In telecommunications, standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line.
The SWR is usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio. For example, the VSWR value 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times greater than the minimum standing wave value. It is also possible to define the SWR in terms of current, resulting in the ISWR, which has the same numerical value. The power standing wave ratio (PSWR) is defined as the square of the VSWR.
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, 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. 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.
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.
