In the above table, a VSWR of 4 has 36% of power delivered by the receiver reflected from the antenna (64% of the power is delivered to the antenna). Note that a reflected power of 0 dB indicates all of the power is reflected (100%), whereas -10 dB indicates 10% of the power is reflected. If all the power is reflected, the VSWR would be infinite.

Note that VSWR is a highly non-linear function of the reflection coefficient . That is, there is very little difference in reflected power when the VSWR increases from 9 to 10; however there is an 11% change in reflected power when the VSWR changes from 1 to 2.

In general, if the VSWR is under 2 the antenna match is considered very good and little would be gained by impedance matching. As the VSWR increases, there are 2 main negatives. The first is obvious: more power is reflected from the antenna and therefore not transmitted. However, another problem arises. As VSWR increases, more power is reflected to the radio, which is transmitting. Large amounts of reflected power can damage the radio. In addition, radios have trouble transmitting the correct information bits when the antenna is poorly matched (this is numerically defined in terms of another metric, EVM - Error Vector Magnitude).

VSWR Specs for Antennas

Often in industry, antennas are screened (pass/fail criteria) based on VSWR specifications (VSWR specs). This is a method of measuring the antennas passively to determine if they are properly tuned in a quick manner. The antenna is measured with a network analyzer, and the VSWR as a function of frequency is recorded. As an example, consider this situation where the VSWR of 5 antennas are measured and plotted, along with 4 lines that represent the VSWR specs for this antenna (in blue):

The VSWR specs in Figure 2 are defined by:

(1) VSWR < 3.8 for 825MHz < f < 910 MHz

(2) VSWR > 4.0 for 1200MHz < f < 1400 MHz

(3) VSWR > 3.0 at f = 1.7 GHz, linearly reduced to VSWR > 2.0 at f = 1.8 GHz

(4) VSWR < 3.0 for 1860MHz < f < 2000 MHz

In Figure 2, the red VSWR curve antenna would fail the second spec, and the light blue VSWR curve antenna would (marginally) fail the fourth specification. This is illustrated in Figure 3:

Setting VSWR specs is a somewhat tricky job for antenna engineers. The idea is to fail the outliers (detuned antennas or antennas with broken connectors, etc). However, it is difficult in practice to determine what the acceptable variance is, particularly in high-volume industries where a tight spec could fail thousands of good antennas.

Note that VSWR is a measure of how much power is delivered to an antenna. This does not mean that the antenna radiates all the power it receives. Hence, VSWR measures the potential to radiate. A low VSWR means the antenna is well-matched, but does not necessarily mean the power delivered is also radiated. An anechoic chamber or other radiated antenna test is required to determine the radiated power. VSWR alone is not sufficient to determine an antenna is functioning properly.

VSWR can also be measured on the Smith Chart. VSWR is only a scalar measurement, whereas the Smith Chart shows both magnitude and phase. See the Smith Chart page for more details.

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