VSWR

Reflected Power

3:1

2:1

1:1

25%

11%

0%

2450MHz

2550MHz

2350MHz

1.543

by

Data Sheet ANT-2.4-WRT-MON-xxx

VSWR Graph

What is VSWR?

The Voltage Standing Wave Ratio (VSWR) is a measurement of how well

an antenna is matched to a source impedance, typically 50-ohms. It is

calculated by measuring the voltage wave that is headed toward the load

versus the voltage wave that is reflected back from the load. A perfect

match will have a VSWR of 1:1. The higher the first number, the worse the

match, and the more inefficient the system. Since a perfect match cannot

ever be obtained, some benchmark for performance needs to be set. In

the case of antenna VSWR, this is usually 2:1. At this point, 88.9% of the

energy sent to the antenna by the transmitter is radiated into free space

and 11.1% is either reflected back into the source or lost as heat on

the structure of the antenna. In the other direction, 88.9% of the energy

recovered by the antenna is transferred into the receiver. As a side note,

since the “:1” is always implied, many data sheets will remove it and just

display the first number.

How to Read a VSWR Graph

VSWR is usually displayed graphically versus frequency. The lowest point

on the graph is the antenna’s operational center frequency. In most cases,

this will be different than the designed center frequency due to fabrication

tolerances. The VSWR at that point denotes how close to 50-ohms the

antenna gets. Linx specifies the recommended bandwidth as the range

where the typical antenna VSWR is less than 2:1.

Counterpoise

Quarter-wave or monopole antennas require an associated ground plane

counterpoise for proper operation. The size and location of the ground

plane relative to the antenna will affect the overall performance of the

antenna in the final design. When used in conjunction with a ground

plane smaller than that used to tune the antenna, the center frequency

typically will shift higher in frequency and the bandwidth will decrease.

The proximity of other circuit elements and packaging near the antenna

will also affect the final performance. For further discussion and guidance

on the importance of the ground plane counterpoise, please refer to Linx

Application Note AN-00501: Understanding Antenna Specifications and

Operation.