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Antenna Feed Line Length

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Old 12-10-2006, 08:51 AM
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Default Antenna Feed Line Length


Iím interested in anyoneís practical experience regarding the proper length for antenna feed lines in a mobile CB installation. The goal is, of course, to achieve the lowest SWR and highest antenna efficiency. Iíve read quite a bit of contradictory advice on the subject which seems to fall into several different schools of thought: (1) the line should be as short as possible, (2) the line should be 1/2 wavelength, (3) the line should equal the effective length of the antenna to which it is matched, either 1/2, 1/4, or 5/8 of a wavelength.

The advice Iíve seen most often is that the lead in wire should be 17 feet long, but this does not seem to make sense if the object is to have the wire be some fraction of a wavelength. Follow my math Ė- and Iím happy for anyone to point out where Iíve gone wrong here. Assume we want to use channel 19 or 27.185 MHz as our starting point: the wavelength (in feet) = 984 over 27.185 or 36.196, the velocity factor of the coax is .82 (assume Belden 9258 RG-8X), thus the wavelength in the coax for that frequency would be 29.68 feet (36.196 X .82), and 1/2 of that would be 14.84 feet, 1/4 = 7.42, 5/8 = 18.55, and 3/4 = 22.20.

Obviously none of these numbers is close to the common wisdom that the feed in line should be 17 feet, which is why I am asking for practical advice. My thought, FWIW, is that the line should be as short as possible without any sharp bends.

[edited to correct math]


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Old 12-10-2006, 09:28 AM
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Ah Grasshopper, you have discovered one of the great mysteries and your BS filter is working well. These people that tell you to use a specific length of coax, don't have a clue. I was reading the back of a packaged Francis 5.5 Hotrod antenna, the other day. It was saying to use different lengths of RG-58, RG-8X, or RG-59 coax, depending upon your installation. While it is true that when you co-phase two 50 ohm antennas, that they be fed with 75 ohm coax, of odd 1/4 wave electrical multiples, you have to know the velocity factor of the coax used. Just because it is RG-59, tells you nothing. Depending upon the construction of the coax, you could have a velocity factor of .66 to .95. A big difference when calculating electrical lengths. It is the same with all coax.

The long and short of it is: With a single antenna, the correct length of coax is the amount it takes to reach the antenna. If you are having to use a specific length, then the coax has become part of the antenna and you don't want any part of it in a mobile installation. Anybody that tries to tell you different is just perpetuating a myth.

Rich
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Old 12-10-2006, 10:04 AM
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Very true..

this BS been going on forever(and many buy into it unfortunatly)

single antenna......shortest needed coax feedline

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Old 12-10-2006, 04:33 PM
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http://www.ocarc.ca/coax.htm

Check this out, it may help you and others further understand the mystery of antenna feed lines a little better.
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Old 12-10-2006, 05:34 PM
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That's a very nifty dipole calculator. Thanks!

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Old 12-11-2006, 02:10 AM
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We have a very good tutorial on our ATV club web page too that deals with testing transmission lines. Everything you might ever want to know from the basics up. There is even a section on why you notice a difference with your SWR meter when you cut the cable to different lengths.

http://www.satvg.org/smf/index.php?topic=9.0

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Old 12-11-2006, 01:06 PM
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A good tutorial

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Old 12-11-2006, 08:14 PM
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Coax length is a case of getting tangled up in the details! Yes, you can see a "difference" in the readings. "Yes", there may be some cases where doing so is necessary. But! It isn't as important as some of us make it out to be! Except for some specific cases, I have never even given the slightest thought to "trimming" coax, indeed, had never even HEARD of doing that until I ran into the CB guys that just swore that you had to do that. Gee! Don't tell my antenna, or it might quit working! :P If you want to fool with coax trimming, that's fine. You won't get an argument from me! But, for the most part, you should trim the antenna elements/whip to resonate your antenna. The coax-cutting formula is:

"A length directly proportional to the distance from the radio to the antenna!"

73

CWM

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