# ARRL

## kj4saf

 Joined: Thu, Oct 1st 2015, 01:47 Roles: N/A Moderates: N/A

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Electric Field Strength Limit Calculation Jul 30th 2017, 14:24 5 6,751 on 31/7/17

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Electric Field Strength Limit Calculation kj4saf on 31/7/17
Zack, W1VT, and Martin, AA6E. Thank you both for your replies.

The FCC rule that I am working out is:
47 CFR 15.249 - (This is not the Amateur service, I'm doing the calculation for the ISM 2.4 GHz band.)

Zack, you and I got the same result from the calculation, that's the value that seems can't be right.

I guess my error is not the calculation, but failing to understand the apparently conflicting information of paragraphs ~15.247 b) 3), and the table in section ~15.249. The former gives a 1 watt limit (for digital modulation summed across all antennas and elements, with other caveats), which we "know" to be the case from work in WIFI, and the later gives 50mV/meter at 3 meters, which, calculates to 750 uW TX power (at 0 dB antenna gain).

It's clear that ~15.247 is the operational regulation being followed; I don't understand what ~15.249 a) is conveying.

Thank you again for your responses.
73
ScottD
KJ4SAF

Electric Field Strength Limit Calculation kj4saf on 30/7/17
FCC sets transmit power limits by electric field strength. Field strength is related to TX power, of course, but how are conversion calculations done?

This site: https://www.craf.eu/useful-equations/conversion-formulae/

provides and explains this equation:

(PG)/(4 π d^2) = (E^2)/(120 π),

which can be simplified under certain assumptions (such as antenna gain = 1 and cable loss = 0, eg) to

P = 0.3 E^2.

( P is TX power in watts, and E is electric field strength in volts/meter at 3 meters)

It seems simple enough, but using it yields incorrect results. The error is more likely mine than the site.

Does anyone have any insights into calculating TX power limits based on mandated field strength limits?