Hi Jeff, Just figured I'd give you an update...
Wasn't sure if the stubs were done for the 166 or 154 mhz incantation of the cans, so re-tuned for 166. Looks like there is 2 dB loss at a 600 khz + spacing (which is where the problems were). (all 4 cans = 2dB) Also did a better look at where the stub's notch was, & it was at 228 mhz. (guess I was seeing things when I wrote down 238)! Next I will size the length of the new stub based upon the 228 & see if the +600 khz is better down @ 147.1. What about the links inside the cans? I have a set of drawings for a lower freq range, but wasn't sure what that would buy for me. Can I just disconnect the link from the SO-239 center conductor & add some copper stock to make it longer (making sure the flat area gets longer)? Also, will the can's interconnect cables be an issue? They are about 9-10" currently. Thanks again! Tim W5FN --- In [email protected], "Jeff DePolo" <j...@...> wrote: > > > 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 >
