VHF Repeater combining
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VHF Repeater combining
I am interested in combining two repeaters to use the same antenna, anyboy have any ideas.
In theory this can be done and is done all the time. What you do is combine the transmitters and couple the receivers, then feed the output of the combined transmitters and coupled receivers into a duplexer.
The rub, as Nand notes, is the frequencies and, in particular, how they overlap. If all of the inputs are below (or above) all of the outputs, life is easier, but still:
1. The engineering is complex (and expensive).
2. The equipment is complex (and expensive).
3. Finding an antenna that covers the composite band spread with acceptable matching can be difficult (and, if possible, expensive).
4. The insertion losses can be huge.
At many sites, you will find that, instead of duplexing into a single antenna, the receivers are coupled into a single receive antenna and the transmitters are combined into a single transmit antenna. The antennas are then mounted coaxially on the tower (with a minimum 2 foot vertical spacing between the lower part of the upper antenna and the upper part of the lower antenna). This does not eliminate the four factors mentioned above, but it can mitigate them.
There are other factors to be considered, of which one of the most important is that this type of arrangement exposes you to the "common cause failure" syndrome, where the failure of a single piece of equipment can take all of your channels down.
If you go this route, it is imperative that you use the best equipment. TxRx is often specified, and they will either do, or confirm, the engineering for you.
The rub, as Nand notes, is the frequencies and, in particular, how they overlap. If all of the inputs are below (or above) all of the outputs, life is easier, but still:
1. The engineering is complex (and expensive).
2. The equipment is complex (and expensive).
3. Finding an antenna that covers the composite band spread with acceptable matching can be difficult (and, if possible, expensive).
4. The insertion losses can be huge.
At many sites, you will find that, instead of duplexing into a single antenna, the receivers are coupled into a single receive antenna and the transmitters are combined into a single transmit antenna. The antennas are then mounted coaxially on the tower (with a minimum 2 foot vertical spacing between the lower part of the upper antenna and the upper part of the lower antenna). This does not eliminate the four factors mentioned above, but it can mitigate them.
There are other factors to be considered, of which one of the most important is that this type of arrangement exposes you to the "common cause failure" syndrome, where the failure of a single piece of equipment can take all of your channels down.
If you go this route, it is imperative that you use the best equipment. TxRx is often specified, and they will either do, or confirm, the engineering for you.
My two bits worth:
Depends on what band you are in, and what the frequency between channels is.
Suppose you had UHF with 3 meg between channels.
You could just use the cook book approach in the Motorola T1500 filter manual. 4 cavities, and some precision cables.
The exact arraingment is determined by many factors, such as band, channel seperation, power level.
It may be possible to make your own, or you could contact any major manufacturer, such as TX-RX, Sinclair, Telwave, EMC, for advice.
Be aware that there is such a thing as "illegal" channels.
For various reasons certain frequencies may not be compatible on the same site, or system.
Depends on what band you are in, and what the frequency between channels is.
Suppose you had UHF with 3 meg between channels.
You could just use the cook book approach in the Motorola T1500 filter manual. 4 cavities, and some precision cables.
The exact arraingment is determined by many factors, such as band, channel seperation, power level.
It may be possible to make your own, or you could contact any major manufacturer, such as TX-RX, Sinclair, Telwave, EMC, for advice.
Be aware that there is such a thing as "illegal" channels.
For various reasons certain frequencies may not be compatible on the same site, or system.
Aloha, Bernie
In the Los Angeles sinkhole of RF, we had a lot of problems on VHF 159-161 frequencies due to co located TX/RX antennas close to each other, these were Base stations simplex.
We have been using master combining in all the larger sites here in the west for more than 20 years.
Most sites have a master wideband receive antenna that covers 144 to 170 and then feed to bandpass filters for the sub parts of the freq range, then to the coupler and on to the receivers.
The transmitters are combined with respect to frequencies, ect, usally up to 4 or more on the transmitt antennas, which are located some thirty feet or so down the tower from the master receive antennas on the top.
One result is that the towers are not overloaded with too many antennas of all types, and the control of interference is easier to do.
Also used on UHF, 800 and 900 also.
We have been using master combining in all the larger sites here in the west for more than 20 years.
Most sites have a master wideband receive antenna that covers 144 to 170 and then feed to bandpass filters for the sub parts of the freq range, then to the coupler and on to the receivers.
The transmitters are combined with respect to frequencies, ect, usally up to 4 or more on the transmitt antennas, which are located some thirty feet or so down the tower from the master receive antennas on the top.
One result is that the towers are not overloaded with too many antennas of all types, and the control of interference is easier to do.
Also used on UHF, 800 and 900 also.