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Thread: Retuning TXRX T-pass combiner

  1. #1
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    Default Retuning TXRX T-pass combiner

    Hi,

    I have a couple of 6 tx t-pass combiners to retune for a customer. I have been working through site details as two of the sites are accessible via chopper so I want to make sure I have the hardware onboard to complete the job.

    after much research I looking some guidance on the cable length between isolator to can - I believe this to be 1/4 or 3/4 wavelength based on what I have seen and measurements I have been provided. The concern I have is the length of cable between can and tee also between tee them self.

    the cable between can and tee looks to be 1/2 wavelength based on freq can is tuned to but the length between tee is unknown, it looks to be an average of 1/2 wavelength of all cans feeding tees.

    I have seen reference to a spreadsheet which provides calculations but I have been unable to source it. If someone has it I would appreciate the help to complete the project.

    thanks in advance.
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  2. #2
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    Cable lengths between cans should always be 1/4 wavelength or odd multiples.
    Avoid 1/2 lengths, these will cause severe mismatch.
    And don't forget the cable velocity factor in your calculations.

    You can easily check the differences with a spectrum analyzer.

  3. #3
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    Thanks, SwissMoto - I got the measurements back from one of one of the sites today. I will be taking a look at the data in excel later today but from what I can see of the figures, you are correct, I will need to tune the spread sheet caculations and factor in correct frequancy and velocity factor.
    I do not believe in political correctness BS leftest tripe
    Suck it up, HTFU and make the place great again! Work never killed anyone who did it safely

  4. #4
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    I can confirm the 1/2 wave issue without going deep into math you want 1/4 or 3/4 wave cables. Most go for 3/4 in so you have a bit of room to move and position stuff.

    On the real world side when doing the math dont forget to add in the connector pin length. Remember though that you just need to calculate to point of contact not the full pin length. The pins usually add about 1/4 inch to the cable length. TO save yourself a load of headaches take about 4 feet of spare cable with you and some connectors that way if you absolutely have to you can make a cable on site. Also in the real world try to be as accurate as possible but there is no need to go nuts trying to cut stuff the 1/128 of an inch type of thing. The cables are there to pass the signal thought them all you want to avoid is having them act as a resonant circuit and messing up the tuning.

    For those wondering why the 1/4 over 1/2 wave issue. its all to do withe voltage/current phase in the circuit crated by the cables and the coupling.

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  6. #5
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    If you contact txrx customer service, tell them you want to retune a combiner system and they'll put you to engineering. If your combiner was ordered complete all you have to do is give them the serial number and they can see the original tuning and can determine the new tuning based on your information you provide and if you have to change any of the interconnect cable between the combiner cavities. If your frequencies are reasonably close to what the original tuning is you can probably get away with it. But if you're changing frequencies on UHF from 460 channel to a 450 channel for example then the interconnect cables will be longer. I retuned a 5 Channel UHF combiner I had about a year ago and the length can be 4 to 5 inches different between a 464 frequency and a 451 frequency. They have always done this service for free as long as I've used txrx and that's been 20 + years now. If you need new length interconnect cables to the cavities, they sell those pre-made as well. A bit pricey but they are made correctly with the right cable and the exacting length needed. They will also tell you if you need to change the insertion loss on a cavity based on your channel spacing. The report that they will give you will show your new tune frequencies, the order of the cavities, the length of the interconnect cables and the calculated loss per channel in dB and if you give them input power levels will show you the projected output power per Channel based on the loss.

    If you let them do the engineering, when you go to your remote site you'll have the correct length interconnect cables you need if you have to have them and all you have to do then is follow their instructions on channel placement to cavity, any adjustment to insertion loss then retune each cavity. The way I've done it is with a transmission test set. You can sweep each cavity to get maximum pass at the frequency you want then when you put it all together in the correct sequence, you can fine tune each can under power for the minimum signal at the tap on the reject load. The exact Factory method is a bit different using watt meters. I believe you can still download the t-pass tuning procedure on the txrx website but it's been awhile. I have the book in my hand and they go into exacting detail on how to tune each one of these cavities and the complete system. It's an invaluable resource if you have to work on these things.

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  8. #6
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    Its nice to see a business providing after sales support. Its very rare, my thanks to txrx for doing this even though im not the one needing it.

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  10. #7
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    I contact the initial Australia reseller and got the whole we need to calculate this out and provide cables, I was willing to do this until the quote come through and customer's heart missed a beat. The customer has been forced to restack the combiner but dod not expect to loose there first bon in do doing so.

    I will reach out to the manufacturer in the US and see what they can do, I don't know if the serial numbers are as per te initial order of kit or if changes have been recorded correctly. I (the company I work with) did not supply the initial kit so my assumption is others have had a go at adding more ovear the years and added more.

    I will advise how I get on talking to the manufacture. - thanks for advice and help.
    I do not believe in political correctness BS leftest tripe
    Suck it up, HTFU and make the place great again! Work never killed anyone who did it safely