[aprssig] PHG and HAAT

hasan schiers schiers at netins.net
Fri Oct 14 06:42:42 CDT 2005

----- Original Message ----- 
From: "Brian Webster" <bwebster at wirelessmapping.com>
To: "N0YXV - Stan Coleman" <n0yxv at gihams.org>; "'TAPR APRS Mailing List'" 
<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 out. 
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 useful.

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 hit 
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 from.

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 choose.

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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