629601 Part 18.305, Distance Factor

Publication Number: 629601	Rule Parts: 18	Publication Date: 04/05/2007
Keyword: Part 18.305, Distance Factor

First Category:	Radio Service Rules
Second Category:	Part 18 Consumer ISM

Question: What is the correct distance attenuation factor which should be applied to a low frequency Part 18 measurement?

Answer: Section 18.305(note 2) states that if a sufficient number of measurements are made at varying distances from the Equipment Under Test (EUT), so that the actual distance attenuation may be calculated, then that calculated factor may be applied to the level of the emission measured at a distance closer, than the specified for the limit.

Multiple measurements have the advantage of using a factor which could be 50 and 60 dB per decade (at 23 kHz) rather then 20 db per decade for a single measurement. Obviously, if the emission is compliant, assuming 20 dB per decade, then only the single 10 m measurement needs to be made.

Note: Measurements at these low frequencies do not require an approved site to be used. You may make measurements at distances greater than 10 m even if: (1) there is no ground screen, (2) site attenuation is not measured at these greater distances and (3) the area is clear of obstructions.

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