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CB Antenna Basics

Author: thephantum

Putting together a CB system

When you are putting together a system for your Jeep, whether on a budget or not, plan on buying the best antenna system (coax, mount, antenna stud and antenna) that you can find and then go buy a radio.

All radios are pretty much the same, with the exception of bells and whistles that do little to improve the performance of your system. The manufacturing of CB radios is FCC the final output power is regulated by law to a maximum of 4 watts. Any wattage that is advertised as higher is audio output, meaning what you hear through the speaker has more power, not what you send. For basic communications, anything beyond a volume, squelch, RF gain and maybe a mic gain control is an extra feature you can do without. See this article for more information on choosing a CB radio.

There is nothing wrong with having all of the extra features, but not at the cost of the antenna system. It is a bad trade off. The best radio with a crappy antenna system will NEVER outperform a cheap radio with a good antenna system. If anyone tries to tell you away. Now, it isn't always just the antenna that is bad. Too often you get a piece of junk coax. Think of junk coax as trying to wash your Jeep with a hose that is full of holes. You can't see RF energy pouring out, or interference pouring into a piece of bad coax, but that is what you can often end up with.

Another part to watch for is the stud mount. The majority of problems in an electrical or electrical-mechanical device will involve poor connections that cause opens, shorts or high resistance. The stud mount is an electrical-mechanical device that must support the antenna and act as a bridge between the antenna and the coaxial feed to the radio. There are plenty of cheap ones out there. They are made of aluminum, or they use non-stainless steel washers (they'll corrode), or they have cheap plastic insulators (that crack or collapse).

When you shop for the antenna system, shop smart and you will only need to do it once. Give the majority of your time and money to the antenna system and you will be on the right track. Before selecting an antenna, you should be aware of the different types available...prior to that it helps to understand what SWR is.

What is SWR?

SWR (standing wave ratio) is a term every Jeeper with a CB should know. SWR (measured with an SWR meter) shows you how well your coax, antenna mount, ground plane and antenna match the output of your CB radio. It is a ratio of maximum voltage or current (the CB radio) to minimum voltage or current (everything from the coaxial connector at the radios output to the tip of the antenna). Generally speaking, the lower the SWR the better. Defective or inferior components, bad installations and antennas not tuned to the specific location on the Jeep increases the SWR. You should try to achieve a ratio below 2:1 on all channels. See this article for additional SWR details.

Before setting SWR, it helps to know some of problems that can be encountered and why they occur. If you take the time to read the information compiled here you will go into your install with knowledge that could save you a lot of time, effort and possibly, money.

Unlike receive-only antennas (AM/FM radio, TV, scanner), antennas that transmit require tuning. While antenna manufacturers can supply you with an antenna that is pre-tuned to the general frequency of the intended equipment, they cannot promise you perfect tuning "out of the box". There are six needed components for a CB radio installation:
  1. CB radio
  2. power feed
  3. coaxial cable
  4. antenna mount
  5. antenna
  6. ground plane or counterpoise (For most installations that we do as Jeepers, the mobile antenna will use the vehicle's body as counterpoise...meaning that while the antenna is the radiating unit, the Jeep is the reflecting unit. All transmit antennas need a reflective unit...a.k.a. counterpoise or ground plane)
Even if the manufacturer supplies you with an entire kit (coax, mount and antenna), this will leave you without the counterpoise requirement. There is an exception to this general rule; antenna systems that use the coax cable as a counterpoise. These no-ground-plane kits are generally marketed to owners of fiberglass vehicles. However, what follows is specific to installations that DO NOT use coaxial counterpoise antennas.

SWR and Antennas

Many people believe that SWR only involves the antenna but that is not accurate. When you place an SWR meter between the radio and the antenna it does not determine one component from another. A defective connector, bad coaxial cable, untuned antenna, etc. may all return the same results on the meter. Even how the antenna is mounted will play a big part in the resulting SWR readings. For instance, if the antenna is mounted on a Jeep in a way that interferes with its ability to radiate applied energy into free space, you will get a higher than desired SWR. Location is important.

