Or one could measure the XL of the loop assembly ( off the resonant frequency of the tuned cavity), plot the measured results on a Smith Chart, determine the WL towards .25 WTG on the circumference circle, and subtract that length in the coax dilectric, from the 1/4 WL coax cable. Allan Crites WA9ZZU
--- On Tue, 9/16/08, Jeff DePolo <[EMAIL PROTECTED]> wrote: From: Jeff DePolo <[EMAIL PROTECTED]> Subject: RE: [Repeater-Builder] Re: Cavity interconnection cable length To: [email protected] Date: Tuesday, September 16, 2008, 11:06 AM > By actual length do you mean the length of the entire conductor, the > circumference of the loop? That could be a couple inches on VHF > cavities. I think some of the comments made in this thread are misleading. The coupling element (loop, probe, whatever) adds *some* electrical length, but it's not correct to say that the actual physical length of the element is what you should use to determine how much to subtract off a true quarter-wave (or half-wave or whatever) length of cable. How much should you subtract? Very difficult to say. The amount may even change depending on the coupling angle (i.e. loop orientation) . It is for this reason that manufacturers, and guys like me out in the field, have an assortment of test cables that we use to find the ideal length. We also use line stretchers and "connector savors" (short male to female adapters) to find the ideal length before cutting the final cable. > My supply of RG-400 and silver-plated UHF connectors (my old DB cans > use UHF connectors) is running a little thin and I don't have enough > to make a lot of trials-and-errors. You can cut the cable a little long, put a little dab of solder the center pin, and just slide the ferrule over the braid without crimping. That way you can re-use the connector until you find the right length, then do the final crimp on the center and ferrule. You can do the same thing with old-fashioned PL-259's and UG-176 reducers too, just soldering the center. > Although we don't have a UHF-connector calibration standard and I > haven't seen any available even if I could convince the boss they're > needed, what I did was I made a set of N-to-N cables, which I can > calibrate, and a set of N-to-PL259 cables the exact same length and > cable material which I use to set the cans. I'm counting on the > small differences caused by impedance bumps and change of velocity > factor in the PL259 won't be critical on VHF. See any big flaws in > this approach? Good adapters with Teflon dielectric will have the same velocity factor as RG400, so the resulting cable length should come out the same. There will be some trial and error involved unless you have a line stretcher, there's no easy way around it. One technique, if you don't have a line stretcher, or a whole lot of test cables cut to incremental lengths, is to start with a piece of cable that's a little short and then add high-quality (very important) connector savers or elbows until you find the right length. If you have a VNA, measure the electrical length (delay) of the resulting cable (with the adapters) and then cut a new cable of the right length. --- Jeff WN3A
