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VK3ABK > TECH 30.06.07 17:22l 50 Lines 2371 Bytes #999 (0) @ WW
BID : 59197_VK3HEG
Read: GUEST OE7FMI
Subj: Re:APRS and Packet-one aerial.
Path: DB0FHN<DB0FOR<DB0MRW<DK0WUE<7M3TJZ<TU5EX<DB0RES<F5GOV<F4BWT<YO6PLB<
CX2SA<VK4TRS<VK3HEG
Sent: 070630/0403Z @:VK3HEG.#WEV.VIC.AUS.OC #:59197 [Ballarat] $:59197_VK3HEG
From: VK3ABK@VK3HEG.#WEV.VIC.AUS.OC
To : TECH@WW
Hello all
The bulletin from M0SCI, asking about operating two packet style systems
into the one antenna has been fully answered, but just another opinion may
be worthwhile.
Ray, VK2TV, has answered from a voice repeater operators point of view, and
there is no doubt about the problems with repeaters that must be able to
transmit and receive at the same time. Even with the Australian separation
of 600Khz on the 2 metre band, it requires lots of filtering and sometimes
physical separation to prevent desensitizing.
But with packet modes, which includes APRS, our TNCs operate by listening
before transmitting. Either by Data or Carrier Detect, the transmitter is
prevented from 'killing' the receiver. I would expect that M0SCI could
operate two TNCs, with appropriate timing parameters, so that only one was
able to transmit at once. This would mean that only TX to TX isolation would
be required, and the receivers would accept anything without a problem.
In high speed data transmission where switching times or RXD are too long,
both TX and RX are on together, with either frequency or physical separation.
Satellite operation allows this, and is necessary for high data rates or path
distortion. As both modes follow the same protocol, One TNC would control the
other and prevent collisions. There would be a delay in each mode while the
received traffic clears, but that's going to happen anyway.
Voice repeaters operate on the same band, and therefore have a co-channel
problem. Translators operate on separate bands, and can cause some
desensitizing, but this can be overcome by fairly simple filtering. Maybe
M0SCI, (tell us your name, this is so impersonal!) is able to do this, but
Packet and APRS usually share a band although this may not be the case there.
I have used two ATV transmitters, on two bands, with the outputs connected
together into a common transmission line without any obvious interference,
and power output from each transmitter was not affected. The respective final
tuning provided isolation and no change-over relay was needed. Some filtering,
covered in Ray's bulletin, or change-over relays, are needed for most broad-
band transmitter finals.
Just some thoughts without full details of the proposed operation.
73. Dick. VK3ABK.
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