Questions regarding RF combiners

[Raccon]

Not very familiar with RF stuff so I wonder if combiners are two-way (transmit and receive)?

I.e. is it possible to combine two radios (single-antenna connector each) and operate them at the same time, on different frequencies? Or will the transmit signal from one radio leak back and block/damage the receiver of the other radio?

See the drawing below:

R1 ---
........|
...... Combiner ---- Antenna (Tx/Rx)
........|
R2 ---

(... used as spacer, no other meaning)

 

[N_Jay]

Not very familiar with RF stuff so I wonder if combiners are two-way (transmit and receive)?

I.e. is it possible to combine two radios (single-antenna connector each) and operate them at the same time, on different frequencies? Or will the transmit signal from one radio leak back and block/damage the receiver of the other radio?

See the drawing below:

R1 ---
........|
...... Combiner ---- Antenna (Tx/Rx)
........|
R2 ---

(... used as spacer, no other meaning)

It depends on the type of combining equipment used and teh specific frequencies.

Most systems use separate receiver multicouplers and transmit combiners.

But most systems are also repeaters.

 

[Raccon]

There are no separate multi-couplers or combiner - just two single-box units with one antenna connector (transmit/receive). Frequency range is around 350-450MHz.

Just wondering if I can use a combiner instead of the RF switch I asked about.

 

[N_Jay]

There are no separate multi-couplers or combiner - just two single-box units with one antenna connector (transmit/receive). Frequency range is around 350-450MHz.

Just wondering if I can use a combiner instead of the RF switch I asked about.

What are the frequencies?
What are the power levels?
What is the acceptable loss?

 

[Al42]

There are no separate multi-couplers or combiner - just two single-box units with one antenna connector (transmit/receive). Frequency range is around 350-450MHz.

Just wondering if I can use a combiner instead of the RF switch I asked about.

Sure - assuming that the two radios are for separate enough frequencies and the antenna is a good match on both. One radio on 350 and the other on 450 could be done, but not with any off-the-shelf combiner I've ever seen - it would have to be designed for the specific application. For 2 adjacent channels on, say, 450? I wouldn't want to be the one who has to design that. And one antenna covering both 350 and 450 with a good match and decent gain isn't going to be simple either.

If this is for a commercial installation try Andrews. If it's for hobbyist-type scanning I hope your hobby budget is huge. It won't be cheap.

 

[N_Jay]

There are no separate multi-couplers or combiner - just two single-box units with one antenna connector (transmit/receive). Frequency range is around 350-450MHz.

Just wondering if I can use a combiner instead of the RF switch I asked about.

Sure - assuming that the two radios are for separate enough frequencies and the antenna is a good match on both. One radio on 350 and the other on 450 could be done, but not with any off-the-shelf combiner I've ever seen - it would have to be designed for the specific application. For 2 adjacent channels on, say, 450? I wouldn't want to be the one who has to design that. And one antenna covering both 350 and 450 with a good match and decent gain isn't going to be simple either.

If this is for a commercial installation try Andrews. If it's for hobbyist-type scanning I hope your hobby budget is huge. It won't be cheap.

All you need is to figure out what loss you can accept and what isolation you want and get some pass cavities.
This will only work for two single frequency stations.

If you want multi-freq stations then you need a set of low-pass and high pass filters with an apropriate cross over point.

Decibel Products, Andrew, Sinclair, TX-RX, all make that parts and pieces.

 

[Al42]

_-Jay, I know, but people who know how to build diplexers and/or combiners don't usually post here asking if it can be done.

 

[N_Jay]

_-Jay, I know, but people who know how to build diplexers and/or combiners don't usually post here asking if it can be done.

You don't need to know HOW to do it.

Just call up and ask for a system engineer and tell him what you want to do.
He will put together the parts list and a sales guy will get back to you.

Now here is the VERY IMPORTANT part: . . . . . Sit down BEFORE you hear the cost!

 

[DaveH]

Not very familiar with RF stuff so I wonder if combiners are two-way (transmit and receive)?

I.e. is it possible to combine two radios (single-antenna connector each) and operate them at the same time, on different frequencies? Or will the transmit signal from one radio leak back and block/damage the receiver of the other radio?

See the drawing below:

R1 ---
........|
...... Combiner ---- Antenna (Tx/Rx)
........|
R2 ---

(... used as spacer, no other meaning)

Better be really careful with this. If you have a combiner with two TXs with sufficient isolation between the two ports at specific frequencies, should be no problem. Same with two RXs.

If you try to TX on one and RX on the other, it is critical that you have enough isolation from one port to the other. Otherwise, TX power could seriously impair the RX, maybe even permanently. The threshold of damage for a RX may be as low as milliwatts.

I once made a simple UHF combiner out of lengths of PCB microstrip. The port-port isolation was only 20dB or so. Not good enough for TX and RX at the same time! You'd need way more, depending on TX power.

There are commercial units called TX combiners, which are for multiple TX's only, and RX multicouplers which are for RX only. There may be combined units, but as already stated, rather expensive, and there would be restrictions on the minimum frequency separation, to keep the TX-RX isolation high enough.

Dave

 

[N_Jay]

Not very familiar with RF stuff so I wonder if combiners are two-way (transmit and receive)?

I.e. is it possible to combine two radios (single-antenna connector each) and operate them at the same time, on different frequencies? Or will the transmit signal from one radio leak back and block/damage the receiver of the other radio?

See the drawing below:

R1 ---
........|
...... Combiner ---- Antenna (Tx/Rx)
........|
R2 ---

(... used as spacer, no other meaning)

Better be really careful with this. If you have a combiner with two TXs with sufficient isolation between the two ports at specific frequencies, should be no problem. Same with two RXs.

If you try to TX on one and RX on the other, it is critical that you have enough isolation from one port to the other. Otherwise, TX power could seriously impair the RX, maybe even permanently. The threshold of damage for a RX may be as low as milliwatts.

I once made a simple UHF combiner out of lengths of PCB microstrip. The port-port isolation was only 20dB or so. Not good enough for TX and RX at the same time! You'd need way more, depending on TX power.

There are commercial units called TX combiners, which are for multiple TX's only, and RX multicouplers which are for RX only. There may be combined units, but as already stated, rather expensive, and there would be restrictions on the minimum frequency separation, to keep the TX-RX isolation high enough.

Dave

Not much different than a pass only duplexer.

You can get all the isolation you need.
All it takes is cash and loss!

 

[Raccon]

Thanks for the replies. I think I will give up on that idea and go with the RF switch solution.

 

[nmfire10]

Good plan. Let me put the price in perspective for you. To put two UHF repeaters on one antenna, we were quoted ~$9,000. We recently put 6 UHF repeaters on a TX combiner... It was in excess of $16,000!!

 

[kb2vxa]

AAAARRRRGGGGHHHH!

Hi Raccoon and all,

Ask the time and they tell you how to build a clock.

"Thanks for the replies. I think I will give up on that idea and go with the RF switch solution.'

"Excellent."
C. Montgomery Burns

Meanwhile, since I don't have room here for a tutorial on RF communications I suggest you do your homework and learn something about it. No harm in a scanner buff or SWL learning about the ins and outs of radio, you don't need a degree in electrical engineering but you'll avoid asking lots of "stupid" questions.

The most intelligent question ever asked is "Where can I get study materials?"