UHF Duplexer - how to establish original TX and RX freqs??

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g8tzl2004
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UHF Duplexer - how to establish original TX and RX freqs??

Post by g8tzl2004 »

I have recently acquired a Sigma UHF duplexer.

In my previous experiments with repeaters I have just used separate TX and RX antennas which work OK as long as you use a wide split. The duplexer will enable single antenna working.

I am reading up on duplexers but initially I want to test the duplexor on its original frequencies so I can get a feel if there is any desense with a correctly aligned unit...but I have no idea of the original TX and RX frequencies.

I don't have a spectrum analyser.

As a very simple alternative, I thought about using a power meter/dummy load and a widebanded HT. If I TX a 0.5W signal at, say, 1MHz intervals into either the "High" or "Low" duplexer input and measure the power level at the duplexer "Antenna" socket...where I get the highest power reading will be the tuned frequency of the duplexer??? Will this work? Is it safe to TX 0.5W into the duplexer at all UHF freqs?

Thanks
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Bill_G
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by Bill_G »

If you don't have the test equipment, then you should go visit a radio shop that does. It will save you a ton of time and effort.
Al
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by Al »

Depending on what type of duplexer you have it would be easier to determine the originally tuned frequencies by identifying where the notches occur(in frequency) because they're much more narrow than the passbands. Obviously, using a sweeper and scope with an RF detector OR a service monitor with tracking generator would be the best way to find the notches, but lacking test equipment of that sort you can make a quantitative but not a good qualitative rough measurement. If you're using something like an HT as a signal source, the frequency steps will have to be much closer than 1 Mhz to spot any notches, probably on the order of 50 Khz. Since the receive leg of the duplexer is tuned to notch out the tx frequency and vice versa, you might try terminating the unused leg, applying your signal generator to the tx or rx leg and looking for minimum signal at the antenna port using an untuned detector of some sort.
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MSS-Dave
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What radios do you own?: XTL5K, NX300, PD782, Spark Gap

Re: UHF Duplexer - how to establish original TX and RX freqs

Post by MSS-Dave »

What the heck is a Sigma duplexer? Can you post a link or something? Google-Fu turned up nothing useful but there is something called a Mega on E-Bay, perhaps this is it? If it is cheap and small, it probably is a notch only type. Looking for max power pass on the ports just isn't going to work. You would have to look for max NOTCH of the opposite channel. Example... If your repeater TX is 444.000 and the RX is 449.000, the TX or Low port would have the notch frequency set at 449.000 and vice versa. Generally, pass/notch type duplexers are pricey new but could be had used for $200-$300 USD if you look. Works lots better but you just can't tune correctly with a RF source and a wattmeter.

I'd agree with Bill G.... spend a little, get it tuned correctly, THEN you could look at the response with your low power HT and a accurate watt meter if you like.

Dave
g8tzl2004
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by g8tzl2004 »

Thanks for the very useful feedback.

The Duplexer is made by Sigma Wireless Technologies based in Ireland. Type DX16FB UHF. The company still exists but there is little info on the internet.

I think Sigma were previously a division of Philips, which was previously a major manufacturer of LMR equipment in Europe...Motorola's major competitor!!

I think Sigma's major European competitor is Procom of Denmark.

OK on looking for the notch frequency.

A few more questions:

- Is the "Low" and "High" inputs interchangeable ie. if the duplexer was set up to TX on the Low and RX on the High input, can you safely switch the TX and RX inputs without any realignment?

- Is this correct : To find the Notch frequency, I inject a signal on the High input, place a dummy load on the Low input and place a power meter on the Antenna input and look for a minimum power reading? I then repeat this the other way round.

- OK on using 50KHz steps but what is the maximum step I can use to initially zero in on the notch? Will say, 0.5 MHz steps completely miss the notch or will I get some indication of power reduction so I can then zero in using 50 KHz steps. Or do I need to go through the entire UHF band in 50 KHz steps?

