Batboard

i got a Motorola Portable Repeater in a carrying case, model# P44SX53180B

it does work but im thinking about changing the bandsplit or at least adding somemore channels , that would be a better option since it's a low split and has the built in duplexer so if anyone can help with a service type manual or just e-mailing the info i would be greatful ... or some surplus low uhf channel elements ...

thanks .

Multi frequency operation and a duplexer just don't go together. A duplexer is tuned to the specific frequency of operation of the TX and RX. When you change channels as your trying to indicate, the duplexer will no longer be tuned to the correct frequency.

When the duplexer is not tuned to the requied frequency, several things happen. First and most important, the isolation between the TX and RX goes down. This basically causes the receiver to be jamed by the TX. It can't hear very good anymore.

Second, the TX will no longer see the desired 50 ohm load and the SWR will go up. The TX will not like that. You chance doing damage to the output stage of the TX.

If you get the duplexer far enough off the tuned frequencies, there is always the chance of putting some of the TX power into the RX front end. This is not desirable as you stand a good chance of cleaning out the front end of your receiver.

Every time you change frequencies, you really need to retune the duplexer.

Jim

My two bits worth:
Certain models switch the duplex filters to enable multi-channel operation.
The Military has one of those here, and I have tuned it up.
You need to open the unit and see exactly what you have.
Multi channel operation in a single channel duplexer is possible providing that the channels are very close, 100Kc or so.
I hope that it is not of the 406-420 variety.

bernie wrote:Multi channel operation in a single channel duplexer is possible providing that the channels are very close, 100Kc or so.

So, if this is correct, you could have say a multi-channel GMRS portable repeater and all you'd have to do is change the freq?

My two bits worth:
I think that it would work just fine.
Tune for 462.625, it should work down to 462.55, and up to .7,
that is only +/-100KC.
This sort of duplexer is very easy to tune with very few instruments.
All you need is a receiver on each frequency, and some sort of signal source.
Before you attempt to tune anything be sure you have the proper screw driver, and lubricate the threads with Kaig D 5.
Be sure that the high frequency goes to the HI port.
does not care if Rec, or TX.
Terminate the antenna port with 50 ohm load.
Generate into transmitter port of duplexer on rec. freq.
Adjust slugs on Transmit side for minimum signal.
Reset signal generator, and receiver to TX freq.
Set slugs on receive side for minimum signal.
Measure desense.
Leak a weak signal into the antenna line with an "Iso-Tee"
allow the transmitter to key, the noise level should not change.
Test all channels.


It might be necessary to "fudge" the tuning, by stagger tuning.
this is one of the very few instances where a tracking generator is very handy.

bernie pretty much summed it up....even those 4 cavity reject only's easily do 100 khz...how else would those good 'ol IMTS phones work...(well, their rx sens. was pretty abysmal )

I've achieved some rather amazing bandwidths w/ 6 cavity pass/rejects
(with negligible insertion loss)