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Duplexer Question


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#1 taco6513

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Posted 19 June 2019 - 06:31 AM

I have a question about duplexers.  I understand what they do. I would like some more understanding about the differences between say a 3 cavity, 4 cavity and 6 cavity duplexer? What would be the reasons to get more captivity vs not? The tower I am on only has a public safety repeater in VHF and 2.5Ghz or above wifi systems. Teach me. I want to lean more.

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#2 berkinet

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Posted 19 June 2019 - 07:31 AM

Well, since it is usually better to learn how to fish than to have to buy fish... Try starting with the Wikipedia. Another excellent source for repeater information is Repeater-Builder.com. Here is a page from them on duplexers.

 

But, to get you started... very basically, each cavity performs one of two functions. It either:

  • Filters out everything except a specific frequency (a notch filter)
  • Filters out just a specific frequency (a band-pass filter)

As you add more cavities (I.e. the more times you filter the signal) the effect of the filter becomes more pronounced. The low cost, so-called mobile, duplexers consist of only notch filters - 3 for receive, 3 for transmit. The transmit cavities (attempt to) filter out any signal other than the transmit frequency. The receive cavities do the opposite, filtering out everything except the receive frequency.  Adding a band-pass filter will boost the effectiveness of the duplexer. For example, a receive band-pass cavity would be turned to attenuate the transmit frequency.  Note that most high-end and commercial duplexers consist of both types of cavities.


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#3 Radioguy7268

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Posted 20 June 2019 - 07:17 AM

In VERY general terms, more cavities = more isolation. Larger cavities generally have a better efficiency - ie: less loss through the duplexer for equivalent isolation achieved.

 

As Berkinet mentioned, the higher quality duplexers involve a combination of bandpass and reject technology. Lower cost "notch" style duplexers are usually limited to 65 to 75 dB of isolation between transmit & receive. Notch duplexers suffer in high power use, and they won't filter out other nearby transmitters very well (They are designed to only "notch out" the transmit frequency of the machine they're used on).

 

Good duplexers have high isolation, low loss, low noise, and can handle high power. They'll also use quality components, and will be built to handle large swings in temperature without de-tuning.


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#4 berkinet

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Posted 29 June 2019 - 11:21 AM

To the OP on this topic. Maybe you did, or maybe you didn't like the responses to your question. But, since two people took the time to give you some useful links and information to help answer your question. I would think some recognition on your part might have been nice.

For all I know, you may never have even bothered to read the responses, in which case we just wasted our time. I suspect the responses weren't horribly inaccurate, since 4 people seem to have liked them. Maybe you wanted better answers, or just didn't find the responses useful, in which case you could at least provide a little feedback so as to the issue.

But, if users here are going to ask people to take their time to try and help them, then at an absolute minimum, they should be prepared to acknowledge the effort.
  • Logan5 likes this

Everything we hear is an opinion, not a fact. Everything we see is a perspective, not the truth.

-- Marcus Aurelius


#5 RickW

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Posted 29 June 2019 - 02:19 PM

I have been wondering how users tuned up the cavities, say 40 to 50 years ago when some of us OT's started using repeaters? Most ham repeaters had some procedures to do this without multi thousand dollar service monitors. I recall some local hams making cavities from large diameter copper tubing and maybe was even silver plated internally.

 

I see some info on repeater builders, but have any forum members done cavity tuning without a service monitor/spectrum analyzer? 

 

If a person made or bought a repeater for GMRS, and then added a commercial duplexer, particularly the very low cost "portable" duplexers, and needed to change channels or to tweak the duplexer, how practical is it to tune it yourself and what is the absolute minimum equipment you would need?



#6 kb2ztx

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Posted 29 June 2019 - 03:57 PM

I have a small mobile duplexer in my motorhome repeater. I have it tuned for the center channel (625).  I can go up or down and not affect it to much. I rarely change it more than one channel away as not many folks are on the channels when i am.



#7 Jones

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Posted 30 June 2019 - 09:29 AM

I have been wondering how users tuned up the cavities, say 40 to 50 years ago when some of us OT's started using repeaters? Most ham repeaters had some procedures to do this without multi thousand dollar service monitors. I recall some local hams making cavities from large diameter copper tubing and maybe was even silver plated internally.

 

I see some info on repeater builders, but have any forum members done cavity tuning without a service monitor/spectrum analyzer? 

 

If a person made or bought a repeater for GMRS, and then added a commercial duplexer, particularly the very low cost "portable" duplexers, and needed to change channels or to tweak the duplexer, how practical is it to tune it yourself and what is the absolute minimum equipment you would need?

 

In the old days, almost all transmitters had pi-network outputs, and would transmit into anything without going into fold-back or shut down.  We would simply tune cavities by transmitting through them into a load.  For a set of pass cavities, transmit on the desired frequency, and tune the cans for maximum power throughput.  For reject cavities, transmit on the unwanted frequency, and tune for greatest null.

 

You can't really do that with modern transmitters, as they will just shut down.


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#8 n4gix

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Posted 30 June 2019 - 07:05 PM

^^^ This! You'd be amazed what a savvy tech can accomplish with just a good Bird wattmeter and a dummy load. As stated above though, such is not really possible with modern transmitters. That's why I own an IFR 1200 Super "S" service monitor.






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