[amsat-bb] Re: true duplex radios

Tony Langdon vk3jed at gmail.com
Tue Jun 2 13:59:46 PDT 2009

At 05:02 AM 6/3/2009, Martin wrote:
>Hi Jim and others.
>For ordinary Repeaters the Rx/Tx in the same band is mandatory. But just
>think of the filtering needed for TX'ing lots og watts just 600kHz away
>from your Rx-frq.
>This is also the reason why most repeaters (at least that I know of)
>uses really big cavity-filters.

Either that or _very_ widely spaces antennas.  For example, to avoid 
the need of rather large cavities, 10m repeaters often use two sites, 
linked by UHF, to separate Rx and Tx.

>But for Rx/Tx on the same frequency - Forget it. What would be the
>purpose anyway?

Nice way to create a feedback loop. :D  You can create an appearance 
of full duplex by using a high speed switching scheme on a digital 
transmission.  In fact, this is exactly what happens if you run 
Echolink over a wifi connection.  The radio itself is half duplex, 
single frequency, but the high data rate and fast T/R switching allow 
full duplex for any VoIP on top of it, at the price of a tiny amount 
of latency.

>In my terms, a full-duplex radio is a radio that listens on one band,
>whilst transmitting on another.

Nope, full duplex refers to any communication channel capable of 
transmitting and receiving information simultaneously.  While I know 
of no radio capable of running full duplex on the same frequency, as 
someone pointed out, the humble analog (landline) telephone does exactly that.

73 de VK3JED / VK3IRL

More information about the AMSAT-BB mailing list