> -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > buil...@yahoogroups.com] On Behalf Of Kris Kirby > Sent: Monday, February 22, 2010 5:03 PM > To: Repeater-Builder@yahoogroups.com > Subject: RE: [Repeater-Builder] Re: DB-201 Measurements for 6 Meters? > > On Mon, 22 Feb 2010, Jeff DePolo wrote: > > > On Mon, 22 Feb 2010, Jeff DePolo wrote: > > > > Kinda along the same lines as "always make the cable from the > > > > connector on the transmitter to the connector on the > > > duplexer an even > > > > half-wave". > > > > > > The reason for doing that is that if the duplexer presents a > > > short-circuit, said short-circuit won't appear at antenna port. > > > > Ummmm....what? > > I was thinking quarter-waves. If you have a tee, connect the antenna at > the center and a duplexer to either side using quarter-wave cables, the > effect I noted should occur, minimizing losses. > > -- > Kris Kirby, KE4AHR > Disinformation Analyst
Well, at the output side of the duplexer that is what is happening already. The cables are a quarter wave length. The one from the transmit cans (to the antenna port) is a quarter wave length at the receive frequency and the one from the receive can (to the antenna port) is a quarter wave length at the transmit frequency. Since the receive can is tuned to the receive frequency, its output loop presents a short circuit to the transmit frequency. And since the cable going from that loop to the antenna T is a quarter wave length "at the transmit frequency" that short at the loop is seen as a very high impedance to the transmit frequency at the antenna T. The same thing happens on the transmit side of the affair but on the other frequency. That's how you get separation between the transmitter and receiver at the T junction. However, what Jeff was talking about was the cable between the TRANSMITTER and the duplexer input. His comment was "tongue in cheek" to make his point about the antenna. That cable in most cases can be any random length. There are times when a selected length will help the transmitter with the load that it sees due to out of band impedances that get presented to it. But you can not say that a certain length will be called for. 73 Gary K4FMX
