§18.305   Field strength limits.

(a) ISM equipment operating on a frequency specified in §18.301 is permitted unlimited radiated energy in the band specified for that frequency.

(b) The field strength levels of emissions which lie outside the bands specified in §18.301, unless otherwise indicated, shall not exceed the following:

Equipment	Operating frequency	RF Power generated by equipment (watts)	Field strength limit (uV/m)	Distance (meters)
Any type unless otherwise specified (miscellaneous)	Any ISM frequency	Below 500 500 or more	25 25 × SQRT(power/500)	300 1300
	Any non-ISM frequency	Below 500 500 or more	15 15 × SQRT(power/500)	300 1300
Industrial heaters and RF stabilized arc welders	On or below 5,725 MHz Above 5,725 MHz	Any Any	10 (2)	1,600 (2)
Medical diathermy	Any ISM frequency Any non-ISM frequency	Any Any	25 15	300 300
Ultrasonic	Below 490 kHz	Below 500 500 or more	2,400/F(kHz) 2,400/F(kHz) × SQRT(power/500)	300 3300
	490 to 1,600 kHz Above 1,600 kHz	Any Any	24,000/F(kHz) 15	30 30
Induction cooking ranges	Below 90 kHz On or above 90 kHz	Any Any	1,500 300	430 430

計算FCC part 18限值： 
以Any type unless otherwise specified (miscellaneous)  Below 500W為例子

Any ISM frequency ISM頻段限值
k=25uv/m @300m distance，FCC官方標準給出的開闊場300m距離的輻射場強限值，當然如果現在已經有了300米的open site就不用限值轉了，否則我們還是要轉換為我們常用的3m、10m開闊場模擬場地限值。另外，我們測試數據呈現方式以對數顯示，所以還需要把25uV/m還要轉換為dBuV/m，既25uV/m=20log25（dBuV/m）≈27.96dBuV/m

又

Limit 2 = Limit 1 + 20log(d1/d2)= 27.96dBuV/m+20log(300m/10m or 3m)
任意距離限值=已知暗室測試距離限值+K（已知距離限值/任意距離限值）
Any ISM frequency 限值 10m限值=27.96dBuV/m+20log(300m/10m)≈57.5dBuV/m
Any ISM frequency 限值3m限值=27.96dBuV/m+20log(300m/3m)≈67.96dBuV/m

Any non-ISM frequency 非ISM頻段限值
k=15uV/m=20log15（dBuV/m）≈23.52dBuV/m
Any non-ISM frequency 限值 10m限值=23.52dBuV/m+20log(300m/10m)≈53.06dBuV/m
Any non-ISM frequency 限值3m限值=23.52dBuV/m+20log(300m/3m)≈63.52dBuV/m

其它以此類推……