The only way to do the close spaced frequencies 447.750 and 447.850 is with a hybrid type combiner. The other channel 464.900 can be added in to the antenna line with an isolator and a pass cavity or you could use a duplexer. But with a duplexer you have to be careful of IM products more so than when using an isolator.
With an isolator and cavity type combiner the cables to the T or multi junction connector to the cavities are ¼ wave length. That length includes the loop length and is best obtained from the cavity manufacturer. The frequencies need to be far enough apart so that the cavity provides at least 10 db of skirt isolation to the other frequencies. You can think of the cavity as performing a switch action or disconnecting that part of the circuit from the other frequencies by enough so that it does not load down the other frequencies at the multipoint junction. The cavity operates as a short circuit (10 db down short at least) and the short gets transformed to an open thru the ¼ wave length line to the junction at the other frequencies. 73 Gary K4FMX _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Mike Mullarkey Sent: Sunday, September 16, 2007 10:58 AM To: [email protected] Subject: RE: [Repeater-Builder] Re: 3 Channel UHF Ferrite Combiner HI Skip, Very tight outdoor cabinet. We need to combine TX 447.750, 447,850 and 464.900 into one antenna. I know this is not optimum but this is what we have to work with. Mike Mullarkey (K7PFJ) _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of skipp025 Sent: Sunday, September 16, 2007 9:05 AM To: [email protected] Subject: [Repeater-Builder] Re: 3 Channel UHF Ferrite Combiner There is more than one type of combiner possible... depending on frequency spacing, band of operation and power level. s. > "Mike Mullarkey" <[EMAIL PROTECTED]> wrote: > > Ok, I been all over the web and cant seem to find any documentation on the > critical cable lengths to build a 3 channel combiner. Does anyone happen to > have such a document. > > > > Thanks, > > > > Mike Mullarkey (K7PFJ) >
