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ANT-2.4-LCW-ccc Data Sheet

About Gain Plots

The true measure of the effectiveness of an antenna in any given application is determined by the gain

and radiation pattern measurement. For antennas gain is typically measured relative to a perfect (isotropic)

radiator having the same source power as the antenna under test, the units of gain in this case will be

decibels isotropic (dBi). The radiation pattern is a graphical representation of signal strength measured at

fixed distance from the antenna.

Gain when applied to antennas is a measure of how the antenna radiates and focuses energy into free

space. Much like a flashlight focuses light from a bulb in a specific direction, antennas focus RF energy into

specific directions. Gain in this sense refers to an increase in energy in one direction over others.

It should also be understood that gain is not “free”, gain above 0dBi in one

direction means that there must be less gain in another direction. Pictorially

this can be pictured as shown in the figures to the right. The orange pattern

represents the radiation pattern for a perfect dipole antenna, which is shaped

like a donut. The pattern for an omnidirectional antenna with gain is shown in

blue. The gain antenna is able to work with a device located further from the

center along the axis of the pattern, but not with devices closer to the

center when they are off the axis – the donut has been squished.

Gain is also related to the overall physical size of the antenna, as well as

surrounding materials. As the geometry of the antenna is reduced below the effective wavelength (considered

an electrically small antenna) the gain decreases. Also, the relative distance between an electrically small

antenna and its associated ground impacts antenna gain.

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.