Introduction to SWR Testing of CB Antennas

Compliments of Firestik^® Antenna Company Technical Support Team

Copyright ^© 1996 Firestik^® Antenna Company

SWR (standing wave ratio) is a term every CB'er should know. SWR (measured with an SWR meter) shows you how well your coax, antenna mount, ground plane and antenna match the output capability of your CB radio. It is a ratio of maximum voltage or current to minimum voltage or current. Generally speaking, the lower the ratio the better. Defective or inferior components, bad installations and antennas not tuned to the specific location on the vehicle increases the SWR. You should strive to achieve a ratio below 2:1 on all channels. Before measuring and setting SWR, it is helpful to know some of the things that can cause problems. Spend the time to read the information we have provided on this site and you will go into your project with valuable knowledge. It 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. Antenna manufacturers can supply you with an antenna that is pre-tuned to the general frequency of the intended equipment, but they cannot promise you perfect performance "out of the box". The six needed components for a mobile installation are the radio, 12V power feed, coaxial cable, antenna mount, antenna, and ground plane (counterpoise). Even if the manufacturer supplies you with an entire kit (coax, mount & antenna), this, in most cases, leaves you without the counterpoise requirement. Counterpoise and ground plane are interchangable phrases used to describe the reflective element required by transmitting antennas. 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, for most applications, the mobile antenna will use the vehicle's body as counterpoise. Unless otherwise stated, the following information pertains to installations that DO NOT use coaxial counterpoise antennas (no ground plane antenna systems). The vehicle, in mobile installations, is just as important as the antenna and other components. The antenna is the radiating unit, the vehicle is the reflecting unit. All transmit antennas need a reflective unit. What effect does the vehicle have? Plenty! If you were to install and tune your antenna on a bumper mount then move it to the roof, you would see a change in the SWR. This is due to the change in the antenna's position relative to the vehicle surface. Location is important. It isn't always feasible, or practical to mount the antenna in the optimum position on any vehicle. Nonetheless, whenever you set aside performance for convenience, you will need to settle for what you can get. Regardless of location, tune your antenna. Tuned antennas 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. Operating transmitting radios when the SWR is over 3:1 can damage the radios circuits. Always check your SWR and tune your antenna. There is a lot to know about transmitting antennas. Even if you have your system installed by a professional, it is helpful for you to know what can affect its performance. The technical staff at Firestik® has compiled 30 plus years of technical support files. Some of our findings seem contrary to theory, but the purpose of this information is to solve real problems, not preach theory. We have tried to concentrate on the areas where most of the performance robbing problems crop up. We highly recommend that you read all of the supplied information in the technical section of our web site. Starting with knowledge is always better than learning from disappointment.