§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 | 25 | 300 |
| Any non-ISM frequency | Below 500 | 15 | 300 | |
| Industrial heaters and RF stabilized arc welders | On or below 5,725 MHz | Any | 10 | 1,600 |
| Medical diathermy | Any ISM frequency | Any | 25 | 300 |
| Ultrasonic | Below 490 kHz | Below 500 | 2,400/F(kHz) | 300 |
| 490 to 1,600 kHz | Any | 24,000/F(kHz) | 30 | |
| Induction cooking ranges | Below 90 kHz | Any | 1,500 | 430 |
计算FCC part 18限值：
以Any type unless otherwise specified (miscellaneous) Below 500W为例子
Any ISM frequency ISM频段限值
k=25uv/m @300m distance，FCC官方标准给出的开阔场300m距离的辐射场强限值，当然如果现在已经有了300米的open site就不用限值转了，否则我们还是要转换为我们常用的3m、10m开阔场模拟场地限值。另外，我们测试数据呈现方式以对数显示，所以还需要把25uV/m还要转换为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)
任意距离限值=已知暗室测试距离限值+K（已知距离限值/任意距离限值）
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频段限值
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
其它以此类推……