Who is a REALLY good installer of cb?

MayhemAutoTransport · Apr 26, 2011

handlebar said: ↑

Jamie,
Mini 8, or RG-8X (or various similar names) is rated for 1,500 watts at 10 MHz, and a little less at 30 MHz. It's hard to imagine a need for larger coax for a mobile installation at CB frequencies. If you were at VHF or higher, and had a long run, it might be worth the hassles (larger required bends, bigger holes) of a larger diameter coax.
As has been noted by others, the 18-foot length is a myth that's been sorta accepted as gospel by folks who remember that it's the length used on each leg of co-phased harnesses (even though that uses 75-ohm line, instead of 50-52 ohms). And, perhaps because of the myth, and the typical length needed to run an antenna lead from under the dash to the rear bumper or cowl on a passenger vehicle is close to 18 feet, it's become kind of a standard length. As it happens, the commercial VHF/UHF antenna mounts I use have standardized lengths, but they vary with the manufacturer -- typically 17 or 18 feet, but again, it's just a practical standard. Nothing to do with physics at all.
And, some specialized antennas that are made to work with no ground plane come with a coax that's close to 18 feet. However, in those cases, the coax is actually part of the tuning network for the antenna, because the antenna doesn't present a 50-ohm "load" at its feedpoint. The special length of the coax in those kits is necessary to work as part of an impedance matching network so that the transmitter will have an acceptable match between its output and the connector of the feedline. If that is the case, the packaging or installation instructions with the antenna will have, in either italics or really large print, something like, "Do not cut this coax".
Except for the special case of that kind of very specific, special antenna, the length of a 50-ohm (or 52-ohm) coax should be exactly what the distance is from the radio to the connector on the antenna. Or it's safe to add some, like I do, because the entire "antenna system" Iincluding the antenna, mount, feedline, connectors, "ground plane" under the antenna, and the literal ground over which we drive, are as close to 50 ohms as we can get them.
Not coincidentally, receivers also work best when their antenna's impedance matches the input of the receiver, which branches off the coax connector on the back of the radio.
Impedance mismatches cause loss of signal transfer efficiency, and it goes both ways (transmit and receive).
There *is* such a thing as an "RF air choke" which is a *good* thing; it keeps stray RF from a balanced antenna (like a wire dipole), that is fed by an unbalanced feedline (typical coax), from running along the outside of the coax. Winding coax carefully around a form that keeps the shield in close proximity to itself on other windings (stacked, not random) cuts down on RF traveling on the *OUTSIDE* of the coax shield, so that it cannot radiate near the radio or other audio devices, for instance.
There is a common misconception that anything that "chokes" RF is akin to closing off one's trachea and preventing respiration, or squeezing a garden hose. As long as you don't make windings tighter than the coax's maker specifies as its "minimum turn radius" or "minimum bending radius", wherein the critical spacing between the outside of the *inner* conductor and the inside of the *outer" conductor, you may shape the coax in any manner you care to. The "characteristic impedance" of the coax depends upon that spacing, and anything that varies it will present a mismatch. If the impedance gets too low, it can behave like a dead short, at least to RF ("radio frequency", a/k/a radio waves), even if it doesn't show up as a DC short on an ohmmeter.
I think my fingers are smoking from friction again, and I realize that once again, someone asked me what time it was and I told them how to build a clock.
Sorry. Hope you're still reading, and hope that helps.
-- Handlebar --
p.s. To summarize: Unless you're going to run over 1,000 watts for long periods, Mini 8 should be fine, of any length that reaches from your radio to your antenna.
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Thanks you are a wealth of knowledge. I could sit and listen to you for hours explain the whole process to me over a 12 pack of beer

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