例如，按EN55022中测试的辐射限制规定，标准中给出的测量距离是10m，相应限值也是10米限值如下： 测量距离10m 频率：30MHz~230MHz 准峰值：30dB（uV/m） 频率：230MHz~1000MHz 准峰值：37dB（uV/m） 但是，10米暗室造价贵，测试价格相应也比3米法暗室贵多了。当然，平时的一些辐射测试大多都是在3米法电波暗室中进行的，那么我们就应该将10米暗室限值转换为3米限值。 测量距离3m 频率：30MHz~230MHz 准峰值：40dB（uV/m） 频率：230MHz~1000MHz 准峰值：47dB（uV/m） 标准中EN55022:2010是这样描述测试距离的转换： 10.3.1 Antenna-to-EUT distance Measurements of the radiated field shall be made with the antenna located at the horizontal distance from the boundary of the EUT as specified in Clause 6. The boundary of the EUT is defined by an imaginary straight-line periphery describing a simple geometric configuration encompassing the EUT. All ITE intersystem cables and connecting ITE shall be included within this boundary (see also Figure 2). NOTE If the field-strength measurement at 10 m cannot be made because of high ambient noise levels, or for other reasons, measurement of class B EUTs may be made at a closer distance, for example 3 m. An inverse proportionality factor of 20 dB per decade should be used to normalize the measured data to the specified distance for determining compliance. Care should be taken in the measurement of large EUTs at 3 m at frequencies near 30 MHz, due to near field effects. 因场强与距离成反比，即10M实验室的20dB/10倍距离的反比因子将测试数据归一化到规定的测量距离上。即： 20log（10/3）+30/37(10M Limit)= 3M实验室的测试限值：10.45+10M Limit = 40.45dBuv/m~47.45dBuv/m=3m Limit 如果已知A距离暗室限值X，要转换为B距离暗室限值Y时，即可用20log（A/B）+X求出其它任何距离限值。如果，用K值代表20log（A/B）的话。那么就有： 3m limmit=10m limit+10 k=20log(d1/d2)=20log(10/3)=10 3m limit=10m limit +k Limit 2 = Limit 1 + 20log(d1/d2) 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） 又如加拿大ICES-005 Issue 2 February 2007 Radio Frequency Lighting Devices (RFLDs)标准中规定辐射为30m辐射场限值，那么依据上述公式就可以计算出下表Class B RFLD在3m场的限值分别对应应该是：40dBuV/m、43.5dBuV/m、46dBuV/m。 Radiated Emissions Limits
| ||||||||||||||||||||||||||||
Helen.ma 发表于 2014-11-15 17:30
30MHz以下的3m与10m场强的换算，FCC part15用40dB做为换算系数，Part18用20dB
FCC限值
深圳陈超 发表于 2015-8-25 15:12
3米法体积太大.空间容不下.有沒更小体积的.作近似值参考;
Conductionlimits
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 |