Thanks

-
Al
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by Al »

The notches are very narrow, usually less than 30 Khz wide at the +3 dB points on the 'sides' of the notch(once again, depends on the duplexer design). You could easily miss them using steps of greater than 50 Khz. Your understanding of the rest of the procedure is correct. Keep in mind that, depending on the design, you might have several notches within the UHF band but the deepest one will reflect the frequency that the opposite leg was originally tuned to pass.
If the design that you have is one of the less expensive mobile types, the high and low are not likely interchangeable(at least on the US-made ones).
Here's one more idea: If you have a GDO(grid dip oscillator), you could terminate the antenna and opposite tx/rx port and couple the GDO signal into the unused port with a single turn loop. Look for the dip, and without changing the GDO, spot the gdo's signal with a tunable UHF receiver or frequency counter coupled to the GDO at the same time that you locate the dip.
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Wowbagger
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by Wowbagger »

Remember: a duplexer is just a filter. It knows nothing of "transmit" and "receive" - just high and low. There is no "transmit" port and "receive" port, just high and low. As long as the transmitter frequency matches the frequency of the port it is connected to, all is well. You don't have to tune a specific port to be the transmitter port.

The reason for the repetition above is that this is a point I keep seeing people misunderstand, and I am hoping by saying the same idea different ways I can get it to stick.

That said: I am biased as I design test equipment for a living, but - if you are going to be working with a duplexer and you don't have the right tools, you are going to have a bad time. Get your hands on a spectrum analyzer with a tracking generator - preferably with a return loss bridge or directional coupler as well - and some 50 ohm termination stubs. Make the measurements properly. Buy, beg, borrow, or rent the tools. All this screwing around with HTs and SWR meters, or GDOs, or all that other stuff WILL LIE TO YOU. You WILL screw it up. You WILL have poor performance because it isn't tuned. You MAY BLOW UP your equipment, due to mismatches or failure to isolate the receiver from the transmitter.
This is my opinion, not Aeroflex's.

I WILL NOT give you proprietary information. I make too much money to jeopardize my job.

I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
g8tzl2004
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by g8tzl2004 »

Thanks for the other ideas and warnings!!

Can I just use the duplexor for RX only? As I mentioned previously, I have done some experimenting with 2 separate RX and TX antennas. This worked OK but to get ZERO desense (on an injected very very weak signal), I had to use a very wide TX/RX spacing. Both antennas are about 20 feet apart. At narrower spacings, the desense was noticeable but not that great (ie. an increase in noise but no wipe out of the weak test signal)...but would obviously effect operation at the margin.

Another basic question - do you normally expect some desense on a single antenna repeater using a duplexer?

Will just using the duplexor for the RX side (ie. using separate TX and RX antenna) help to reduce any desense...and I guess its safer!!! Do I need to terminate the TX side if I just use the duplxer on RX?

Thanks
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Wowbagger
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by Wowbagger »

"Can I just use the duplexor for RX only?"
If you are wanting have 2 receivers at different frequencies, you could use a duplexer for that, IF the frequencies are spaced far enough apart that the duplexer can be tuned for that split. The question is, why?

For your 2 antenna setup - you really don't have enough distance between your antennas to get good isolation. You could split the duplexer and use half on each antenna, to help your desense.

"Another basic question - do you normally expect some desense on a single antenna repeater using a duplexer?"
Not if it is properly tuned, and if the antenna and feed line are in good condition.
This is my opinion, not Aeroflex's.

I WILL NOT give you proprietary information. I make too much money to jeopardize my job.

I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
g8tzl2004
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Joined: Wed Sep 08, 2004 10:13 am

Re: UHF Duplexer - how to establish original TX and RX freqs

Post by g8tzl2004 »

If I continue to use 2 antennas for RX and TX, if I use the duplexer on the RX side (and ignore the TX side), will this help to reduce any desense so I can use the repeater on a narrower TX/RX split?

How do I use the duplexer with 2 antennas...there is only 1 antenna input?
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Bill_G
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by Bill_G »

I'm sensing some misconceptions. G8ztl, in your own words, what is a duplexer?
tvsjr
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by tvsjr »

I think he's talking about using just one side of the duplexer as a preselector for the RX side...
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Bill_G
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Re: UHF Duplexer - how to establish original TX and RX freqs

Post by Bill_G »

tvsjr wrote:I think he's talking about using just one side of the duplexer as a preselector for the RX side...
I think so too. What I can find about Sigma Wireless Technologies is they no longer make duplexers, but back when they did, they made the low cost notch type. As you and I know, those are not suited for that purpose. Wow did an excellent job trying to steer him in the right direction, and then he took a tangent. That's why I think he has a slight misconception of what he has. We'll try to turn it into a teaching moment.
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