In a recent discussion with a fellow ham about where to use a RigExpert antenna analyzer for tuning an antenna for minimum SWR, we couldn't decide if "antenna only" or "feedline+antenna with the analyzer at the transmitter end". For example, if I tune my antenna for minimum SWR right at the antenna itself ... that's perfection, right? But if I then analyze the feedline coax PLUS the antenna, the "whole antenna system" ... and realize that I need to tune the antenna again for minimum SWR (as seen by the radio end), then that tune would result in the highest power efficiency 'into the air'. But wouldn't that be a de-tune of the antenna itself? I'm confused on where I should connect the analyzer, and whether or not the "de-tune of the antenna" is real or best or not? Can any of you, smarter than me, explain and explain what I should do? BTW, calling different antenna manufacturer tech support lines gave us different answers ... one said to tune at the antenna, the other said tune from the transmitter end of the coax.
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2welcome to the site! at first glance, it looks like this may be a duplicate of another question, check this out: https://ham.stackexchange.com/questions/20825/vna-basics-antenna-coax-cable – webmarc Aug 21 '23 at 18:16
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1Your transmitter cares what the SWR is at the terminals on the back of the box - that includes the cable and anything else beyond there. If the reflections are too high, you may blow the final amp. Maximum power transfer from the transmitter to everything beyond the connection on the back of the box occurs at the minimum SWR. Whether the system actually emits anything, or just how much it emits, from the antenna is dependent on what happens between the transmitter and the antenna. – Jon Custer Aug 22 '23 at 18:40
1 Answers
Tuning at the antenna will give you the most accurate antenna characteristics, but a good VNA can cover up some of the inaccuracies from tuning at the end of the coax instead.
However, you have to realize that the coax is also part of the antenna system. You shouldn't have to tune the coax, but you can.
If SWR is lower at the end of the coax, that's normal loss in the coax. If it is higher, the coax is acting like part of the antenna and a choke to stop common mode current will help.
If there is common mode on the coax, that means the coax is radiating and part of the antenna. You can tune the coax by making it a multiple of a quarter wavelength (corrected for velocity factor) long. However, if you do this, you have to realize the coax is radiating like part of the antenna which can be undesirable and possibly dangerous. If the coax is not tuned, this will raise your SWR. A balun will prevent (or at least reduce) common mode current and (hopefully) make the coax length irrelevant.
Some antennas (like the carolina windom) intentionally allow part of the coax to radiate by careful placement of the balun (or unun). However, you don't want the coax in your shack radiating, as that adds extra RF exposure, and the possibility of RF burns.
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1Thanks. If tuning at the antenna gives you the most accurate antenna characteristics (good resonance), then tuning at the transmitter end of the coax means the antenna may be detuned a bit so that it's reactance components cancel out the reactance in the coax, but then the antenna system (coax+antenna) is optimized. That sounds like the best overall answer to me. Thanks for the info on the SWR being higher at the TX end of the coax means you've got some common-mode current going on. I'll check mine ... – K4JUL Aug 22 '23 at 16:42
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Coax doesn't have reactance, but it will rotate the phase of any reactance. Actually, there's other pitfalls. If your coax has common mode current and is radiating, that's energy that is not going into the antenna, and may not be going in the direction you want. – user10489 Aug 22 '23 at 22:37