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Combining dispatch and tac freqs

This forum is for discussions regarding System Infrastructure and Related Equipment. This includes but is not limited to repeaters, base stations, consoles, voters, Voice over IP, system design and implementation, and other related topics.

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webby52
Posts: 63
Joined: Sun Dec 10, 2006 3:17 am
What radios do you own?: HT1250, XTS2500, APX6500

Combining dispatch and tac freqs

Postby webby52 » Wed Jan 29, 2014 8:51 pm

I would like to add an MTR2000 (dispatch channel) to my tower site without spending $3000 to have the tower guys come out to install an additional antenna. Would it be possible to just install a combiner so the Dispatch base (new MTR) would use the same VHF antenna that is currently on the tower? This antenna serves a 5 site voted (repeated) system that is the ems ops channel (quantar) and transmits from this tower location which happens to be one of the highest towers in the County. If I would be able to install this base at this site I would maximze my dispatch channel coverage area 50%. Would the combiner be able to send both signals at once if someone happens to be transmitting on the repeated ops frequency?

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Bill_G
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Re: Combining dispatch and tac freqs

Postby Bill_G » Wed Jan 29, 2014 9:32 pm

That is the purpose of a transmitter combiner - to sum all transmitters in a band to a single antenna .... generally speaking and with some caveats. They cut down on tower clutter, and simplify site planning. And yes, all the transmitters can be keyed at once as long as the antenna and line are rated for that power. That's one of the caveats. There are other caveats like the freq separation between transmitters versus insertion loss. You have to review the specs for each model available. Generally speaking, the further apart the freqs, the lower the insertion loss. You can expect 2db or more. ie: Put in X power, and get about 2/3 out to the antenna. If you're lucky. I've seen greater than 5db in some 800Mhz combiners because of the freqs selected were close together (1/3 power out to the antenna).

Your system planning has to account for those losses. It becomes a balancing act of performance versus cost versus future expansion versus self interference versus tower loading. Don't expect to save much money going for a two channel combiner. Tessco lists a Sinclair 2 channel VHF combiner with 60khz spacing for $5500. At 60khz spacing, you get 4db insertion loss. At 275khz spacing you get 2db. Comes in a 21x21x38 steel open frame, and weighs 70lbs.

Hope that helps

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webby52
Posts: 63
Joined: Sun Dec 10, 2006 3:17 am
What radios do you own?: HT1250, XTS2500, APX6500

Re: Combining dispatch and tac freqs

Postby webby52 » Wed Jan 29, 2014 9:36 pm

dispatch is 154.13 ops is 155.52...seems like its the same amount to have the climbers and the cost of the antenna/cabling. FCC license allows me to transmit at 100W on dispatch and 50w on ops.

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Bill_G
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Re: Combining dispatch and tac freqs

Postby Bill_G » Wed Jan 29, 2014 10:16 pm

You have plenty of spacing for these two channels to minimize insertion loss no matter which make/model of combiner you chose, but the cost is about the same. On the other hand, you said the dispatch channel is a repeater. Is the repeater using a duplexer and just one antenna? Likewise, are you expecting to receive on the TAC channel on this single antenna that you are thinking of using a combiner on?

Here's one more caveat - When you use combiners, you separate the transmit and receive paths to their own antennas. You still end up with two antennas for the band on the tower. In most site designs, all the receive antennas go to the top of the tower, and all the transmitters goes 50 or more feet below. The receiver lines go to a receiver multicoupler shelf that filters, amplifies, and distributes inbound traffic to all the receivers in that band. Likewise, all the transmitters in a band combine, and go to their appropriate antenna. Between the vertical separation, and the filtering in both the combiners and the multicoupler shelf, you no longer need a duplexer on a repeater. They should accomplish the same thing.

So, in your two station scenario, you may not save any money at all because you will still need the second antenna. Plus you'll need a multicoupler as well as the combiner.

If the site has room for expansion, and there is interest in adding a third or fourth VHF station to the site, then planning now will save you trouble later. But, if your crystal ball says expansion is not likely, then a dual antenna solution is the best one. You should add some reject filtering to the TAC receiver to prevent the repeater from clobbering it. Depending on the model of duplexer, you may need to consider a filter for the repeater rcvr a well to prevent the TAC channel from clobbering the rptr. ie: pass/reject for the TAC to pass TAC and reject the rptr, and a pass/reject for the rptr to pass rptr, and reject TAC.

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webby52
Posts: 63
Joined: Sun Dec 10, 2006 3:17 am
What radios do you own?: HT1250, XTS2500, APX6500

Re: Combining dispatch and tac freqs

Postby webby52 » Wed Jan 29, 2014 10:56 pm

The ops channel is recieved via a 2 channel PTP600 system (1 ch uhf/1 ch vhf) voted into a comparator then the audio is fed into the Quantar on the recieve side (154.71)of the repeater. It then transmits (155.52) using the antenna I want to use with the combiner. The new MTR I want to use (conventional not repeated) for dispatching would be keyed over an existing IP system (Mototrbo) my radio vendor would supply (Or I'm told could go over the Fios located at the site). (The police dept is not within line of sight of this tower and there is a big dense mountain in the way, though only a few miles away as the crow flies) So in essence I'm just using the one antenna to transmit from both bases correct?

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Bill_G
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Re: Combining dispatch and tac freqs

Postby Bill_G » Wed Jan 29, 2014 11:12 pm

I'd have to see it drawn out. I can see the the satellite receivers coming in over wireless backbone to the voter. But, I can't imagine not using the local Quantar receiver too. I'll bet it's in the mix someplace. So, there is probably a duplexer on the repeater meaning the antenna is used for tx and rx. Combining your MTR TAC station to that antenna with a duplexed repeater could be done, but it will take some planning, and I don't think you'll save any money. Putting up a second antenna will be your lowest cost solution, and you will need to consider pass/reject filters no matter what to prevent self interference.

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webby52
Posts: 63
Joined: Sun Dec 10, 2006 3:17 am
What radios do you own?: HT1250, XTS2500, APX6500

Re: Combining dispatch and tac freqs

Postby webby52 » Wed Jan 29, 2014 11:35 pm

Thanks for the insight. Much appreciated.

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Bill_G
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Re: Combining dispatch and tac freqs

Postby Bill_G » Thu Jan 30, 2014 6:42 am

You're welcome. Glad to help.


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