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A simple Repeater -Duplexer question

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NYCone

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I'm setting up a repeater for MMDVM using old CDM Motorola radios. All is on track, but I have a question about duplexers. If I set up my repeater with a wider frequency spread than is typical, can I get away without a repeater on UHF? For example, if my receieve is at 420Mhz and my transmit is at 450MHz, would they system work well without a duplexer?

Thanks
 

Mr_Boh

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So glad you are asking a question to learn. First, make sure you follow all local frequency band plans, including ones from your local coordinator if they exist. I am going to assume that you were just using this as an example.

There's a lot more that can be easily answered in a forum post. You can use physical separation/mounting methods to avoid the need for a duplexer - typically vertical separation of the antennas for a vertically polarized antenna. There is basically a diminishing risk that depends upon mathematical formulas, and lots of RF theory as well.

If you are trying to connect one antenna to two transceivers without a duplexer, I would just throw that idea out the window right now. Even people with the "automatic" switches complain about them and say they are far from perfect.
 

mmckenna

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You can have separate TX and RX antennas and not run a duplexer, but there's some issues that will make a duplexer look like a really attractive option.

To run without a duplexer, you need a LOT of vertical separation between the TX and RX antenna, not a few feet, but tens of feet. That might make your system coverage very lopsided unless you have access to a tall tower. The lower antenna isn't going to work as well. Then you'll have a bunch of coaxial cable and its losses to deal with.
You can do horizontal separation, but the distances will be greater. Again, more cable to deal with. To overcome feed line losses, you'll need to spend a lot of money on good cable, like 1/2" or larger Heliax, and that can be a few bucks a foot. Connectors are expensive, too.

On the other hand, finding a used duplexer on line and getting a knowledgeable person to tune it for you will save you some money. You'll only need one antenna and one feed line.

If the antennas are too close, the transmitter will desensitize the receiver and your coverage will positively suck.
 
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To add to what @mmckenna stated, you can also run into desense issues if you run too much power through a duplexer.

The bigger issue is it is simply a matter of transmitter power and isolation. This can be done via filters (such as a duplexer) or via physical antenna separation. To achieve proper UHF antenna separation you need roughly 50 ft of vertical separation for a 20W repeater. It translates to about 100 yards of horizontal separation.
 

prcguy

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With a 30MHz split at UHF you can sometimes get by without a duplexer depending on the quality of the radios. If this is a low power repeater like 5w or less and the radios actually auto tune the front ends then it might work out ok. You would need some physical (vertical) separation of the antennas. It will certainly work and repeat, the question is how much desense will it have and what will be acceptable for you.
 

NYCone

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Thanks for the feedback.

Yes, the proposed frequencies were just to understand the issue.

It sounds like I need a duplexer. I have the possibility to buy a bandpass only 4 can from Motorola (T1507A), or a cheap 8 can Chinese piece rated for 100W (SGQ-450K). I think the Chinese piece has 3 notch and one BP for each side, but I'm not sure.

Does anyone have experience with the 8 cavity Chinese duplexer? I've seen scans from 6 cavity versions, but not the 8 cavity. The Motorola would be great if it were the T1504A, I don't know anyone who has experience with the T1507A.
 
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