<<< see responses/questions embedded below >>> --- In Repeater-Builder@yahoogroups.com, "Gary Schafer" <[EMAIL PROTECTED]> wrote: > > Cables between cavities include the length of the coupling > loops in the cavities. They are measured from the bottom of > one loop, the cable, to the bottom of the loop in the next > cavity. You need to figure the velocity factor of the cable > and the velocity factor of each loop separately as the cable > has a specific velocity factor and the loop will have a velocity > factor of air. > > The cables that go to the T are measured from the bottom of > the loop again as above to the center of the T connector. > > A notch cavity will provide about 30 to 35 db of notch depth. > Each interconnecting cable provides an additional 10 db or so > of notch depth when its length is right.
W6NCT: Ok, I understand that they include the lengths inside the cavities; but how can I determine how much is inside the cavity without disassembling them? If I measured an unterminated (test) cable before I connected it to the cavity, could I connect it between the MFJ analyzer and the cavity and measure the additional electrical length added by the cavity? If I made the actual cables that much shorter would the system be resonant; ot do I need to do it iteratively? Would I need to start off with the test cable being a specific wavelength by itself, so that the in-cavity length (and associated velocity factor) would be more easily determinable? > > ... Don't forget that the transmit side cables are tuned > to the receive frequency just like the transmit notches are. W6NCT: Why are the cables in the Tx cavities tuned to the Rx frequency? This seems counter-intuitive to me. I would expect that the Tx cables and cavites should be resonant at the Tx frequency to have the best performance and SWR match. Similarly, I would expect that the Rx cavities would be resonant at the Rx frequency for the best RF throughput to the receiver. What am I missing? > > The cable on the transmit side to the T provides a high > impedance at the receive frequency because the notch in > the transmit cavity is a short circuit at the receive > frequency. That reflects a high impedance or open circuit > at the center of the T connector thru the quarter wave > length at the receive frequency. > > The cable on the receive side provides a high impedance > at the transmit frequency at the T because the notch in > the receive cavity is tuned to the transmit frequency > and provides a short there. The quarter wave length cable > reflects that short to a high impedance or open circuit > at the T to the transmit frequency. W6NCT: I don't quite understand this; and it doesn't help me understand why the Tx cavities' cable lengths should be for the Rx frequency. I'm still missing something, I guess. Sorry. > Ideally you could disconnect the receive cable at the T and > it should not effect the transmit power flow to the antenna. W6NCT: I don't understand this either; since I'd expect the unterminated stub of the T to act like a really short antenna. Since it is not an odd multiple of the Tx quarter wavelength, I'd expect its open circuit reflections to impact the Tx path's operation and SWR. Again, I must be missing something. > > When the transmit energy gets to the T connector it sees > a high impedance going to the receiver and a lower impedance > (50 ohms) going out to the antenna line so it takes that path. > > When a receive signal comes in it sees a high impedance in > the transmit path at the T and a low impedance (50 ohms) > to the receiver path. W6NCT: This piece makes sense to me; and it is fundamentally why the duplexer works. Right? I suspect it should be giving me a hint what I am missing above; but unfortunately it doesn't just yet. > > The cables that connect to the T connector are necessary to > provide transmitter/receiver isolation so power flows in the > right direction. If they were not properly tuned there would > be a lot of suck out of transmit power and receive energy > going to the wrong path. W6NCT: Ok, I can see/understand this. > > The same thing is done on a transmit combiner where each > transmitter has a pass band cavity (it could be done with > notch cavities instead). The cavity isolation between > transmitters needs to be around 10 db on each side in > order to provide the isolation so that the power goes to > the antenna and not to the other transmitter. So the other > transmitter must be 10 db down on the skirt of the opposite > transmitters pass band skirt. You can think of these > cavities as switches. The same type of thing goes on at > the T connection of a duplexer. W6NCT: I can also understand this for a combiner for multiple transmitters. > > 73 > Gary K4FMX W6NCT: I'm sorry if I seem a little dense on a couple of these notions; some of the concepts seem counter-intuitive to me. That being said, I still need a methodology to determine the proper physical length of the interconnecting cables; and to test them after they are made. I appreciate your patience. Vern (W6NCT)
