EMC Testing Services

Site attenuation

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There are two ways to describe site attenuation:

The validation procedure used to qualify a test site or
A measure of the transmission characteristics between the point where the equipment under test is located and the receiving antenna.

Site attenuation is considered acceptable when measured values are within +/- 4 dB of the theoretical ideal site.

Ideal site

A perfectly flat, infinitely large, continuous (gap free) ground plane located in free space and having infinite conductivity.
Provides uniform performance over the entire test volume that the equipment being tested will occupy.

Free space

An open area, free of objects which can reflect electromagnetic waves such as buildings, electric lines, fences, trees, hills etc.

Possible causes for exceeding site attenuation limits

Inadequate ground plane construction, size and properties.
Reflecting objects to close to the test site.
Degraded performance of all-weather sites due to inadequate construction and maintenance, and long term effects such as penetration of residue from airborne contaminants.

If weather protection is used, it should be constructed of moisture resistant, RF transparent materials and should be shaped to allow easy removal of snow, ice and water.

UL Sites

UL conducts detailed measurements of site attenuation. Below is a typical example of the site attenuation data for a UL 10-meter chamber.