I
will be telling that the coverage is shorter that in reality is. On
the other hand I understand that will be not the case for a scenario
with lot of mountains with different height. I which case I really
will declare my self unable to produce a value for PGH.

So please, I want to understand better how to calculate an decent
value for the PGH. Sooner or later my digi will get to an IGATE
(probably sooner because I intend to do so) and I want to be online
with all the best practices.


On Tue, 14 Dec 2004 09:23:01 -0600, Jason Winningham <jdw at eng.uah.edu>
> Looking through the data Bob posted I find 46 digis with advertised
> heights of 5120ft (a mile!) and 140 with heights of 2560 ft.
> Are there 186 towers over half a mile high with digis on them?  Are
> there even 186 towers over half a mile high in the world?
> Or are there a bunch of operators who don't know the difference
> height and altitude?
> OK, the last one was rhetorical, but the first two were serious: are
> there really that many tall towers around?
> -Jason
> kg4wsv
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