CB
Radio Infoby Maurice
Riggins
December
1993The
102" whip antennas with a 6" spring give a full 1/4 wavelength ground-plane
antenna at CB frequencies (discounting velocity factors of various conductive
materials). The higher the frequency, the shorter the antenna should be
and vice versa. When tuned for minimum Voltage Standing Wave Ratio (VSWR)
on Channel 19 (mid-band), the VSWR does not rise appreciably as you move
to Channel 1 or 40... such antennas are relatively "wideband".Full-length
whips have better reception and a fairly low angle of radiation (transmission)
for a 1/4 wave ground-plane antenna (meaning more power pointed along the
surface of the earth where receiving radios are located, instead of up
in the sky). HOWEVER, full-length 1/4 wave whips are seldom practical...
especially in the woods. The
1/4 wave ground plane antenna depends on a conductive plane perpendicular
to the antenna. Ideally, the ground-plane antenna would be mounted in the
center of an all metal vehicle, up on the roof. With the birds eye view
of the vehicle, the radiation pattern would be oval with more radiation
out the front and back of the vehicle than out the narrower sides. If you
mount the antenna on the left rear, the unit will transmit strongest to
the right front. Kind of impractical to put a full-length whip in the center
of the vehicle, on the roof, especially on a softtop.Bowing
a full length whip makes it overall less efficient than, say, a 3-5' continuously
loaded whip, but you do have the option of unclipping the front if longer
range transmission is necessary. The reason deals with polarization. While
TV transmissions are horizontally polarized (the reason TV antenna elements
are horizontal), most CB or other radio transmissions are (supposed to
be) vertical. For maximum reception of received signals, and max transfer
of your transmitted signal to a receiving vertical antenna, your antenna
should be vertical. Also, if you look at the radiation pattern of a bowed
antenna, there's very little energy going to the front or back of the vehicle
(which are basically looking into the hole of the donut shaped pattern)
and most of the energy is directed up in a wide weak area, down into the
vehicle in a small, highly focused area (health risks?), and out the sides
of the vehicle in a wave 90 degrees to most receiving antennas. The
radiation pattern is a donut with the 1/4 wave whip up the center. More
ground plane on any side (up to 1/4 wavelength) will concentrate energy
in that direction. Less (or none) on any side will reduce the radiation
in that direction. Tilting (or bowing) the antenna from vertical "squeezes"
the pattern on the side the antenna is being pulled towards (and lowers
the radiation pattern) on that side and does the opposite on the other.
And finally, loading, as previously described effects the angle of radiation,
or how high the donut is off the ground. You want it as low as possible
so the fattest part of the donut, looking at it from the side, is along
the ground, giving you more range. There are other (omnidirectional, vertically
polarized) antennas with lower angle of radiation than the 1/4 wave ground
plane (which also allow them to be mounted higher, clearing obstacles),
such as the 5/8 wave, or center-fed coaxial 1/2 waves, but they're WAY
too long for vehicle use.Shorter
than 1/4 wl antennas can be made to appear electrically longer (to the
radio's output circuits) with parallel capacitance, or series inductance
the former being seldom used. This tricking of the circuits does narrow
the bandwidth of the antenna (VSWR on Channels 1 and 40 is higher), but
it makes the antenna a much more practical length. The shorter the antenna,
and the more you depend on loading, the worse the reception/transmission,
and the narrower the bandwidth (other factors being equal).The
loading coil (inductance) can be placed anywhere along the antenna. Typically,
you'll see a coil at the base of the antenna. This has the worst (highest)
angle of radiation, but puts the coil where it places the least wind drag
on the antenna. There are also center loaded antennas with the coil in
the middle, which offer a lower angle of radiation. Top loaded antennas
have a lower angle of radiation, but aren't too practical on vehicles.
Another style of loading is continuous loading, usually done by wrapping
the coil around the full length of the antenna... you see this with the
shorter fiberglas whips. It has about the same angle of radiation as a
center loaded antenna.But
as I stated before... vehicle installation is a compromise... a lot of
folks are using a variety of less than ideal setups to meet their needs
just fine. The best compromise I've seen was a 3' center loaded whip permanently
mounted on the roof, and in the center of a Pinto station wagon. Signal
strength was an nice strong oval, with a little more strength in the front,
due to the lowered ground plane (the hood lower than the roof) pulling
the radiation pattern down a bit in the front. The problem with this setup
was that it could transmit farther than it could receive from most other
vehicles... partly due to their less than ideal antenna setups and reduced
transmit range, and partly due to the Pinto's short antenna length putting
less metal up to grab signals. An Oldsmobile wagon or full-size van with
that setup would be even better due to more ground plane. Dual
antenna's interfere with each other unless they are 1/4 wavelength (9')
apart... their radiation patterns aren't round, and are affected by the
length of the cables feeding them... forget 'em. RG-58 is the appropriate
cable for vehicle CB radio's (RG-8 has less loss for long runs but is too
stiff and thick for vehicle use).For
drawing the power for a CB, the best bet is a direct connection (fused)
to the battery or closest terminal. Shielding the power cable also prevents
RF noise from the engine compartment getting into the radio (but coax isn't
a good power cable... buy separate shielding). Good ground connections
are essential... scrape any paint or rust off before making grounds. I
prefer to get radio ground at the battery. Make sure ground straps for
the hood, trunk, doors, frame, etc are on good, clean connections.Another
hint is to make sure the engine is running when you want power... i.e.
making an emergency call from deep in the woods. CB's (and other vehicle
electronics) are designed for an average input of 13.8 or so volts... without
the engine running you only have 12 volts or so from the battery. I've
run tests with an RF wattmeter that showed radios putting out a full- legal
4 watts with the engine running, but only 2 - 2.5 watts with it off. If
you absolutely must mount the antenna on the rear of the vehicle, point
the vehicle in the direction you want the broadcast to go (preferably nose
slightly downhill) before making your emergency call.
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