Good Sam Club Open Roads Forum: Truck Campers: CB Antennas?
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burningman

Seattle, WA USA

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Posted: 03/01/22 09:47pm Link  |  Quote  |  Print  |  Notify Moderator

There’s no magic antenna that works better than another, it’s all about size.
They need to be a quarter wavelength (or half wave or full wave, but that’s huge at 27 MHz).
A 1/4 wave CB antenna is 9 feet long. The way shorter antennas are able to tune and resonate like a full 9 foot 1/4 wave is by coiling up the length of it.
If you look at a short antenna like a Firestick, you’ll see it’s a fiberglass pole with a wire spiral wrapped up it’s length.
That’s called continuous loading.
Other short CB antennas have a coil at the base or in the center about halfway up. It looks like a cylindrical lump, but there’s a coil of wire making up the rest of the required length of the antenna inside it. That’s “base loaded” or “center loaded”.
The coil just allows it to tune and resonate correctly for the frequency it’s being used for.
The shorter the antenna, even if it’s loaded correctly, the worse it works. The longer the better.

They also need a groundplane. The metal car body they’re mounted on acts as the groundplane.
Electrically, it mirrors the antenna to make it behave as if it were a full half-wave antenna. Without it, the radiation pattern is upward toward the sky.
It needs the groundplane to lower the pattern so it’s stronger in a more line of sight pattern where it’s more useable.
Note, a groundplane isn’t a “ground”, totally different thing.
The radiation pattern becomes directional toward the mass of the groundplane.
If you mount the antenna at the front of the vehicle, it will be more directional and strong toward the rear of the vehicle, following where the groundplane is.
Ideally you’d use a 9 foot whip on the top and center of the vehicle, but obviously that has practical problems.
You want it as high as possible, as long as possible and as centrally located as possible, and you compromise down from there as needed.

You also need to tune it with an SWR meter.
Standing Wave Ratio. That’s the amount of power that just bounces back at the radio rather than efficiently resonating and radiating out from the antenna. Most CB antennas have a set screw and a fine height adjustment to allow this. You want the reading on the SWR meter as low as possible. 1.5 to 1 or less is great.


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BradW

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Posted: 03/02/22 08:23am Link  |  Quote  |  Print  |  Notify Moderator

Read this: FIRESTIK INFO

The second advantage of co-phased antennas is there ability to perform where there is little reflective ground plane for the single antenna to radiate its energy from. For instance, on fiberglass vehicles or those with light aluminum sheeting over a non-metallic frame. In those situations a co-phase set-up allows the antennas to use each other's radiation field to direct the combined energy across the horizon. In order to be effective at least 2/3's of each antenna must have unobstructed line-of-sight to the other antenna.


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