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3:1
2:1
1:1
318MHz 418MHz 518MHz
VSWR
25%
11%
0%
Reflected Power1.386
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 has 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 is 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.
by
ANT-418-CW-HWR-ccc Data Sheet
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.
VSWR Graph
