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negative offset repeater -dupolexer tuning

offset duplexer repeater

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#1 wqzw301

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Posted 18 June 2019 - 06:19 PM

Point of curiosity....

In the amateur radio world I noticed repeaters with negative offsets -5 mhz.... in 70cm band

do you need a different duplexer, factory set for negative offset?

I can't imagine that your standard cavity and notch duplexers would be able to be tuned in reverse....

for example instead of 440/445   a negative 445/440 offset....

can't find any info on this, or even experiment (do not have skills or equipment for tuning)

Anyone have some knowledge in this area?

thanks

301  



#2 WRAK968

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Posted 18 June 2019 - 07:29 PM

not at all. All duplexers use 1 lower frequency and one upper frequency. It doesn't matter which one is TX and which one is RX. All that matters is that those frequencies are isolated from one another which is what a duplexer does.


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#3 Corey

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Posted 19 June 2019 - 11:17 AM

Point of curiosity....

In the amateur radio world I noticed repeaters with negative offsets -5 mhz.... in 70cm band

do you need a different duplexer, factory set for negative offset?

I can't imagine that your standard cavity and notch duplexers would be able to be tuned in reverse....

for example instead of 440/445   a negative 445/440 offset....

can't find any info on this, or even experiment (do not have skills or equipment for tuning)

Anyone have some knowledge in this area?

thanks

301  

 

As long as the duplexer covers that frequency range you can tune it any way you like. I have tuned a duplexer to work like a combiner for 2 transmitters. Works great as long as you include isolators in your design.


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Just My $.02

 

Corey

 

Midwest GMRS

https://mwgmrs.com


#4 WRAK968

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Posted 19 June 2019 - 03:46 PM

Interesting to hear Corey, I would think that would still cause distortion in the RF signals when they hit the antenna. I know combiners exist (That is after all how trunking systems work) but I didn't think a duplexer would be able to do the same job.


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#5 marcspaz

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Posted 19 June 2019 - 05:07 PM

Interesting to hear Corey, I would think that would still cause distortion in the RF signals when they hit the antenna. I know combiners exist (That is after all how trunking systems work) but I didn't think a duplexer would be able to do the same job.

 

 

I see even the cheap $29 MFJ duplexers are advertised as a splitter/combiner.  Seems reasonable that a high quality duplexer would, too.


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#6 WRAK968

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Posted 19 June 2019 - 05:24 PM

I've only seen VHF/UHF splitters/combiners, didnt know they worked in same band. But we be getting off topic, The more we learn each day :)


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#7 kb2ztx

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Posted 21 June 2019 - 06:28 AM

Interesting to hear Corey, I would think that would still cause distortion in the RF signals when they hit the antenna. I know combiners exist (That is after all how trunking systems work) but I didn't think a duplexer would be able to do the same job.

 

I do the same thing at one of my GMRS sites. I had a commercial LMR system (452.xxx) and combined our .650 repeater via a duplexer and isolators. In all reality if you look at a real transmit combiner all it is is a filter of some kind, alot of times the same exact filter that a duplexer is made of. 

 

My RX combiner is the same as I filter 457.xx and 467.xx only.


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