Duplexer Tuning Question

[SpugEddy]

Does anybody here know the step-by-step method
of tuning a duplexer with a spectrum analyzer (without
a tracking generator) and handheld radio(s)?
Don't have tracking generator or signal generator.

Any help would be great. I refuse to spend $80+ for
somebody to tune a $95 duplexer.

 

[buddrousa]

Then it will never work correct.
You will lose power may not have a swr that the repeater will live through.
You will lose input thus poor receive range.
Do not cheap out for a repeater you will spend more money in the long run $80 is CHEAP FOR TUNING DUPLEXERS.

 

[Ubbe]

Do you got a picture or model number of the duplexer? Sometimes you can adjust the trims to a mechanical position to match the frequencies and then you program a radio with a TX frequency for the duplexers TX frequency and another channel for the RX frequency.

Set the radio to the TX frequency and connect to the duplexers antenna connector. Set a wattmeter to the RX port and a 50 ohm load to the TX port that can tolerate the power from the radio. Adjust the trimmers on the RX side of the duplexer to minimum power when you transmit with the radio. Switch the wattmeter and load and set the radio to the RX frequency and transmit with the radio and adjust the TX side to minimum power.

You would then probably need to use double shielded coax and a radio with a metal case with good shielding and an additional receiver like a well shielded scanner like Unidens latest scanners. Program the radio to lowers possible power, 0,5Watt or something like that. Make the adjustment procedure again with a scanner instead of a wattmeter and probably some external attenuator as well as the internal one in the scanner to be able to tell when the signal level decrease and gradually use less attenuation. You are in the ballpark when there's no attenuation and you can hardly hear the signal from the scanner or are at least very noisy.

Set the wattmeter to the TX port while transmitting on the TX frequency and try and get as much power as possible on the wattmeter while trying to minimize the signal out the RX port to the scanner at the reciever port.

I've done this a couple of times and have worked really well. It helps tremendously using a network analyzer that can sweep the duplexer as you immediatly see where the trimmer are affecting the frequency.

Usually you have to make the final adjustment at the site with the actual antenna, having a wattmeter in line and a T-connector with some 40-60dB attenuation to a connected RF signal generator set to the RX frequency and adjust the duplexer to max output power and the reciever to max sensitivity while the repeater are transmitting. There shouldn't be a noticable change of receiver sensitivity if the repeater are keyed or not.

/Ubbe

 

[prcguy]

What kind of duplexer? Mobile notch only or big pass/notch? It would be possible to tune one using a radio with a stable transmitter and some power attenuators to a spectrum analyzer but it would take a lot of typing on my part to put together step by step instructions which I'm not in the mood for at the moment. Maybe later.

If you mess up you can easily wipe out your spectrum analyzer.

 

[SpugEddy]

It is a Fumei flatpack 6 cavity mobile duplexer.

The original auction said to leave a comment
if you want a different frequency. Well, the unit
came tuned to 462.000 and 467.000. The seller
said that the comment was for a different range.
Whatever.
Unfortunately, I am outside the limits of my knowledge
with something like this. A friend has the analyzer
but without the tracking generator.
Could I use a cheap Signal Generator? Found
one that does .5 to 470mhz for $129

 

[cmdrwill]

That is a knockoff copy of the Phelps Dodge 'mobile' duplexer. They are notch/notch. So all you need is a signal generator some RF attenuators and a receiver. Each 'side' is tuned to reject the other frequency,

 

[Ubbe]

That type of duplexer are the easiest to adjust. The original came with instruction that had a diagram that said how much each trimmer would stick out to match a specific frequency to make a coarse adjustment as a start.

One problem with those duplexers are that they are very fragile and you absolutly do not want to strain the connectors with heavy cabling or tighten the connectors with too much force, the recommendation are 1-2 newton meter, that's finger tight without using any tools, as the metal sheet the connectors sit on will bend and the brass tubes weldings will crack and ruin the duplexer. Be sure to attach the duplexer to a perfectly flat surface if you want to fix it in place with screws and bolts or the sheet metal will bend.

/Ubbe

 

[radioman2001]

NEVER use TX power to adjust as you can destroy the duplexer with an internal arc or smoke the finals on your radio. Also make sure you have a 50 ohm load on the open port as it will affect your tuning. For reject types of duplexers just tune the opposite port for max loss of sensitivity with a receiver on the channel to reject.
With the CCR duplexer you should never get to a point where you don't have enough signal from your monitor to adjust from. To eliminate the coax length you are using from the monitor use a 10-20db pad in between to balance the line. I did this for many years and it worked quite well even with the Mot T-1500 series.
While I don't recommend this you could buy one of those white noise generators from fleabay and tune for the dip of rejection on your channel. The only problem I see is to the accuracy of the spectrum monitor to the frequency you want to reject.

 

[Ubbe]

You can never destroy any kind of duplexer with a low power 5 watt portable radio. This kind of duplexer only use cavities and have no capacitors that can arc. A cheap new signal generator could be ok (you've got a link?) but you can usually buy a second hand cheap that are of a well known brand. The local amateur club usually have a stack of instrumentation and thinks it's fun to use their knowledge to help people tune things, even if they are not members.

/Ubbe

 

[radioman2001]

Using power is not good practice, go ahead if you want to.

 

[Ubbe]

A duplexer are designed to be transmitted into with high power when correctly tuned and using low power while tuning are absolutly complety safe. I've used that method with zero problems in my 35 years as a professional radio technician.

/Ubbe

 

[SpugEddy]

Thank you for all the replies guys.
I decided to bite the bullet and pay the $50
to have the duplexer tuned professionally. It
didn't make sense to "cheap out" at this stage.