Two repeaters, one antenna

[W9AMT]

I believe somewhere this may have been discussed before, but not in the exact application I am wanting to do. I am wanting to put an amateur repeater (already have my freq pair from my state coordinator) and a GMRS repeater up at the same location. I have two antennas, but if I can get away with one, I'd like to. My frequencies between the two repeaters are 21.35 MHz apart. Obviously, I can see that finding an antenna with that wide of bandwidth might be a problem. Other than that, how could this be accomplished. Both my duplexers are BpBr, and have more that -100 dB of rejection. Would a "T" suffice, or am I running the risk of smoking the repeater with stray RF into the other???

[N8OHU]

Based on similar discussions I've read on the Repeater-Builder Yahoo Group, no can do, for the very reason you mentioned. It might seem doable, but there is a very high risk of burning out the receiver in one or both systems doing that.

Matthew Pitts
N8OHU

[prcguy]

Generally you would use two antennas, one being a master receive and the other a common transmit. Then you would have receive band pass filters for the ham and GMRS band then combine them for a master receive distribution.

For transmit with a couple of MHz or more separation you can combine the transmitters with a single cavity filter and isolator on each transmitter and critical lengths (multiple of 1/4 wave) of coax to a T connector for combining then to the transmit antenna.

If the transmit freqs were within a few MHz you would combine them with a 3dB hybrid combiner and isolators, but this eats up half your transmit power in the process. This is how most repeater sites deal with multiple systems using a common receive antenna and distribution, then a separate antenna for each transmitter or combining multiple transmitters or a combination of both.

I can't think of a safe way to combine the two systems at 462MHz transmit and 440MHz transmit using duplexers without an expensive and custom dual band pass filter that would have a window to cover transmit and receive range of GMRS and the same for UHF amateur.
prcguy

[rfradioconsult]

If you were doing a 2 Meter and a GMRS repeater this would be a super easy project.

[W9AMT]

It's not that big a deal really. The transmitters are going in a "penthouse" of a downtown office building and only 50 feet or less of transmission line is being used. I will just vertically stack the antennas. How much vertical separation would you all suggest being the transmit and receive freqs are 21 MHz apart.

[mmckenna]

This is frequently done on commercial sites. Like PRC said, Master RX antenna and shared TX antenna. You will need to spend a lot on the combiners to do it right, but it'll still likely be cheaper than new antennas, coax and tower space lease.

[prcguy]

If they are real duplexers like full quarter wave bpbr then you could get away with very little vertical separation (a few feet) because the duplexers will have some reasonable band pass qualities. If they are mobile duplexers then you want more. Depending on the type of antenna 10ft may be ok or it may need more, hard to say without knowing how much coupling there is between them when stacked vertically.
prcguy

Originally Posted by W9AMT

It's not that big a deal really. The transmitters are going in a "penthouse" of a downtown office building and only 50 feet or less of transmission line is being used. I will just vertically stack the antennas. How much vertical separation would you all suggest being the transmit and receive freqs are 21 MHz apart.

[zz0468]

There's no reason why you can't build a 4 legged "duplexer", but not with just a pair of bpbr duplexers. You'd need dual circulators and band pass cavities for each transmitter, a set of notch filters for each transmitter frequency in front of each receiver, a set of notch filters for each receiver frequency on each transmitter, and other cavities as dictated by specific tx and rx performance levels, etc.

Another approach is a pair of window filters (expensive) tee'd together, with each repeater using it's own bpbr duplexer. Dual circulators would still be required.

I won't forget to mention that you would need a scalar network analyzer for measurements, know how to use it, and be handy with a Smith chart to calculate the critical length cables that would be needed to tie it all together.

One thing you won't likely get here is the length's of the critical length cables soe you can just hook stuff together and call it good.

Barring all that complication, as mentioned by others, two antennas would be easy. If there's a separate level or location for receive antennas, use a multicoupler for sharing a receive antenna, and make a transmitter combiner for the two transmitters.

The concept of combining lots of transmitters and receivers onto a single antenna isn't at all unusual. Trunking systems can have as many as 28 channels TX and RX combined, multicoupled, and duplexed onto a single antenna, although there is usually several TX antennas. I've also done 700, 800, and 900 onto a single broadband antenna. Works good.

[W2MR]

We used quadraplexers when we used one antenna for two repeaters. One site has a repeater on 452.65 sharing with another repeater on 464.400 with no problems.

[n0rc]

Originally Posted by W2MR

We used quadraplexers when we used one antenna for two repeaters. One site has a repeater on 452.65 sharing with another repeater on 464.400 with no problems.

Can you elaborate more on how to implement this? I'm wanting to do the same thing as W9AMT

[zz0468]

Originally Posted by n0rc

Can you elaborate more on how to implement this? I'm wanting to do the same thing as W9AMT

You might want to read my post, where I elaborate on how to do that.