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Thread: Newb Question: Mobile Antenna Coax Cable Length

  1. #1

    Default Newb Question: Mobile Antenna Coax Cable Length

    I'm new in the game so I turn here for expert opinions: I've got a new antenna mount on the way and I believe it comes with 11-17' of coax cable for the NMO mount. The antenna is a Comet SBB-224 tri-band 2m/1.25m/70cm. I read some where online (can't find the link) that an antenna cable must be 17' long, but I believe this advice was only for single-band 2m antennas so I'm not sure how this would apply to dual- and tri-band antennas.

    So what's the rule of thumb in regards to cable length? The radio will be installed behind the rear seat on the floor of my truck, with the antenna right above it at the rear of the roof, so it won't require a very long cable. Should I cut any excess, or find a lazy, meandering path to spread out the extra length (avoid coiling/looping, of course).

  2. #2

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    I'm no expert, but I have an opinion. Coax should be as short as possible as VHF and higher frequencies see loss in every foot of extra cable (as do all frequencies, but the loss is worse at higher frequencies).

    I believe the reason people favor half wavelength coax cables is because getting a near perfect match is almost impossible at anything other than half-wave lengths of coax. The reason for this is often attributed to the loss. My theory is the likely fact the coax is not perfectly matched to the radio or antenna. For example, you could have a coax at 52ohm or 47.5ohm and key into a 50ohm dummy load. The transformative effect of the coax only rotates around a constant SWR circle if the smith chart is normalized to the coax cables impedance. Because of this slight mismatch, you will see the impedance transformative effect appear as small SWR humps that peak every odd quarter wave multiple.

    There are a few reasons that I am aware of for wanting a cable at a half wavelength, I will list the ones I know: 1 - If the cable is a multiple of half wavelengths, the impedance transformative effects of the cable have come full circle on a smith chart and the impedance measured at the coax input is the same as that at the antenna feed point (neglecting coax loss). It makes using an MFJ antenna analyzer to measure R+jX values of an antenna feed point easier (as measuring at the feed point directly is a pain). Some analyzers account for the effects of coax transformation via OSL calibration and the coax length won’t matter once set up. 2 - Getting lower than 1.2:1 SWR. This was mentioned in the paragraph above. 3 - Preventing common mode currents. If the coax (and radio ~ the end of the electrical path) connects to either a short ground connection or has no ground (either a short or open circuit), the same impedance will be presented at the other end of the coax shield and act, in a sense, as a cheap balun. This is often not worth the hassle, just use an antenna presenting a balanced load to the coax.

    As for coiling up the excess cable, there are pros and cons that can get complicated. One could call it an “ugly balun” and assume the inductance of the coiled shield will stop common mode current. People who make this assumption (and sometimes have success) don’t always consider the impedance of the common mode path before adding an inductor (coiled coax) to it. Let’s say the shield path is already reasonably capacitive, would partially cancelling out that already-present reactance be good? Likely not. It all depends on the situation.

    If you have the equipment and skill to properly shorten the cable, by all means do so. It will help for UHF for sure. If your soldering skills are sketchy and you have never swapped connectors, keep the extra wire (and feel free to coil it up).

  3. #3
    gnuuser's Avatar
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    general practice for a cb antenna was to make the coax ( usually rg58 or rg 8) 18 feet in length. this afforded the best chance of obtaining a match with ground plane type antennas. But with the mag mount antennas they made ground plane by capacitive coupling and usually the cable attached to the antenna was sufficiently long enough to closely match the antenna at the factory.
    the biggest issue with todays vehicles is that the steel is much thinner than older models or car body is made of aluminum and makes for a poor ground plane. to counteract this ground plane plates of various sizes are manufactured.

    mobile antennas can be a trial in themselves with either ground plane or non ground plane depending on the location on the vehicle.
    a good example my 4 foot firestick was tried in various locations (including a very short ground strap to the body).
    after numerous place changes the best place was just in front of the drivers side front door on my toyoya highlander.
    beautiful signal and 1:1.1 swr.

    swr meters are good tools but in all truth they are brute force devices and cannot point out problems with the antenna system.
    for this its better suited to use a quality antenna analyzer or a vna (calibrated for the middle frequency of course)
    a vna can point out a short or open coil all while testing the system, they can even point out a broken center conductor in the coax by measuring its length electronically.

    and as far as coiling the cable! Dont! loop excess in a figure eight and if you must
    Im so old dirt was my apprentice

  4. #4

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    Thanks for the very informative responses, though it's way above my pay grade

    I only have a Yaesu SWR YS-500 meter, which I haven't learned to use yet (hey, did I mention that I'm a green-horn?). I can definitely solder and crimp coax, though this experience is only with video cables so I don't even know if I have the jaws for my crimper to handle PL259 connectors.

    In the meantime, I'll just stick to a lazy-eight behind the cab until I can better understand what I'm doing, how and what to measure for signal-wise...

    I'll also be searching for local clubs to pick the brains (and maybe lend a hand to cutting/measurements) for first-hand knowledge...

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