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[aprssig] PHG and HAAT

Robert Bruninga bruninga at usna.edu
Fri Oct 14 15:57:28 UTC 2005


Brian,
I dont think I would have any trouble with this
for the purposes of APRS...

>I sit on a 1420 foot high mountain and look down 
>on a lot of area, 

That says a lot.  The effective height in that direction
is simply how high you are above that "area".

>I also have many other hills that are higher
>around me.

THen I would definately add a PHG directivity term
in the opposite direction.   This will drastically
reduce your indicated range in the direction of those
hills.    Done.

>In using software to determine my HAAT (Radio 
>Mobile and a web site that has since disappeared) 

Such software is practically useless for this purpose.
Its value is to do the "HAAT" calculation as required by the
FCC.  This number is practically meaningless as far
as RF propogation is concerned on un-even terrain.
A site like yours will have a high HAAT in one direction 
and a very low one in other directions.  That is why 
the single HAAT "calculated" value is of little value here.

>my location comes up with a negative value.

As well it should, if it is blocked in all but  one direction.

>This is not a simple calculation for others to guess 
>unless they live in relatively flat areas. 

I disagree.  APRS PHG lets you indicate a directivity
direction and in your case, you simply estimate
your Height over  that direction where you cover
a "lot of area" and indicte that as your direction
and APRS will plot a circle that is offset in that
direction.  This also then significantly reduces the 
range in the opposite direction where you have all 
those hills and I infer, -a-lot-less-area.

So the APRS greater good is served by your PHG
circle being pretty good in the direction where the
most area is, and the coverage in the hilly 
direction is displayed as much less and that is 
a smaller area with fewer users, and users in
hilly areas expect spotty coverage, etc..

So it seems that a proper PHG value with the
directivity properly chosen can do a pretty good
job of displaying your station's relative capability.

>"In complex terrains, this guessing game 
>will make the PHG circles useless. 

I dont see it that way.  People have to simply be 
aware of their situation, and enter the PHG numbers
including directivity in the favored direction if they
are not in a homogeneous area.

> In my case there is no provision to show 
>directionality in multiple directions.

No, but the way you describe it, the *majority*
of the vast many square miles of coverage of
your station is clearly in one general area,  and
the concept of PHG is not trying  to match
every little lobe, it is only trying to show a
relative indication of performance.  When people
see your PHG circle CLEARLY offset in a given 
direction, they see all they need to know, that that
is that you are on the side of a hill and that you 
clearly have good coverage in one direction and 
less coverage in the other.   What could be 
simpler?

>I do RF engineering and coverage mapping 
>for a living and can tell you in hilly terrain the 
>PHG circles are mostly useless.

I disagree.  You seem to be trying to compare
apples and oranges.  PHG is only a very *simple* 
concept of communicating relative antenna
effective height (PHG) as a relative display
of your stations capability.  You should not try
to compare that to terrain mapping of
RF coverage which as you know is a very complex
situation.  It  simply is -not- what PHG is all about.

I dont care how "hilly" it is, there simply is no
better predictor of station performance than
effective ANTENNA HEIGHT in the direction
of interest.  There is a world of difference between 
someone with a 19" whip at roof level in a valley 
to one with a 100' tower on top of a 500 foot hill.

I cannot believe that because the result is
not *exact*, that therefore users choose to 
be completely blind to this difference.

> The simple answer to this, in the case of UI-View 
>is for folks to examine the effective path of 
>each station they can see on their map, since 
>they can SEE a digi on a map (this is APRS) and 
>if they see a lot of surrounding stations that 
>have this call sign as the first hop in their
>effective path they can assume the coverage 
>of a digi much better than a PHG circle. 

Ah, let me see.  So you are content in an RF network
to assess coverage only on the basis of other users?
Try that in New Mexico where there are digis with
100 mile ranges and over 10,000 mile
coverages and not one single user on the air.  So
to you, the range of that digi is zero?

>If an operator were to take things further and 
>install UI-path or use Radio Mobile as an APRS 
>client they can actually see the path hopping
>from site to site and get the idea of the coverage 
>as well.

Absolutely,  But remmember, PHG is not for the
exact solution you seem to be focused on , it is for
the RELATIVE visual instantaneous conveiance to 
the user, the relative merits IN THE RF NETWORK
of a stations relative performance.

Try visiting Logan utah, where there is only one digis
on the local map.  Its at the local EOC with an 
effective range of about five miles.  But when you 
visit logan, you find that all your packets are diigpeated
by digipeaters over 60 miles away that are
on top of thousands of foot mountains but not even
showing on your map.  The casual observer of that
map is simply not getting the info that-is-available
when he just looks at green stars on a map.

I cannot believe that APRS users are content to 
look at maps of little icons with no clue to their
effective range, when such data was fundamental
to APRS in the first place.

>It would also be a good thing... to create a plug-in...
>that would use ... the first hop in the path and 
>create an irregular polygon to show the footprint 
>of the particular station/digi. While it won't be
>completely accurate it would be much more like 
>reality than a PHG circle.

I disagree completely.  It will be TOTALLy dependent
on where the users live.  And the resulting polygon
will ONLY show the location of USERS and will be
TOTALLY missleading while "implying" that it is 
a better depiction!  

ALl this nitpicking is just missunderstanding
the concept.  Trying to NAIL down accuracy of
RF range is ludicrous without the very tools you
use.... and even then is still very imprecise...
BUT APRS PHG is not trying to do that.

It is simply a display of relative performance
based on the most important parameters in RF
range and that is ANTENNA HEIGHT AND GAIN.

HAMs should understand that after position, the
next  most important paramater affecting range
performance  are antenna height and antenna gain.

I simply cannot believe the resistance being 
expressed here to using these fundamental
APRS parameters for viewing station performance.

> Having too many people create this without strict 
>standards applied gives inconsistent results thus 
>making it's value much less.

Ah ha!  That is why APRS has a standard for PHG
and it DOES give consistent circles for antennas
at given heights and gains.  And if users properly
assess their relative height relative to the
area they cover the most and indicate the offset
if there is any, and you would be surprised at
how valuable this can be.  Even in varriable
terrain.

de Wb4APR, Bob





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