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]