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[aprssig] PHG and HAAT

Walden, Lee LWalden at ebmud.com
Fri Oct 14 16:28:31 UTC 2005

The Height above Average Terrain can indeed be "negative". Put your
location on the floor of the Yosemite Valley in Ca. Draw eight radials
(0, 45, 90, 135, 180, 225, 270, 315) from the center point out 16
Kilometers. Measure the ground height (AMSL) along the radials every 100
meters. Add up the heights and take the average. I can guarantee you
that your HAAT will be a huge negative number. Conversely, if you had a
ground mounted antenna on the north peak of Mt Diablo in Ca, your HAAT
would be 809Meters.   

-----Original Message-----
From: aprssig-bounces at lists.tapr.org
[mailto:aprssig-bounces at lists.tapr.org] On Behalf Of hasan schiers
Sent: Friday, October 14, 2005 4:43 AM
To: TAPR APRS Mailing List
Subject: Re: [aprssig] PHG and HAAT

----- Original Message -----
From: "Brian Webster" <bwebster at wirelessmapping.com>
To: "N0YXV - Stan Coleman" <n0yxv at gihams.org>; "'TAPR APRS Mailing
<aprssig at lists.tapr.org>
Sent: Thursday, October 13, 2005 9:21 PM
Subject: RE: [aprssig] PHG and HAAT

"I still somewhat of a newbie so help me out here. How can you get a
negative number for HAAT? Did you dig a hole and put an antenna in it?
It would seam to me the lowest HAAT number you could come up with is
zero. Zero meaning your not gaining anything over the average height of
the surrounding terrain."

Getting a negative is easy to visualize. You live in a valley or
significant depression area. You are surrounded by elevations higher
than you by say 80' 
(out to a few miles, not right next to you). In this case, with a 30'
tower, you would have a negative HAAT, most likely.

HAAT is the best predictor of how well you are going to hear and get
Bob is absolutely correct about this. The problem is a lot of people
have no idea what it is, have no idea how to guess at what might be a
realistic number, and have no idea how to enter the data into their
program.....and add to this, are they interested in adding it?

HAAT is the key to PHG. It's the "H" in PHG.

Here is a simple set of instructions on how to do PHG so it might be

1. Start with H = the height of your tower. (not right, but a STARTING
2. Start with P = the output of your radio, forget feedline loss (not
3. Start with G = 3 dB

Look at your own coverage circle and the coverage circle of your local
and distant digis.

a. Now...watch to see how your signal is digi'd by digi-peaters (do you
b. Watch to see what digi-peaters you hear DIRECTLY. (Same as

Once you see the "advertized" PHG circle from other digi's, adjust your
HAAT upward so that your circle touches or overlaps the digi's your
hear/hit directly. If the opposite is true, then adjust your HAAT
downward until you lose contact with the digi PHG circle. The ideal is
to get your PHG circle to just touch the most distant digi's that you
can use effectively DIRECTLY...assuming you have enough digi's to pick

Basically, you make your HAAT in PHG correspond to the REAL world where
your circle touches or overlaps (if the digi is real close to you) the
PHG circle of the distant digis that you hear directly. Don't bother
playing with Power or Gain.

Once you have adjusted your HAAT to produce a PHG circle of yours that
touches the distant digi's you can access (RX and/or TX) directly, you
have the COVERAGE circle that reflects you real world situation. You can
play at this over time to get things more and more accurate as you

CAUTION: Do this under "Normal" band conditions, not during a big band
opening, or during the typical morning and evening tropo extensions.

The above will get you real close and will reflect that ACTUAL coverage
that you are getting from your qth. ....and that is what PHG is supposed
to do: 
produce a COVERAGE circle that informs the network.

This same exercise needs to be done for all the digi's as well, so they
put out realistic (not perfect) PHG/Coverage circles.


..hasan, N0AN

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