It isn't always practical to mount the antenna in the optimum position on any vehicle. When you set aside performance in favor of convenience, you will need to settle for what you can get. Regardless of location, tune your system/antenna. When tuned properly, the antenna will give you the best performance you can expect from an antenna mounted in any given location. Most of all, remember that untested installations can cost you big bucks. Checking your CB antenna system for continuity, shorts and opens is a MUST and should always precede checking SWR. Operating transmitting radios when the SWR is over 3:1, or with opens/shorts can damage the radios circuits.

Tuning principles

If CB radios only transmitted on one channel, it would be much easier to setup antennas. Most CB's have 40 channels, but mobile antennas can only be made to resonate at one specific frequency or channel. For example, on a 40 channel CB, channel 19 is the center frequency (27.185 MHz). If the antenna is tuned to be resonant on channel 19, the further away from that channel you are, the higher the SWR will be. This is also known as "Center frequency dip". CB antennas are therefore made to resonate at a frequency in the middle of the available band (channel 19 on CB) and are made broad-banded enough to keep the off-frequency related SWR at the two extreme ends of the band (Channel's 1 & 40) below 2.0:1.

This important, because it needs to be taken into account when measuring and setting SWR. Essentially, you will need to take 3 readings: At channels 1, 19 and 40. With these three readings you can determine all kinds of things about the system...including whether the antenna is electrically long or short. More on actually taking SWR reading and adjusting all this later. (If you communicate on one or two adjacent channels anywhere within the band, you can tune your antenna to achieve optimum performance on those channels. Most people, however, prefer to use the entire bandwidth when tuning.)

Types of Antennas

All CB antennas are tunable in one way or another. Antennas that have a tuning device (usually mechanical), are referred to as a "tunable antenna". Because of this, lots of people think the following types of antennas are not tunable and so they make no attempt to have the antenna tuned.

Wire-wound antennas (without mechanical tuners), can be tuned by removing wire from the top (to fix a electrically long condition) or the coils at the top of the antenna can be separated and spaced further apart (to fix an electrically short condition). In addition, parts that increase the physical length of the antennas (springs, quick disconnects, etc.) can be added.

Solid fiberglass antennas (which come in straight or helical wire configurations) that are electrically too long can have the cap removed and the top can be cut off with a hacksaw. If the SWR test shows that it is electrically short, the addition of a spring or quick disconnect (or both) is the only way to correct it.

Wire Whip antennas have a set screw (or two) just below the area that the whip is inserted into the loading coil. By loosening up the set screw(s) you are able to slide the whip up or down as required to adjust it

Choosing an antenna

There are some basic rules to follow when choosing a CB antenna to install on your Jeep. Before you even consider buying the antenna, mount, etc. figure out where it will be mounted on the Jeep and take the following points into account.
  1. Top loaded antennas perform better than center loaded antennas, center loaded antennas perform better than base loaded antennas  - I'm not going to get into a discussion of what loading a CB antenna means, you can go here for more info.
  2. At least two-thirds of the antenna should be above a vehicles roof line for optimum performance.
  3. Taller antennas perform better than shorter antennas
  4. Antennas mounted higher perform better than if mounted lower
  5. Single antennas perform better when mounted as close to center as possible on metal vehicles - this is neither practical nor even possible on most we are already at a disadvantage
  6. The physical length of the antenna should be determined by the likelihood of it hitting of overhead objects as well as the type and strength of the antenna mount itself
  7. The bandwidth of the antenna should always meet or exceed the needs of a 40 channel CB.
Once you've used the previous criteria to figure out where you're going to mount you're antenna, how long it needs to be, etc., you are ready to start shopping. One other thing to note: If the antenna/mount combination is available in a kit form, you will normally save money by purchasing it as a kit. Just make sure that the components are all of high quality.

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