Re: I want to document some RF techniques, help please


David GM6BIG
 

Hi,
A bit of topic this, but the general techniques are useful..

The trick with the duplexers is to have a good return loss at the PASS freqy, even at the expense of notch depth.
23db ret loss is a minimum, preferably better at the PASS. it all starts to work far better when there are several cavities connected.

Use a circulator to join Tx and Rx cavities together (assuming single antenna working).
The traditional T-Piece phasing is critical, and will detune the notch cavities.

Also, if this is for a "digital" type of signal like dstar, DMR etc, the group delay distortion caused by notches degrades the decodability.
I have seen undecodable signals though a duplexer which was fine for FM speech.

If you are looking at a wide spaced repeater, say 7MHz split or more, consider bandpass filters so they mutually reject each other (filterplexor) , nice and flat response, so no group delay issues, and cover several channels (maybe all) at once requiring no retuning..

Be wary too of poor local osc's in the Tx and Rx, mostly phase noise. Some are so poor they jam themselves badly.
Even good LO performance requires filtering, that's the point of the filters in the Tx chain, to remove Tx noise power at the Rx freqy.

The microwave connection...
The 10GHz FM repeater diplexer we had here worked perfectly, and fitted in the hand !

Cheers David

On 01/06/2020 10:16, Robin Szemeti - G1YFG wrote:
Try this: http://www.repeater-builder.com/antenna/w6nbc-duplexer-book/ch3.html
And a couple of good YouTube videos on the subject:
https://www.youtube.com/watch?v=NKw6-x4gFJI
https://www.youtube.com/watch?v=2Y1aMhCcxXY
The problem may be that you are trying to achieve a narrower spacing than the notch is set up for, if you are trying to achieve a 1.6 MHz spacing from a setup deigned for a 5MHz spacing there may not be enough range on the trimmers to move the notch where you want it to be.   You will struggle to achieve 1.6MHz spacing on 2 cavities a side .. 3 is the minimum and 4 is better.
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