Herold,
I agree that it would be a ton easier with a network analyzer.
What happened in my case was the loop in the PD-1173 can was about
8.5 inches long (8.5" down and 8.5" back up). It seemed that was way
longer as compared to what was seen in other bandpass cans. After
reading the theory page on Repeater-Builder from John Portune W6NBC,
he says it really doesn't matter the size or shape of the loop (as
far as the response goes).
But as you mentioned, the loop length DOES effect the jumper length
required.
So after taking a few other bandpass cans apart and measuring their
loops, I decided to cut the PD loops down to 3.5 inches. This allowed
the jumpers that were the same length as on the duplexer to give a
good match.
In the end, it was a lot of reading, followed by some good
suggestions from this group (and recognizing the need for a network
anlyzer).
Dwayne Kincaid
WD8OYG
>
> Not exactly right.
>
> The length of the cable between the pass cavity and a notch or pass
> reject cavity has to be 1/4 wave length including the length of the
> loop in the bandpass cavity and that is why the cable lengths are
> different for different cavity brands that have different loop
sizes.
>
> If the length of that cable is too short for the mounting
arrangement,
> then add 1/2 wavelength (in cable) to that short cable.
>
> With a Network analyser, one can see that the length is optimised by
> viewing the effects of different cable lengths with respect to the
> effect of the total return loss, insertion loss, the bandpass
response
> (whether it adds properly as opposed to having 2 pass bumps with a
dip
> in the middle like a camel) and the effect of the individual tuning
of
> the cavities. With the proper length cable, one can tune the
cavities
> individually and when placed together, the tuning is not effected.
> With the wrong lengths, the tuning of the cavities is affected.
>
> I am not familiar with those products mentioned but in Sinclair top
> mounted loops between a Q can and a Pass can, the length is 3 to 4"
> similar to the feedthrough harness end connection length.
>
> Life is easier with a Network Analyser.
>
> Harold
>
>
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