The loop size of 8.5" down and then back up if used on VHF would produce a very low insertion loss cavity with a very wide pass response and if both of the loops were that dimension then the return loss would be very good assuming the loops were adjusted to the same coupling.
Such a large loop may have been needed for a special wide band preselector that had multiple cavities using that size of loop. Or it might have been Low Band loops in a VHF cavity by someone not knowing. Loop size is dependant on the frequency and loaded Q desired in the bandpass cavity and the type of coupling method. In the standard Sinclair cavities, the pass and notch loops are identical for the VHF band. The Q loops are one style for the 7" diameter cans and the Reslok Cans are another. Having special requirements for non-standard bandwidths or notch depths would produce different loop sizes. The old side mounted loops that were fixed mounted had many different sizes depending on the frequency and insertion loss and hence there were many different cable lengths to do the same thing at different frequencies and loss settings. Harold, VA3HF --- In [email protected], "skipp025" <[EMAIL PROTECTED]> wrote: > > > 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). > > Well.... not really. The proper loop length is a big > deal. Too small is really bad news and too large is > really hard to work with. If your cavity depends on > re-entrant energy it would matter a great deal how > big the size and shape of the loop/probe is. > > If the loop/probe length was really out of line, you > might not get a good return loss (swr) through the > reactive sections. > > > But as you mentioned, the loop length DOES effect > > the jumper length required. > > Quite a bit and it's considered a mostly air dilectric > element. > > > 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. > > Smart move... and probably the right move. > > > 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). > > Open thy wallet.... and hold thy breath > > cheers, > skipp > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
