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Practical Wifi Antenna Measurement

Practical Wifi Antenna Measurement

Describes a simple way to determine the main RF characteristics of a Wifi (IEEE802.11b/g wireless LAN) antenna. Although this procedure is written specifically with Wifi antennas in mind, it can also be used on other antennas.
You can measure the following…

Voltage Standing Wave Ratio – VSWR
Maximum Gain
 -3dB (Half power) Beam width
 Front to Back ratio

…with relatively simple homebrew equipment like:

A Wifi SWR/Field strength Meter
An RF Generator
A High Gain transmit antenna
A Reference antenna of known gain
An RF Power meter

With this procedure some influences are not taken into account, leading to a reduced measurement accuracy. The results however are accurate enough to compare homebrew antennas and determine if the antenna performs close to the theoretical characteristics or not. When Wifi equipment is used as an RF signal source, the SWR and Field Strength (power) meter must be able to accommodate the pulsed RF signal. My own SWR meter has a Peak-hold circuit for this purpose.

The  VSWR of the antenna is determined as follows:

First, connect an RF source to the RF Input of the bridge.
Switch ON the VSWR meter.
Connect a known unmatched load to the Zx port on the bridge like the supplied short circuit connector.
Set the Sensitivity control so that the meter indicates the correct VSWR (S for the short circuit)
Remove the load from the Zx port on the bridge.
Connect the Antenna Under Test (AUT) to the Zx port. Use a single, good quality coupling (Male-N / Male-N or other) as the use of coax cables or inferior RF connectors/ adapters may yield inaccurate measurement results.

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