Mobile CB Radio Info

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Mobile CB Radio Info

Post  Cobra on Tue Jan 29, 2008 3:26 am wrote:The 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.


Number of posts: 16
Location: Yuma, AZ
FRN#: 0015687585
Radio Model: Cobra WX75
Registration date: 2007-10-17

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Radio Shack 102" Whip Antenna endorsement.

Post  brujo de la colonia on Fri Mar 07, 2008 2:58 am

I run the full length stainless steel whip from Radio Shack, catalog number 21-903. Link:

I have it on a mount on the spare tire carrier on my Jeep Wrangler. It really "gets out" and works very well. It's durable and flexible. Low overhanging branches and other such stuff does not damage it, is just takes a licking and keeps on ticking. It does not exhibit a "shape memory" when being bent over by low overhead obstacles, or when being tied down over the top of the Jeep: it always spring back to it's original shape, no kinks or bends happen to it.

I fashioned a tie from a length of parachute cord and a carabiner clip. If I need to park in a very low parking garage or carport, I simply bend the antenna over the roof of the Jeep and clip the carabiner to the footman loop on the hood of my Jeep. I don't operate the radio with the antenna tied down like this, but it's a very convenient way to get the antenna out of the way when I need it.

Antenna is plenty strong enough to dangle an off-road flag from the top of it, at normal off road driving speeds. I take the flag off when I'm on pavement.

Overall a great antenna at a good price, highly recommended. You won't regret it.

You won't see the antenna in the show room of any of the local Yuma area Radio Shacks. They have them in the back room, it comes in a long, heavy protective cylindrical cardboard tube. So don't be discouraged if you don't immediately see it in the store. Don't be shy, ask for it, they have them in the back room.

And as if all that weren't enough, it looks cool sticking up from the back of the Jeep. And not only that, but it makes it easy to find the Jeep in a parking lot full of huge, tall full-sized Snowbird 5th-wheel pulling pickup trucks. Age is getting the better of me, I suppose, and I often forget exactly where I parked it. Antenna makes it easy to find again. Saves me from walking up and down the isles looking like a lost fool Laughing

Last edited by brujo de la colonia on Fri Mar 07, 2008 3:20 am; edited 1 time in total

brujo de la colonia

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Registration date: 2007-11-15

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I beg to differ . .

Post  brujo de la colonia on Fri Mar 07, 2008 3:20 am

putting out a full- legal 4 watts

There is no such thing as "full legal output power". The FCC's legal specification is that a CB transmitter can use no more than 5 watts input power to the final amplifier stage in the transmitter. Input power means the DC power being supplied/consumed by the transmitter's final amplifier, as in (for example) 13.8 volts X .3623 amps = 5 watts of DC power.

Now, there is no such thing as a 100% efficient electronic circuit, at least in consumer priced electronics (we won't get into theories about cryogenically cooled superconductors as that is out of the realm of cost for all consumers except for the likes of Bill Gates), so we will always have losses due to heat and other circuit inefficiencies. However much power the manufacturer can squeze out of 5 watts DC input power, is the "legal output power". Some radios are better than others when it comes to efficiency. On average, most radios manage to squeeze out 4 watts of RF output power for 5 watts of DC input power.

Just felt like picking nits, hope nobody is offended.

brujo de la colonia

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