Low Power Desense?

[box23]

I know with moderate to high power repeaters a duplexer is needed to isolate the receiver and transmitter when using a single antenna. My question is: Is there a power limit at which below you would not need a duplexer? I have an idea for a low power 440MHz repeater using a Hamtronics exciter and receiver. Since the exciter transmits only 2-2.5 watts would a duplexer be needed?

 

[N_Jay]

Do the math.

Find the level of TX signal at the RX frequency and the sensitivity of the RX at the TX frequency, and then if you are cabling them together you have to deal with some way to keep the TX power from the RX and match the impedances.

Or you can use two antennas and assume some safe coupling loss to use.

What was the questions again?

 

[rescuecomm]

You wont be able to use the transmitter and receiver on a single antenna without something. Separate antennas can be used with the appropriate separation.

Bob

 

[kb2vxa]

There are three ways to build a repeater.
1) Single antenna with a duplexer.
2) Separate antennas with plenty of vertical separation, receive on top being it's more critical.
3) Separate transmit and receive sites.

 

[N_Jay]

There are three ways to build a repeater.
1) Single antenna with a duplexer.
2) Separate antennas with plenty of vertical separation, receive on top being it's more critical.
3) Separate transmit and receive sites.

What about a TX with a really tight output filter and a receiver with a really good preselector, all cabled up with some carefully cut jumpers and a "T" connector?

 

[EngineerZ]

You wont be able to use the transmitter and receiver on a single antenna without something. Separate antennas can be used with the appropriate separation.
Bob

Since no one has addressed what the appropriate separation would be, I pulled out an old Decibel Products catalog and looked at the graph in the engineering notes. You'd probably want at least 55-60 dB of isolation between the two antennas, and the graph says for 450 MHz you need about 15-20 feet of vertical separation. I would take that as an absolute minimum and try for 25-30 feet if I could get it. Also, if you don't mount one antenna directly over the other, you'll get less isolation.

BTW, those old (pre-Andrew) dB Products catalogs are a wealth of practical information. If you're into building repeater and other tower-based systems, you'll find a lot of good stuff on building land mobile systems in the real world that you won't find in a typical textbook...

--z

 

[EngineerZ]

What about a TX with a really tight output filter and a receiver with a really good preselector, all cabled up with some carefully cut jumpers and a "T" connector?

That's called a bandpass duplexer.

--z

 

[N_Jay]

That's called a bandpass duplexer.

--z

Or the "Duplex Filtering" option on the MSF 5000 repeater.:twisted::twisted:

 

[box23]

Do the math.

That's what I'm trying to figure out how to do. It seems to me that with lower power less isolation, or separation, would be needed. The idea is to have it be compact so it would have to be a single antenna. I'm just not sure of how to go about solving this. If you could point me to a good source for the math behind this I'd appreciate it.

 

[N_Jay]

That's what I'm trying to figure out how to do. It seems to me that with lower power less isolation, or separation, would be needed. The idea is to have it be compact so it would have to be a single antenna. I'm just not sure of how to go about solving this. If you could point me to a good source for the math behind this I'd appreciate it.

Power out of TX at RX frequency less all losses in that path should be below the anticipated noise floor.
Power out at TX frequency less all losses should be less than the receive sensitivity at the TX frequency.

Yes, low power helps (a little).
Do all your work in dB. Figure that a 100 watt radio needs about 163dB of total isolation including the losses from the frequency separation.

A 1 Watt radio needs only 143 db.

If you want one antenna you need a duplexer (or separate filters that make up a duplexer)

 

[kb0nly]

Considering your at UHF you can go with a small duplexer and accomplish what you want. You could get one small enough to go in an enclosure with the exciter and receiver. Check out eBay, usually some sinclair or phelps dodge models on there to choose from. With the low power your using a simple low power mobile type duplexer would be more than enough.

Just as an example..

TX RX Vari Notch UHF Repeater Duplexer - eBay (item 140309594537 end time Mar-30-09 09:12:00 PDT)

I bought a small phelps dodge four can model a while back, these are a notch type duplexer, its smaller than the average mobile radio in size. Put together with two HT's in a small briefcase it made a nice portable repeater that only needed a mag mount antenna and 12v to operate.