How do I connect 2 repeater duplexers on a single band UHF antenna,

[Logan005]

How do I connect 2 repeater duplexers on a single band UHF antenna, or 1 UHF and 1 VHF repeater on a single dual band antenna? I am going to have a 50 foot tower with LDF4-50a run to my garage, but I also have a 50 foot mast in a backyard tree. I want to maximize the use of both, and I am willing to spend the money and even have it site tuned. I currently have a "Phelps Dodge" UHF 4 can duplexer, can I not use a single can notch that tunes out , or just a spliter since I would have a properly tuned duplexer on both repeaters. I also have a cellwave UHF mobile duplexer tuned to a different frequency. There is a lot I do not know, so I am I asking in the right place? any help is greatly appreciated. Best Regards..Jim..

 

[zz0468]

So, what is it you're actually trying to accomplish, 2 UHF repeaters on a single antenna, or 1 VHF, and 1 UHF repeater on a single antenna? How you go about each one would be different.

For 2 UHF repeaters on a single antenna, the proper way to do this is use a single duplexer. The receive side of the duplexer feeds a multicoupler going to both receivers. The transmit side of the duplexer goes to a transmitter combiner, which consists of a dual isolator, and a band pass cavity for each transmitter.

The receive side of the duplexer could require additional filtering to get sufficient isolation from both transmitters, since the duplexer can't be optimized for either frequency unless they are very close.

For one VHF and one UHF repeater on a single dual band antenna, the antenna will go to a diplexer that splits the antenna feedline to separate UHF, and VHF ports. Each separate port could then go to the appropriate duplexer. To do it right, you would need an isolator and pass cavity on each transmitter in addition to the duplexer.

Without the isolators and additional pass cavities, you risk creating a severe case of intermod that could potentially be heard for miles. Do NOT leave that out EVER, if you put multiple transmitters on a single feedline.

 

[Logan005]

So, what is it you're actually trying to accomplish, 2 UHF repeaters on a single antenna, or 1 VHF, and 1 UHF repeater on a single antenna? How you go about each one would be different.

For 2 UHF repeaters on a single antenna, the proper way to do this is use a single duplexer. The receive side of the duplexer feeds a multicoupler going to both receivers. The transmit side of the duplexer goes to a transmitter combiner, which consists of a dual isolator, and a band pass cavity for each transmitter.

The receive side of the duplexer could require additional filtering to get sufficient isolation from both transmitters, since the duplexer can't be optimized for either frequency unless they are very close.

For one VHF and one UHF repeater on a single dual band antenna, the antenna will go to a diplexer that splits the antenna feedline to separate UHF, and VHF ports. Each separate port could then go to the appropriate duplexer. To do it right, you would need an isolator and pass cavity on each transmitter in addition to the duplexer.

Without the isolators and additional pass cavities, you risk creating a severe case of intermod that could potentially be heard for miles. Do NOT leave that out EVER, if you put multiple transmitters on a single feedline.

I have not decided on UHF/UHF or UHF/VHF, that is why I ask about both. I gather it is easier to isolate if UHF/VHF? However I am partial to UHF/UHF. I assume all this would need to be tuned by a technician at the site location to be efficient and effective? May be a good time to refresh on using an Oscilloscope.

 

[zz0468]

I have not decided on UHF/UHF or UHF/VHF, that is why I ask about both. I gather it is easier to isolate if UHF/VHF?

Yes, isolation becomes easier as the frequencies become further apart. Running a separate UHF and VHF transmitter with a diplexer is no big deal. Running duplex with two repeaters that have the potential to be transmitting at the same time IS a big deal.

However I am partial to UHF/UHF. I assume all this would need to be tuned by a technician at the site location to be efficient and effective?

A technician could theoretically put it together and tune it on the bench, and then have it installed elsewhere. But to really optimize it, it's best to be on site with the right instrumentation and know-how.

May be a good time to refresh on using an Oscilloscope.

An oscilloscope isn't going to help you here.

You're going to need either a network analyzer, or a spectrum analyzer with a tracking generator for tuning the duplexers and cavities. You'll need a service monitor, you'll need that spectrum analyzer for verifying that your combined transmitters aren't creating any mixes (intermod), you'll need an iso-tee, some 50 ohm attenuators, a watt meter, and a directional coupler.

You'll also need to be well versed with a Smith chart for calculating the critical length cables that will be needed. The network analyzer will be perfect for measuring things, but lacking that, one of the MFJ antenna analyzers will do to get you the data needed to plug into the smith chart.

As you can probably see, doing what you want to do is quite possible, and it can be made to work quite well. But it's pretty involved, especially without the background and the tools.

You might be better off with separate antennas. It'll certainly be cheaper.

 

[flecom]

For 2 UHF repeaters on a single antenna, the proper way to do this is use a single duplexer. The receive side of the duplexer feeds a multicoupler going to both receivers. The transmit side of the duplexer goes to a transmitter combiner, which consists of a dual isolator, and a band pass cavity for each transmitter.

sorry to thread hijack how would you do this if the RX frequencies of the two repeaters were far apart?

say you had two repeaters in the following config:

TX: 442Mhz
RX: 447Mhz

TX: 426Mhz
RX: 421Mhz
(or the other way around this one doesn't matter)

possible?

I have seen band filters for 420/440 before on ebay at some point but now that I need one of course no luck!

I was thinking of making some band pass/band stop filters but not sure if the 10+mhz guard band is enough

 

[zz0468]

say you had two repeaters in the following config:

TX: 442Mhz
RX: 447Mhz

TX: 426Mhz
RX: 421Mhz
(or the other way around this one doesn't matter)

possible?

Yes, that's possible.

It might work to diplex the antenna ports to the two duplexers together. That would be a critical length cable harness. It's also likely that you would need additional pass and notch cavities on all 4 ports to the transmitters and receivers. You'd also need dual isolators on each transmitter.

I have seen band filters for 420/440 before on ebay at some point but now that I need one of course no luck!

DCI sells good band pass filters. perhaps one each for each band segment, duplexed together with the appropriate critical length cables, each feeding the separate duplexers for the two band segments.

You could also just build up a "4 legged duplexer". That would require multiple notches on each leg, band pass cavities, and dual isolators on the transmitters.

I was thinking of making some band pass/band stop filters but not sure if the 10+mhz guard band is enough

Considering that the UHF band TX/RX separation is 5 MHz, the 10 MHz between band segments is more than enough to be workable. It'll just take a bunch of hardware, and careful measurements to get the critical length cables right.