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

Robert Bruninga bruninga at usna.edu
Fri Dec 17 13:39:27 UTC 2004


>...the HAAT must be calculated...

I was out of town, and so I may be all out of sequence,
but if I may get up on my stump, there are 2 answers to 
the question of HAAT.

1) It is a very clearly defined averaging process of precise
numbers defined by the FCC for the purposes of entering
DATA into their database of transmitters.  It gives everyone
an exact way to come up with a number, and any two people
folloowing that method WILL get the SAME answer.  THus
perfect for bureauracies.  Perfect for calculators and
perfect for computers and bean counters.

But practically completely impractical if taken at face value
 and attempted to be applied in the real world without any 
additional application of common sense in the area of 
interest.

2) Hence the HAAT to which APRS expects or any HAM or
RF engineer wants to apply in the real world is a more
subjective number that approximates the best guess for
most of the area over which the person wants to apply it.

Using the FCC definition and a very common HAM radio
application of a repeater on the side of a  1200' mountain
range /high plain area serving a city down at 200' on one
side of the mountain, the FCC answer might get you 
an answer of only 500'.  But anyone with common sesnse
knows the HAAT in the direction of the city is 1000.
(Off by a factor of 4! (because a factor of 2 in height equals
a factor of 4 in power))

Similarly, to the people that live out on the high 1000' plain
on the other side of the repeater the HAAT is only 200,
and not 500.  A Huge difference. (off by a factor of 6 in
power!)  Yet obvious to the most casual observer with
common sense..

So again, forget the FCC definition unless you are filling
out forms.  Use common sense.

APRS even takes this typical example into account when
you include the DIRECTIVITY digit in your PHG and 
offsets your PHG circle by about 6 dB in the direction
indicated.  This accounts for the typical example above.

THe FCC's 10 mile area is good for antennas with 
HAAT's in the hundred feet or so, because that
is the terrain that will have the biggest impact on the
range of that transmitter.  But if the transmitter is way
up there in the many hundreds of feet, then the
first 10 miles (if typically lower) are probably meaningless, 
since there is no way that terrain is going to block
propogation out to the horizon.

In common sense terms, simply focus on the terrain that
is about half way out and beyond to where you think 
the useful radio range of the transmitter might be.  The terrain
in that area will have the BIGGEST contribution
as to what is really going to mostly affect the signal
out there...

In this age of computers and calculators and numbers
it is too easy to forget that precise numbers can
be absolutely meaningless depending on where they
came from, how they were calculated and how they
intend to be used.

Bob.






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