> I did a check on the can with the original stub, it had
> a broad notch at 238 MHz (in addition to the tunable smaller
> notch at the 154 mhz).
The notch you're seeing at 238 is the effect of the shorted stub (I think
you said it was shorted, correct me if I'm remembering wrong). Your
measurement indicates that the stub is 1/2 wavelength long at 238 MHz, so if
you do the math, you can determine its electrical wavelength at the original
frequency of the filter section; e.g. at 154 MHz, it would be 0.32
wavelenghts.
> Does the application of the stub (evidently cut for some
> x amount higher frequency than the notch) actually pull
> this side up, giving less attenuation?
Yes, but I'm talking about improving the pass performance at frequencies
very close to the notch i.e. from a few hundred kHz to a few MHz, depending
on what your Tx-Rx spacing is). The 238 MHz indication is helpful only in
that it tells you how long the stub is electrically.
> Do you know what the formula would be for cutting this stub?
While the factory may have had a magic formula for fabricating the harness
and stubs, most of the time emperical data and/or iterative cut-and-try (or
adjust-and-try if you have line stretchers, sliding shorts, etc.) is
required, either in conjunction with, or as a substitute for, impedance
measurements (typically obtained via a vector network analyzer) and Smith
chart solutions.
> I didn't check the return loss.... it was obvious that there
> was a problem!
I think if you look at both transmission and reflection response with one
cavity plus the stub as originally configured you'll get a better feel for
how well the harness and stub are going to work at your new frequencies of
interest.
--- Jeff WN3A
