Simplex Duplex Repeaters

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Re: Simplex Duplex Repeaters

Postby Davie Boy » Sun Nov 12, 2017 5:29 pm

It would be very useful to have one of those living on the top floor of a high rise building.... You would be able to communicate on the ground on 446 city wide especially with an outdoor antenna connected to the PMR connected to the parrot....

Is there a more effective way of using this or more effective model....? If it could re transmit with just a second delay you could surely have near instant talk with little delay if you used 2 PMR channels....? The tech looks minimal and simplistic enough to have your own duplex repeater at a very low price....
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Davie Boy
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Re: Simplex Duplex Repeaters

Postby G4RMT » Sun Nov 12, 2017 5:51 pm

The problem is simply receiver desensitisation. Your transmit frequency has the unfortunate property of making a receiver on a different frequency appear to be deaf. In the ham bands, this was always a bit tricky because the band is small, so a repeater transmitting on 145.600MHz listens on 145.000MHz - 600KHz away. Putting out even a few watts on that frequency makes the receiver act as if it is very, very deaf. The solution is filters - and for 600KHz shift big, expensive and often multiple ones! On the UHF band, it's bigger, so they picked a 1.6MHz shift, which enabled less radical filtering as the space is greater. Business radio systems often have more than 4 MHz split, helping yet again.

My van has loads of radios in, and if I transmit on 145.5, for example, while a weak ship transmits on 156.7, it vanishes - and that's over 10MHz away.

If you rig up a repeater, you don't need a delay - the real problem is just desense. the closer the frequencies, the less effective the system is. If I run a spare repeater in my van with the usual 4.6MHz OFCOM split and use separate antennas with no filter, the system works locally - but has little more range than portable to portable. Adding some filters and a duplexer gives a bit more range, but two antennas on the same level fire AT each other. Commercial repeaters never do this, they always stack them, because minimum output from vertical antennas is up and down, so sticking antennas one above the other work far better than side by side. The parrot repeaters have maximum range because the never have to receive while transmitting, removing desense issues.
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