Communication – CB Antenna Basics

 CB Antenna Basics

 

 

Confused by all the different CB antennas available for your home or car? Which one is better? Which one suits your needs better. Why do some cost so much compared to others? What the heck is “gain”. Why does SWR matter, and what is it? Here is all you need to know to take the mystery out of your antenna:

This is the most basic antenna you can get for a CB, short of hooking a random length wire to your radio: The Dipole Antenna.You’ll notice it consists of 2 “elements”, a radiator that is attached to the center wire of the coaxial cable from the radio, and a ground element that is hooked to the shield (chassis grounded) part of the coax cable.The correct length of these elements makes them resonantly tuned at the desired frequency, which means the antenna is correctly matched to the radio so it is most efficient. If the tuning of the elements is incorrect, some, or even all of the radio signal gets reflected back to the radio, instead of leaving the antenna, which can damage the radio, and reduces the range of the signal.Although from a visual perspective this does not much resemble most CB antennas you see, from an electrical standpoint, this is basically what most types of CB antennas are. You’ll see how, in a moment. 
Next, we look at a standard “Ground Plane” antenna, the type that is commonly used (in varying forms) for CB Base Stations as a rooftop antenna. If you compare carefully, you’ll see it is basically an improved Dipole Antenna. It still consists of 2 parts, since it has a radiator element, and ground elements. By adding more ground elements and putting them horizontal to the ground, the radio signal leaving the radiator element is focused better to extend the range.
The shape of this signal “focusing” is known as the radiation pattern, and the angle the signal is focused to leave the antenna at is known as the radiation angle.The more ground elements you add to this antenna, the more multidirectional the radiation pattern becomes. Usually a minimum of 3 – 4 ground elements will give a fairly circular pattern (as seen from above the antenna).Placing them at a 90 degree angle to the radiator will usually give a radiation angle of about 45 degrees – which means the strongest part of the signal is actually going up into space, instead of following the ground. Fortunately, there is enough signal in the pattern at other angles to still be fairly efficient, and the “lobe” of the radiation pattern is a great improvement over the one for the Dipole.If you look at the diagram at left, which shows the radiation pattern for a Dipole Antenna, you can see a lot of the signal is radiated uselessly toward the ground, or almost straight up. By using a Ground Plane configuration, more of the signal is sent in a useful direction. Both of these diagrams are as if viewing the radiation pattern from beside the antenna. 
Here we see a “mobile” antenna, the standard setup for use on a vehicle. Again, by comparing, you’ll see the basic arrangement is the same as a groundplane antenna: a vertical “radiator” and a horizontal ground plane, which is the metal vehicle body in this case, instead of a wire as in the dipole. Using the vehicle body as part of the antenna has it’s advantage in cutting the size of the antenna, but has some drawbacks. If the antenna is not placed in the center of the body, it can cause the radiation pattern to be uneven in shape, which can cut range in some directions while increasing it in others. Also, the antenna ground connection to the body becomes very important, because a bad connection or lack of enough metal body works the same as cutting off half the antenna. This is why non-metal body cars like the Corvette have to have a CB antenna hooked to the car’s metal frame in order to work correctly.
Mobile antennas come in all shapes, sizes, and mounting configurations. Most work with about the same level of efficiency, so that test equipment is needed to tell the performance apart. As you might guess, the extreme ends of the design spectrum also give the most extreme results: the ultra-tiny antennas tend to perform miserably, and the larger antennas tend to perform the best. This is primarily because of the physical properties that a radio wave has at a certain frequency. The lower the frequency, the longer the physical wavelength is. A full wavelength for CB radio frequencies is approximately 36 feet long. Since this is a bit long for an antenna mounted on your car’s bumper, manufacturer’s scale down by dividing the length. Your CB radio will match with an antenna that is an even fraction of a full wavelength, but the shorter it is, the less efficient it becomes, so manufacturers use 1/4 wavelength (8 feet) as a standard. All mobile CB antennas are electrically this long, even if they are physically much shorter. Shorter antennas “cheat” by using a built in coil to simulate part of the length. It works well as far as the radio matching is concerned, but a lot of signal is wasted in the coil. If the antenna is so small that a large portion or even all of it is in the coil, the performance drops drastically.
Last, but certainly not least, we have the “Beam” antenna. Again, it is just an improved dipole antenna at heart. The actual portion connected to the radio is noting but a standard dipole, and is referred to as the “driven” element. The improvement comes from adding a “reflector” dipole, which cuts interference in reception from the rear, and helps reflect your transmitted signal forward in a tight “beam”. The “director” elements act like a lens and further concentrate the radio beam even narrower – the more “directors”, the narrower the signal is beamed, and the more range it has. All of this makes a “beam” antenna very good to use for long-distance communications since it not only extends range but cuts down interference from undesirable directions at the same time. The design requirements and the need for a tower and antenna rotator also make this the most expensive type of antenna, as well. To get an idea of the difference in how a beam antenna performs versus an omni-direction “ground-plane” antenna, think of your radio signal as if it were the light from a bulb. If the bulb sits in the open by itself, the light covers a circular pattern on the ground for a short distance. However, if you put the bulb in a reflective housing like a flashlight, the beam of light travels much farther but must be aimed carefully to shine where you want it to illuminate. These antennas work the same way.
Base Station “Beam” Antenna

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