[aprssig] PHG calculators

[Robert Bruninga]

> There is another weakness in the PHG 
> notation where the front-to-back
> ratio is left out to the reader's 
> imagination.

Actually, it was originall specified as about 6 dB so that the
range in the forward direction is assumed to be at least double
the range in the back direction.  But again, it is only an
indication that a given station has a favored direction, nothing
more.

> Consequently the offset or directional 
> patterns plotted out of PHG are quite 
> speculative and not even close to useful.

By that definition, nothing short of a full terrain mapping RF
propogation system would be "close to useful", which is beyond
the scope of APRS.  The  indication of PHG directivity is only
intended to indicate a favored direction, not the degree of
propogation in the unfavored direction or any kind of plot.  In
fact, the indication of PHG was never intended to be a
line-in-the sand propogation model, but simply a discriminant so
that casual observers would be able to see the HUGE difference
in coverage of a digi on a 5000 foot mountain and one on a 10
foot mast at the local firestation.

HAMS have the good feel for VHF propogation and are able to use
that knowledge to infer the general larger coverage of a higher
station compared to a lower station, and one with a favored
direction compared to omni's.

Or said another way, in the realm of RF propogation models, the
more we would try to make sure to have enough variable
parameters to make sure the computer gives us a perfectly good
answer, the closer we are to always getting a perfectly wrong
result most of the time.

Just my thoughts.
Bob, WB4APR



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