Well yes the T is sort of a magical device that makes the OTHER SIDE of the T disappear electrically. Actually it is not the T itself that does the job (that is just where IT happens) but it is the quarter wave length cables that perform the magic!
Without the quarter wave length cables between the T and each set of cavities the duplexer would not work! That is what provides the 50 ohm isolation between tx and rx cans so the feed line still sees 50 ohms. The quarter wave cable effectively "disconnects" the transmitter from the feed line at the T (at the receive frequency). The quarter wave cable on the receive side of the T effectively disconnects the receive side from the feed line (at the transmit frequency). Without doing this each would load the other down and there would not be 50 ohms at the antenna port of the T. Once you are on the other side of the T (the antenna port) the feed line length has no effect on the duplexer operation. All that the quarter wave lines do on the duplexer side of the T are to give isolation to the opposite side (tx-rx) so each does not short out the feed line. A similar thing happens between can cables in a duplexer but rather than using them for isolation they are used to enhance the notch of each can by presenting a high impedance at each cans T from the previous cavity. Working with a high impedance is easier to notch out than a low impedance. The notch in the first cavity presents a short (low impedance) at the unwanted frequency and 50 ohms at the wanted frequency. By coupling the next cavity with a quarter wave length cable (at the unwanted frequency) that short is transformed to a quite high impedance at the next cavity while at the same time the wanted signal being at 50 ohms is passed to the next cavity where it sees 50 ohms and goes on its way unatenuated. But we are left with the high impedance at the unwanted frequency that was transformed by the quarter wave cable. The second cavity notch is also tuned to the unwanted frequency which it pulls down to a short (low impedance) to give further attenuation. When I say the notch presents a "short" it is not really a short but a very low impedance of say a few ohms. But by having the unwanted source impedance high rather than at 50 ohms it is much easier to pull the high impedance down with the "few ohms" short circuit than it would be if we were working at 50 ohms for the unwanted. It works like a voltage divider between the two impedances. The higher the source is (from previous cavity) to the short the more loss there will be which is just what we are looking for. In the case of the quarter wave cable to the T on the output of the duplexer we want to transform the low impedance up to a very high impedance at the T so that it does not load the circuit at that point on that frequency. 73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Dan Hancock Sent: Wednesday, March 10, 2010 12:50 PM To: repeater builders Subject: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site One thing was missed regarding cable lengths. The loops in the cans are part of the equation for figuring the 1/4 wave length. I've seen that discussed here many times in postings related to inter-cable lengths on duplexers. But the 1/4 wave length issue only applies to the inter-cabling between the cans. It is my understanding that the antenna to duplexer lengths are irrelevant since the T connector and the rest of the feedline are all part of the equation. It's not like the T is some magical device that makes the rest of the feedline disappear electrically. The only time length might be a problem is if the entire feedline happens to be a resonant length. If by some chance that happens, then changing the jumper a couple of inches will clear that. Dan N8DJP Posted by: "n...@no6b.com <http://us.mc1104.mail.yahoo.com/mc/compose?to=n...@no6b.com> " n...@no6b.com <http://us.mc1104.mail.yahoo.com/mc/compose?to=n...@no6b.com> no6b Date: Tue Mar 9, 2010 8:29 pm ((PST)) At 3/9/2010 20:12, you wrote: >OK, question... > >If you put a cable which is 1/4-wavelength at VHF between the T and the >UHF cavity, it's 3/4-wavelength at UHF. Since any odd multiple of a >quarter wavelength will invert the impedance, what will this really >accomplish on the UHF cavity side? Doesn't matter at UHF, since the cavity "looks" like (hopefully something close to) 50 + j0 ohms @ UHF, so the cable length has no effect (other than plain ol' cable loss) @ UHF. At VHF, the short at the UHF cavity connector (I'll take Gary's word that it looks like a short off-resonance, though to be sure you'd want to put the can on a VNA to get the actual phase angle at the connector) needs to be transformed to an open at the T so it has no effect & VHF. The short-to-open transformation @ VHF is accomplished with a 1/4 wavelength of coax @ VHF. > The dual-band diplexers are usually high-pass/low-pass arrangements, and > lose something like 0.2 dB while providing 40 dB or more isolation. > Assuming you get a real one, and not something made with PIM-prne > materials, would this not be a safer bet? It's true you wouldn't need to mess with cable lengths if a cross-band diplexer were used, but OTOH it would be another piece of hardware in the system that really isn't necessary, since the cavities are already there. Plus if you're really worried about PIM, you'd probably have to move up to something like a cross-band coupler from TX-RX, which IIRC runs over $300. Bob NO6B
