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Mobile vs base station UHF duplexers

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LakeMan2

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Joined
Feb 21, 2014
Messages
110
I have been looking at UHF duplexers in case I decide I should "upgrade" or not after I do some range and desense testing. I currently have a UHF repeater with a Celwave 6 cavity mobile duplexer. Looking at the specs it says it has 75dB isolation. Looking at Sinclair mobile and the larger Res-Lok 4 cavity the isolation specs are similar (again 75 dB). The obvious difference I see is power handling capability and maybe some insertion loss differences. The mobiles are limited to 50W and the bigger ones are 350W or so.

Bottom line is that for a low power station (25W) in a low RF environment, I am not seeing that the benefit is there. You have to step up to very expensive Sinclair 6 cavity Res-Lok or PD526 before you get to 90+ dB isolation. If your running 100-200W, sure I can see it.

Everyone says to use a external duplexer, but unless you go top of the line, I don't see the benefit over a good mobile duplexer (assuming lower power). I feel like I must be missing something, At lower power, why would the Sinclair Q322E be better then their (or Celwave's) mobile duplexer?

Thanks
 

petnrdx

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Joined
Feb 5, 2004
Messages
384
Location
Hudson, FL
There is a LOT of misunderstanding about duplexers and filters.
The greatest difference between the simpler, cheaper, duplexers is the "protection" that a given model will give you at frequencies NEAR your two frequencies.
MANY duplexers that advertise VERY low insertion loss also don't afford much of the "protection" I am describing.
The most common models any more are truly just "reject" duplexers.
If the coax leading into a cavity goes essentially "across" a cavity, thru a coax Tee, then it really is a reject/reject duplexer.
If the coax goes "in" one side of a cavity, and out another, then it is PASS or true PASS / REJECT.
Sounds like your system is a low coverage, low level site.
And a reject duplexer is just fine.
Where things go badly is (for example) hams that want to use a simple, cheap, lower loss duplexer on say a 444/449 pair.
If you are at a mountain that also has a commercial 450/455 or 451/456 repeater, those transmitters are VERY close to the ham receives at 449.
This is where a reject duplexer MAY not be the best thing.
The "base" type the the Sinclair Res-Loc reject duplexer will be BETTER than a mobile duplexer design, but might not be enough.
It really is a case of needing to know what is at your site ( or very near ) and getting the right duplexer / filters to do the job.
 

RFI-EMI-GUY

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Joined
Dec 22, 2013
Messages
6,859
Many years back I had the opportunity to evalute some low band Base Tech repeaters that were equipped with TX-RX Psuedo-Pass duplexers. Although the rejection and isolation was excellent in band, I discovered some awful desense. This even when tested into a clean dummy load. It turns out that the 100 watt power amps used by Midland had a very high noise floor at about 14 to 17 MHz. To make matters worse, the receiver had a spurious response in that range. on top of this the duplexer had no isolation to this out of band frequency. The receiver also did not meet the minimum EIA-603 criteria for spurious response, so back to the vendor they went.

To me it was a head scratcher to look at the spectrum analyser and see virtually no noise at the receive frequency. A bit later I scanned the entire band and found the elevated noise floor at 14 to 17 MHz. A few reverse IM calculations involving the first LO freq and first IF, revealed a potential matching spurious response that I then verified with a signal generator.

The lesson here is that the repeater and duplexer are a system, and the selection of those parts is critical. Having sufficient bandpass protection is good practice.
 

freddaniel

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Joined
May 6, 2011
Messages
113
Location
Newport Beach, CA
Duplexers are not the perfect solution to all repeater problems. Antenna systems are almost always a compromise. The best solution is to use a separate receive and transmit antenna. This works if you have one or 100 repeaters. Always keep the receive antenna at least 20 feet above the nearest transmit antenna. This will provide at least 60 dB receive antenna isolation from the nearest tx antenna.
With the first repeater at the site, the receive antenna system may only require a simple notch filter to remove strong local tx signals. With two or more repeaters, a pre-amp is required to avoid the loss from splitting the receive antenna signal. A simple attenuator on the output of the pre-amp is required to avoid over-loading your receivers. If the pre-amp has 17 dB gain, and you are using a 4-way splitter with loss of 7 dB, then add 4 db for cable losses, and insert a 6 dB attenuator. with a 16-way split, you do not need an attenuator.
As the local transmitter count increases, additional notch filters may be required to limit the signal into the pre-amp. Most people would also suggest several pass cavities in front of the pre-amp, but if you want superior handheld coverage, be very careful about limiting the loss before the pre-amp. This cannot be made up with any amount of amplifier gain. If necessary, a combline filter works best as a front-end pass filter with wide bandwidth, low loss, and steep skirts. Limit filter losses ahead of the pre-amp to 1.5 dB.

As your site grows from one UHF repeater to several, you will need to start using circulators to limit transmitter mixes, that will cause intermod. To prevent intermod, you will require at least 60 dB isolation between transmitter antennas. This is very hard to achieve with horizontal separation of antennas. So you bite the bullet and buy dual-junction circulators with more than 60 dB isolation. This works, but you are 70% on your way to transmitter combining. Just add a cavity with each circulator, and you can now connect 4 to 6 transmitters to a single antenna.

Now back to duplexers. They work great for one or two repeaters at a site. More than that, you will be fighting receiver overload, and transmitter noise from the "other" repeater(s). Yes, the larger Sinclair is better than the mobile duplexer, as the mobile duplexer only protects the associated receiver from the associated transmitter. The Sinclair will provide some broader protection, as it is a pass-notch design.

Very few sites with more than 5 repeaters work well with duplexers, while I have operated sites with more than 100 repeaters using a multi-coupled receive antenna system, and combined transmitters. More info available for the asking.
 

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