[amsat-bb] FW: [FWD: RE: Southern CA Satellite Presentations]

Jim Walls jim at k6ccc.org
Tue Jun 28 15:33:50 UTC 2016

Jerry Conner said (in part):
 > I have a hard time with all the purest, my way or the highway mentality 
I am hearing.
 It's not that it's a elitist mentality, it's simply a technical 
requirement for successful operation.  Even on a FM satellite, operating 
without full duplex would be about like trying to use your local 2M 
repeater with a transmitter and no receiver turned on until AFTER you 
called someone.  You have no way of knowing if someone else is talking.  
Since on most FM satellites, the idle time between conversations is 
measured in milliseconds, your chances of successfully hitting an idle spot 
without knowing if your are getting through is quite slim.  On SSB or CW on 
a linear satellite, without full duplex, you will forever be chasing trying 
to figure out where your downlink is.  Even with complete computer control 
of dopler shift, you will still be chasing the other guy.
 On an FM satellite, the situation goes something like this.  A contact 
ends, and for example three people key up to make a call at essentially the 
same time.  Depending on signal levels, maybe one person has a strong 
enough signal to capture everyone else, but more likely there is a massive 
hetrodyne between several signals.  With  everyone operating full duplex, 
we all hear that either someone else is capturing the uplink or the 
hetrodyne and drop carrier.  If you're not using full duplex, you blindly 
keep transmitting and creating interference for the rest of the users.
 The exception to all this is that if you are operating in a location that 
has almost no one to talk to (Hawaii comes to mind) since you are almost 
the only one around, you might get away without full duplex most of the 
time.  A decade or so ago, every night there was a SE to NW pass that was 
well off the southern California coastline - such that the footprint only 
covered 50 - 100 miles along the coast.  There were a small enough number 
of us in the footprint that we could actually hold several minute 
conversations with one or two other people.  Non full duplex would likely 
have worked in that case.  However for the previous pass that covered most 
of the US, I would not have even considered it.
 BTW, note that in my first sentence, I said "it's simply a technical 
requirement for successful operation."  The key word there was successful.  
Without full duplex you will occasionally make a contact (especially as 
mentioned earlier if you are almost the only person in the footprint), but 
if you want to routinely be successful on busy passes, there is no 
alternative to being full duplex.  Period.
 Jim Walls - K6CCC

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