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Author Topic: Question for our HAM friends (reguarding coax length). Josh Jones Redhat Posts: 1207 From: Plano, TX Registered: Apr 2000 posted 12-16-2002 12:40 PM                        Is it necessary to cut coax to multiples of wavelengths of one's operating frequency, or is this largely a myth? Josh  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 12-16-2002 12:52 PM                     If you have a loading coil to tune out a mismatch, probably no. Transmission lines can be either inductive or capacitive. If you use two antennas, the cable lengths should be cut to provide the correct matching and should not be changed. Open end lines have a characteristic of which all odd quarter wavelengths the transmission line acts as a series resonany circuit. That is, it acts at these points as a low resistive impedance. At all even quarter wavelengths, an open-end transmission line acts like a parallel resonant circuit. That is, it acts at these points as a very high resistive impedance. If you use a phase comparison device, the sampling coax has to be the same electrical length. There can be some substancial attenuation. For example, RG-59/u coax has about 3.75db loss per 100 foot at 100mhz. That is a hefty loss.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 12-16-2002 01:11 PM                     It's neither necessary, nor a myth, Josh. A lot depends on what frequency you're running at, and what kind of match you have, and how much you care about SWR (is ther an antenna tuner?), what kind of power you're running, etc. Most people don't worry about it, and it is not a problem. But if you're matching two components in-phase (e.g. feeding a stacked pair of Yagis), then it is critical. Tell us more about your application. --jhawk (KB1CGZ)  |  IP: Logged Josh Jones Redhat Posts: 1207 From: Plano, TX Registered: Apr 2000 posted 12-16-2002 01:43 PM                        I am feeding some RG-8 at 12 watts to a j-pole antenna. my SWR is hovering around 1.1:1. I was just curious if it was worth the trouble. JJ  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 12-16-2002 02:45 PM                     Almost certainly it doesn't matter then. Frequency is important, don't leave it out. "What's the frequency, Kenneth?" --jhawk  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 12-16-2002 04:03 PM                     Yeah, Josh...just leave it alone. If you are getting 1.1:1 SWR, that means you are getting 99.9% power transmitted through the antenna. I don't think you can do any better than that! Great job on your building of that antenna! http://www.sugardelta.com/swrcode.php  |  IP: Logged Oscar Neundorfer Master Film Handler Posts: 275 From: Senoia, GA Registered: May 2000 posted 12-17-2002 06:50 AM                        Josh, If I recall correctly, as long as the terminating impedance (your antenna) is the same as the characteristic impedance of the coax, then line length does not matter except for loss considerations. However, if there is a substantial impedance mismatch between line and load (coax and antenna) the coax length can nake a large difference. Again this is from long ago memory, but I believe that as long as the coax is a half wavelentgh or multiples thereof, the impedance mismatch largely does not matter. But if you use a quarter wavelength, then mismatch gets very serious with high attendant losses due to high swr. Now to go out on a limb, I think it is possible to get a low swr reading at one point in the coax, (the feedpoint right out of the transmitter for example) and still have a bad mismatch at the antenna/coax interface wich will result in high swr and poor power transfer into the antenna. Also you can use the coax length to actually act as a transformer for the purpose of matching an antenna to the line. This however is very frequency dependent. The ARRL handbook goes into a lot of detail on this. You may want to check it out. Also, if there are any guys out there with a strong background in transmission line theory, please correct me if I am wrong. I don't wamt to pass out incorrect information.  |  IP: Logged Jeff Stricker Master Film Handler Posts: 481 From: Calumet, Mi USA Registered: Nov 1999 posted 12-17-2002 06:10 PM                     If the line is flat (terminated in it's characteristic impedance), then no matter where you cut the line you will measure Zo, the characteristic impedance of the line. If the line is not terminated in it's characteristic impedance, then you will get different values of Z depending on where you cut the line. Thus you can find a "sweet spot" where the source (transmitter) "sees" the correct impedance that it wants. It the missmatch is small, then the variations in impedance at various points along the line won't vary too much from Zo, and there won't be much of an issue. Jeff W9GY  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 12-17-2002 11:18 PM                     Thank you for explaining that, Jeff. You did a much better job in explaining that than I could have.  |  IP: Logged Jeff Stricker Master Film Handler Posts: 481 From: Calumet, Mi USA Registered: Nov 1999 posted 12-18-2002 06:13 AM                     Paul, I can remember at one time having a 75 M dipole that was quite significantly mismatched to the coax (at least at the freq. I was wanting to use) My old Heath Hw101 (antique nowadays) just didn't want to load properly. That's when I learned the adding some length to the cable trick. I found a 20 foot length of coax and connected it to the end of the feedline, and then the transmitter was happy. What happened in practice is adding that extra length got aprrox a 50 ohm point at the transmitter's output. Of course that didn't do a thing to improve the loss in the feedline associated with high SWR, but it made the transmitter happy. When I was asst CE at the local directional 5KW AM station, the coax feeding the #1 tower was actually warm to the touch owing to the high SWR on it. As I understand it, the line couldn't be matched to the tower load owing to the need to preserve the correct phase and current needed to get the correct pattern. But at the transmitter end, the transmitter "saw" 50 ohms as the result of "tuning" effect of the phasing network that combines the feed to all the towers in the array. I'm getting off the subject  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 12-18-2002 12:49 PM                     Jeff, we faced the same thing in the 5KW station I am with. Result is, the transmitter sees a 56-ohm load on 5KW non-directional, and a 50-ohm load in 1KW directional.  |  IP: Logged

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