APRS MOBILE RANGE                                         WB4APR
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In 2001 we strongly recommended that all APRS software display
PHG range circles at HALF their previous value as described in
the APRS SPEC.  The SPEC value approximates the reasonable range
of two fixed stations.  But it turns out that mobiles in motion
experince less than HALF this reliable range.

Mobiles in motion travel though a complex field of interacting
waves and reflections.  These multiple fields add and subtract
at any instant giving from 6 to 30 dB of "mobile flutter".

When the signal is strong enough, these variances are not 
noticed at all, because they are a superimposed AM modulation
to the receiver and since we use FM, these modulations are not 
normally noticable until we get to weak signals.  Then they 
are quite noticible and pronounced.  Every mobile operator is 
familiar with this phenomenon and it usually occurs in multiples 
of wavelength.  That is why you can move your car only 18" on 
2 meters and see a change in signal.

For voice communications, a lot of mobile flutter can be 
tolerated and still have good communications between humans.  
A missed sylable or millisecond of voice is not noticed.  But for 
the mobile DIGITAL signals, and AX.25 packet, a single millisecond
hit in a packet will cause an error bit and that will cause the 
entire packet to FAIL the CRC check.  Thus the packet is lost.

To account for this 10 to 30 dB of signal fluctuation, a digital 
signal needs about 10 dB or more stronger signal to be reliable 
than for voice.  Since mobile range is proportional to the SQUARE 
of the power, this means to improve the signal by 6 dB, you have to
be half as close to the repeater.

Thus, we strongly recommend that ALL APRS software display the 
half-sized MOBILE PHG circles on all maps when the operator asks
for a PHG display.  This give the more realistic map of coverage
for mobiles, protables and other things on the move.  Then as an
option, the operator can request a FIXED PHG display that will
display the original equation PHG circles.

As most people have learned the PHG circles in APRS are optimistic
and so they were giving people a missleading sense of coverage.
I hope all authors will apply this HALF-SIZE PHG fix.

Further, since there is no such thing as zero range for a radio,
and since the path between two stations is the sum of their
PHG ranges, there were some defaults that should be used for 
displays for stations not reporting their PHG:

Fixed stations: PHG5130 25W, 20 feet, 3 dB gain Omni
Mobiles:        PHG3020 10W, 10 feet, 2 dB gain omni

These result in range circles (half-size) of about  4 and 2 miles 
respectively (8 and 4 for fixed paths according to the original 
formula)..

de WB4APR, Bob