Amplifier before or after combiner and why?

[Frmn85]

A while back we were building a 2 channel conventional repeater system with 2 NXDN repeaters each with its respective 100 watt amplifier. The amplifier was connected to a 2 ch combiner and then to a duplexer. The duplexer was connected to a antenna system.
The receive was connected from the duplexer to a multicoupler and into each respective repeater.

A question came up that was never clear to us back then and is still not clear today. Hopefully someone here can explain.

I understand redundancy but is there any other reason on why a single amplifier is never connected after the combiner then into the duplexer?
Can any damage occur from doing so? Any frequency mixing?

Looking forward to this answer.

Thanks

 

[buddrousa]

A simple answer is the amp can only do 1 at a time so for both to work at the same time you need the 2 amps with 1 connected to each repeater.

 

[buddrousa]

The duplexer plus combiner with 1 antenna is very lossy 2 complete antenna systems and do away with the combiner option for better range.

 

[prcguy]

You can use an amplifier after a combiner if its linear and designed for multiple carriers. You also need to balance the carrier levels so if two or more are on air at the same time the total power of all carriers is within the power rating and IMD spec of the amplifier.

If you have 100 watt amplifiers on two repeaters before the combiner you will get a little less than 50 watts out of the two way combiner for each carrier. If you have a 100 watt linearized amplifier after the combiner you will get no more than 50 watts for each repeater channel. Up that to four repeaters and now you will have less than 25 watts each carrier for a 100w linearized amplifier.

 

[R8000]

The duplexer plus combiner with 1 antenna is very lossy 2 complete antenna systems and do away with the combiner option for better range.

We don't know the OP's variables to require a combiner. Yes, combiners do have loss but in the pursuit of "50 watts of clean RF is better than 100 watts of dirty RF" play into the equation.

In keeping RF sites clean, proper engineering is critical. In order to keep mixing, intermod and generally being nice to your frequency neighbors, proper filtering is a good thing.

There are times this setup the OP has is needed. If you think this has loss, try a control station combiner. Properly engineered radio systems will take into account combiner losses. When I walk into a site that has two 7 foot racks of TX/RX filters, I at least know someone tried to keep it clean and did it properly.

The fact the OP has proper filtering is a good thing. Let's not give out bad info and turn another site into a ham engineered IMD disaster.

 

[prcguy]

Combining before an amplifier is not "ham engineering", its what I did for a living for 18yrs as a principal engineer for a huge company and I had upwards of 16 clean carriers out of each amplifier. Its not the easiest or cheapest way of doing things but sometimes its the only way you can do it and that's when they call me.

We don't know the OP's variables to require a combiner. Yes, combiners do have loss but in the pursuit of "50 watts of clean RF is better than 100 watts of dirty RF" play into the equation.

In keeping RF sites clean, proper engineering is critical. In order to keep mixing, intermod and generally being nice to your frequency neighbors, proper filtering is a good thing.

There are times this setup the OP has is needed. If you think this has loss, try a control station combiner. Properly engineered radio systems will take into account combiner losses. When I walk into a site that has two 7 foot racks of TX/RX filters, I at least know someone tried to keep it clean and did it properly.

The fact the OP has proper filtering is a good thing. Let's not give out bad info and turn another site into a ham engineered IMD disaster.

 

[R8000]

Combining before an amplifier is not "ham engineering", its what I did for a living for 18yrs as a principal engineer for a huge company and I had upwards of 16 clean carriers out of each amplifier. Its not the easiest or cheapest way of doing things but sometimes its the only way you can do it and that's when they call me.

18 years is a good number, I been doing it for that long as well.

My quoted text was concerning the recommendation of removing the combiner to get more power. If the combiner was needed to mitigate interference, the recommendation to remove it is not a good idea.

 

[buddrousa]

The OP never stated seperate TX RX Antennas I said more range as in total RX TX.

 

[Frmn85]

So it seems from the posts above, connecting 2 repeaters to a combiner then to a single linear multi frequency amplifier and then to a duplexer can work but losses will be higher and of course redundancy will be lost.

The reason I brought this up was just purely out of curiosity that was never answered a long while back. For all multi channel system installs I always went the regular route-combiners, multicouplers , duplexers etc.

Also there are times where, for in building radio system setups, running another cable is not possible due to budget and physical constraints.

To answer doing away with combiner systems, I've run into situations ie a humongous hotel was upgrading to a trunked system from their current conventional system.

Instead of running another line, I just added a combiner and filters and reused their existing antenna infrastructure. This was to avoid accumulating man hours to hunting down existing antennas and then run another parallel antenna system. In large buildings tracing an antenna system can seem like a maze that can require multiple keys to different rooms which the building engineer may be reluctant to provide to you.

 

[R8000]

not possible due to budget and physical constraints.

Radio systems on community water towers is another good example.

Some water towers were built to host several antennas, some were not. Those that did not often become cost prohibitive when you start adding up the water tower manufacture engineer's fee to approve modifications, draining the tank, welding, repainting and resealing.....etc. That adds up fast. Almost all well ran local governments will want to do these types of modifications properly, and get engineering approval to do the job right.
Some local governments won't and just slap on some metal for an antenna. That usually comes back to haunt them in one way or another.

This example is where using an existing antenna to host more than one radio system may come into play, and complicated combing systems is the solution. Yes, your 100 watt repeater may only actually get 40 or 50 watts to the feedline, but that's better than 0 watts from having nothing at all. This happens quite frequently when public safety agencies add voted receivers or simulcast sites.

Oh, a VHF 5/8 wave mag mount works friggin awesome slapped to the top of a water tower. Comes in handy when the two bay dipole antenna fails and your need to get back on the air NOW to keep the fire pagers going for a community while you wait for a new antenna to be shipped