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 --- In [email protected], "Coy Hilton" <[EMAIL PROTECTED]> wrote: > > Dwayne, you are right... The cable length Mst be 1/4 electrical wave > length to come out right just like the ones on your exesisting > duplexer. This is shorter than the calculated wavelength in free > space because the RF propegates slower in coax than free space. You > have to determine the velocity factor of the cabel that you are > using between the duplexer and the cavity and then use it to > determine the correct length. Cut the cable as exactly as possible > to that length THEN put the connectors on it. > > I have added a bandpass and and an additoinal reject filter to the > receive side of my Duplexer and it works very well. I use a preamp > to make up for all the loss. > > Good luck > AC0Y > > --- In [email protected], "ldgelectronics" > <ldgyahoo@> wrote: > > > > Bob, > > > > I think you got it right on. Placing in random lengths of cables > (out > > of the junk drawer) between the band pass and the duplexer got > > different results. > > > > Re-reading the VE2AZX duplexer theory paper on the Repeater- > Builder > > web site verified that the cable should be shorter than a 1/4 > > wavelength. > > > > It seems it's pretty random on what the proper length should be > > depending on the can used. The PD-1173 can used a shorter jumper > than > > a DB-4001 can. This is probably where the right angle connector > trick > > would come into play. > > > > Thanks for you input, that really helped track it down. > > > > Dwayne Kincaid > > WD8OYG > > > > > > > > > > I think the length of coax you use between that BP > > > filter and the rest of the duplexer is critical. > > > > > > I have a 900 MHz duplexer that has a BP filter and a > > > BP/BR filter on each side. The BP filter gives me 25dB > > > attenuation 25 MHz away, and the BP/BR gives me about > > > 45dB. But when connected together with what seems to > > > be a 1/2 wavelength coax, I get 90dB attenuation. The > > > whole is greater than the sum of its parts, somehow. > > > > > > I'd suggest trying a piece of coax that's the same > > > length as what's being used now, which might be 1/4 or > > > 1/2 wavelength, to couple your BP filter to the > > > duplexer. > > > > > > Bob M. > > > ====== > > > --- ldgelectronics <ldgyahoo@> > > > wrote: > > > > > > > This seems like a perplexing problem. Maybe someone > > > > can point me in > > > > the right direction. > > > > > > > > I have a TPD 1554, 4-can VHF duplexer. It provides > > > > about 77 db of > > > > isolation with 1.4 db of loss. This is just about > > > > what the spec says, > > > > so that all looks good. > > > > > > > > I happen to have some extra PD-1173 VHF band pass > > > > cans, so I'm > > > > thinking of adding a can on each side to get the > > > > rejection up to 85 > > > > db or so for use on a VHF repeater (Exec II base > > > > station at 25 > > > > watts). > > > > > > > > The bandpass can by its self shows about 0.5 db loss > > > > and 8 db of > > > > rejection at 600 KHz. The curve on the spectrum > > > > analyzer looks good > > > > and the return loss is very good (-40). > > > > > > > > Here is where it gets messed up. I added a can on > > > > the duplexer on the > > > > radio side (let's just say the RX side). The loss > > > > goes up to 1.9 db > > > > as expected and the return loss is still good. But > > > > when I go to > > > > measure the rejection, it now shows only 73 db. How > > > > can this be? > > > > > > > > Everything uses double sheilded cable and attenuator > > > > pads are used on > > > > the input and output to the analyzer. > > > > > > > > It seems like the rejection should be additive, but > > > > for some reason > > > > it's not showing up that way. Anyone know why? > > > > > > > > Dwayne Kincaid > > > > WD8OYG > > > > > > __________________________________________________ > > > Do You Yahoo!? > > > Tired of spam? Yahoo! Mail has the best spam protection around > > > http://mail.yahoo.com > > > > > > 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/
