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Kenwood HT's via AO-27



Earlier I had suggested enabling one's 0.3 sec APRS burst on the end of
occassional voice transmission via AO-27.  THen simple non-tracking,
non-doppler downlink sites could capture these ID/POSITS  over to live WEB
links via the worldiwde APRS infrastructure.

Building such an automated ground station is nothing more than an $88 UHF
HT in a water proof plastic box at the focal point of a 4 foot chicken
wire corner reflector oriented towards your best coverage sector.  Just
tune the UHF receiver for doppler.  North = 436.805, E/W = 436.800,
and South = 436.795.  Run any length speaker zip cord down to your
existing APRS station and connect it to the TNC input.  This completes
your automated downlink gateway (assuming that someone is operating an
APRS Igate in your area.)

As with anything, however, human nature and human error suggest that using
these ID/POSIT bursts via AO-27 should be used with caution (and rarely
on weekends!).  Example, on my first transmission, I was appalled by how
long the packet was.  What should have been 0.3 seconds was in fact over a
second!  All packets from the THD7 contain your POSITION, COURSE, SPEED,
and COMMENT encoded into only 9 bytes.  But you can make the packet much
longer if you aren't paying attention:

W4APR-7>LLLLLL:'OOOCSP$/Bob            <= The ideal 0.3 sec burst

W4APR-7>LLLLLL,R0MIR:'OOOOCSP$/Bob

W4APR-7>LLLLLL,RELAY-0,WIDE-15,WIDE-15:'OOOCSP$/Bob

W4APR-7>LLLLLL,RELAY-0,WIDE-15,WIDE-15:'OOOCSP$/Bob using Kenwood in
Baltimore

The first one is the desired burst.  The second one had R0MIR left over
from the last MIR pass, and the 3rd still had your terrestrial path left
in it.  The last one is just verbosity of zero benefit because the
LAT/LONG already equals Baltimore and it has to be a Kenwood HT!
Again, I'm not suggesting routine use of the burst, but only when needed.

P.S.  Until we get all of the USA covered (about 9 stations), some of you
can add more receivers and other corner reflectors.  Just parallel the
receive audio to the single TNC.  Due to doppler and antenna gain, only
one receiver will have an open squelch at a time.  In other words, three
reeivers covering three 120 degree sectors would provide hemispherical
coverage... for almost half of the USA!

de WB4APR, Bob

P.S.  Due to paralleling the UHF downlink to your existing APRS TNC, there
your APRS operations will be effectively lost while AO-27 is in view for
about 5 minutes twice a day around noon.  ALso, in busy areas, there is a
good chance that an AO-27 packet might collide with a local packet and be
lost.  Fixes are to use a separate TNC or to use a COR relay on the UHF
receiver to disable the APRS terrestrial audio during the 5 minute sector
pass.


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