[aprssig] UHF/VHF Voice Alert (rev 1)

[Stephen H. Smith]

Robert Bruninga wrote:
> For your UHF/VHF Cross Band regional APRS Voice Alert System.
>
> You recall last month or so, I announced our local plans to put
> up a 445.925 RECEIVER cross-banded to a 144.39 T100 Voice
> Transmitter on a major mountain top so that APRS mobiles could
> call Voice Alert over the entire central part of the state.  It
> was one-way only, of course, because if the return path was
> implemented, then the UHF output would be wall-to-wall T100
> packets.  The caller simply announced "WB4APR cross-band Voice
> Alert, listening on the 147.xx repeater"...  It was a one-way
> call. 
>
> But there is a better way... Make the return path be listening
> on 146.52 T100!
>
> Here is the cross band repeater plan:
>
> 445.925 input PL 100 goes to 144.39 PL 100 output.
> 146.52  input PL 100 goes to 445.925 PL 100 output
>
> This way, any APRS operator needing to do an emergency call-up
> of ALL APRS mobiles within central Maryland can put out a call
> on 445.925 and get relayed to EVERY APRS mobile running Voice
> alert (listening wit CTCSS 100 on 144.39).
>
> The standard call is "WB4APR crossband Voice Alert, listening
> 146.52 PL 100"
>
> This arrangement gives the same long-range call-back capability
> as the outgoing call.
>
>   

No it doesn't !    Assuming anyone actually responds to this incredibly 
confusing kludge,  they transmit back to you on '52, but you hear them 
but miss their response.  Now when you transmit "Please repeat that" on 
UHF, it goes out on 144.390, not on '52.   So they miss your response 
since they have dialed their 2M rig off of 144.39 (unless they have TWO 
2-meter radios, or a D700-type "dual-dual-bander"; i.e. capable of 
having both sides on different frequencies in the SAME band.)


Further, you will have an acute "hidden transmitter" problem on '52.  
'52 is relatively heavily occupied and fairly active. 

1)  The mobile APRS user hears your call on  '39.

2)  He quickly fumbles around and selects '52 with PL transmit.

3)  Being a low-level station (i.e. mobile) he CAN'T hear the activity 
on '52 over a large area that your cross-band repeat site can, so he 
keys up.  

4)   Assuming he is lucky and arrives at your mountain top stronger than 
other activity elsewhere on '52 to successfully and positively capture 
the '52 RX, the PL tone on his transmitted signal now unsquelches the 
mountain top '52 RX.

5)  While his PL tone holds the RX squelch open, other NON-PL'ed signals 
on '52 will also get picked up by the RX, resulting in a howling mess of 
non-capture heterodynes retransmitted on the UHF downlink. 

[This is not hypothetical -- it happens all the time here in the west 
with cross-band remote bases at high locations. A PL'ed signal opens the 
RX, but then a non-decisive capture of multiple non-PL signals compete 
for the RX.]



I think the only practical way for this to work is to have the UHF 
system be a full-blown standard UHF-to-UHF repeater, with an auxiliary 
transmitter on 144.39.   Use one PL tone on the user's mobile to repeat 
to 144.39.  Use a different tone to repeat to UHF.  

Now when the mobile user hears voice alert from this system on 144.39, 
he just selects a standard UHF repeater pair on the other side of the 
radio, and talks back like any other repeater to coordinate the QSY.  
You would have two adjacent memory slots on the UHF half of the radio 
with the identical freq and splt--- one with the PL tone to key the 
144.39 TX downlink, and one with a different PL tone for the normal UHF 
repeater.

After all, just this week, you commented about discovering how nice 
conducting voice ops on UHF was, while the APRS goes on without conflict 
on VHF.

------------------------------------------------------------------------

--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

JavAPRS Filter Port 14580 Guide
  http://wa8lmf.net/aprs/JAVaprsFilters.htm

"APRS 101"  Explanation of APRS Path Selection & Digipeating
  http://wa8lmf.net/DigiPaths

Updated "Rev H" APRS            http://wa8lmf.net/aprs
Symbols Set for UI-View,
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