What is the maximum allowable VSWR?
VSWR (voltage-standing-wave-ratio) represents the degree with which an antenna is “matched” to the system impedance. Most modern antennas do not require any tuning for optimum performance. VSWR is one of the easiest parameters to measure and VSWR meters are becoming very popular antenna installation tools.
Most modern antennas, receivers and transmitters are designed for peak performance when operating into a 50 Ohm transmission line. If the VSWR is too high, the transmitter power may be reduced as well as the strength of the received signal.
The typical commercial standard for maximum allowable VSWR across the entire bandwidth of a system is 1.5:1. This means that the antenna impedance must be somewhere between 37.5 and 75 Ohms. You should specify the maximum VSWR and the operating frequency bandwidth when specifying your antenna. A VSWR of 2:1 or greater usually is considered unacceptable (or for receiving purposes only) since it increases losses in the transmission line. Incidentally, decreasing the VSWR below 1.5:1 will often be expensive and will have little noticeable performance improvement.
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This information should be used as a guideline only to help you in the appropriate selection of an antenna.