1.-- Impedance repeats itself every odd multiple half wavelength of coax. This is why it's perfectly OK to use 75 ohm coax (or other impedance) to feed your single-band antenna. You can trim ANY coax, no matter what the impedance, so that it will repeat the 50 ohm impedance shown at your antenna AT A PARTICULAR FREQUENCY .

2.-- While tuning your antenna, it makes sense to use a tuned piece of coax that is an odd multiple of a half wavelength because it will present the exact same impedance at the end away from your antenna. This makes sure that you tune your antenna properly.

3.-- Once your antenna is tuned properly, coax length makes absolutely NO DIFFERENCE WHATSOEVER. It's 50 ohms at the feedpoint of your antenna, and 50 ohms at ANY POINT ALONG THE PIECE OF COAX, NO MATTER WHAT THE LENGTH !

4.-- This pertains to a SINGLE BAND antenna. Multi-band antennas -could- be tuned in this manner, but it's not worth the extra effort of swapping several pieces of coax. The antenna designers have made the antenna so that it ought to present a 50 ohm load after installation, so if you use a pre-manufactured antenna, coax length should make NO DIFFERENCE . (Key words- "ought to"! This does not take into account any problems you may have with the antenna itself or grounding differences, and if you're making a home-brew antenna, you may want to use a piece of coax cut to an odd multiple of half-wavelengths for tuning purposes.) If you're copying an antenna design of proven performance, then you can probably assume (if you followed the instructions on how to build it correctly) that your finished product should be close enough to 50 ohms so that you won't need to use a cut length of coax for SWR purposes.

5.-- All this is pretty much null and void anyhow- if you have a 50 ohm antenna, and use 50 ohm coax. When you find the settings for matching the antenna for minimum SWR, since the coax is 50 ohm coax, if the antenna is 50 ohms, you should have a match regardless of the coax length! After all, that's what you're looking for- a match! If your antenna is 50 ohms, and your coax is 50 ohms, your radio is wanting to see a 50 ohm impedance, and your SWR presents as 1:1, what other condition could you possibly have except a correct and proper match? Think about this. If you have a 50 ohm antenna and your coax is 50 ohms, unless the antenna is mismatched you will see the exact same impedance anywhere along the length of the coax! If the antenna is matched, it won't matter how long the piece of 50 ohm coax is!

6.-- Some (much older) radios want 75 ohm impedance antenna systems. Keep this in mind if you're using a very old tube radio. I can't think of an example right now, but I have a Johnson SWR meter that is switchable between 50 and 75 ohms. A dipole antenna is (approximately) 72 ohms, so don't expect to see a flat match when using a dipole. If you do, the antenna is NOT TUNED PROPERLY ! When properly tuned, if you are using a 50 ohm SWR meter and 50 ohm coax, you SHOULD see an SWR of 1.5:1 (approximately) . Yes, you can use a transmatch to lower the SWR perceived by your radio. Transmatch use is another subject I'm not going to cover here.

7.-- If the antenna -is- mismatched, you -could- see a resonant point at some point along the coax, but not at another. This is why people believe that using an odd multiple of a half wavelength of coax is the only "proper" way to tune an antenna. To a point, this is true. But once the antenna is tuned, you may use any length of coax necessary to reach the antenna, and none additional. But if the antenna is designed to present a 50 ohm load, and is a factory designed and built antenna, and is not defective, then requiring an exact odd multiple of half-wavelengths is unnecessary.

I may have repeated myself in some places in this post, but I did so in order to clarify various points of information.

73, -Web

Many thanks to Web Willams at
11meteramps

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The Copyright Minefield By Jacqui Kramer, Contributing Author
10/5/2000 9:19:53 AM PDT

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