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EMF: Selective Band VS Wideband Measuring

 

 EMF safety: What are the advantages of Selective Measuring Devices compared with Wideband Measuring Devices? 

Wideband measuring devices like the Narda Broadband Field Meter (NBM-550) gives blanket coverage of large frequency ranges for one to check the electromagnetic fields exposure levels. They are generally very easy to use for one to check the overall exposure measurement against the  applicable permitted exposure limit values value which is important in ensuring safety to people and the environment.

On the contrary, Selective measuring devices such as the Selective Radiation Meter (SRM-3006) is capable of breaking down the fields into their individual frequency components, allowing further detailed analysis of various signal sources This is especially important when a site is exposed to multiple emission sources such as broadcasting, cellular radio or within industrial plant.  How do one know which source is contributing to the field exposure level? Having this analysis is important not only to the relevant authorities and safety representatives, but also to the service providers so as to determine the appropriate emission source output power to be reduced to adhere within approved limit values.

Do You Know?

In actual fact, the SRM can perform both wideband and selective measurements. However, it can also analyze which emission source is ultimately responsible for the limit being exceeded. Selective measurement can replace wideband measurement, but not the other way around! And, because it is seldom possible to predict whether a critical situation could occur at the measurement location, the SRM should always be available. You can reduce time and costs with the NBM, but should the need arise for you to perform electromagnetic field spectrum analysis, you should always take the SRM along with you.

Do you get the same results with a Selective Measuring device and a Wideband Measuring Device?

Yes and no! Why not just plain yes? If both devices are calibrated and in good condition, why should there be any differences?

Scenario 1: You are making measurements close to an FM transmitter, which is the dominant field source. Both the selective Narda SRM as well as the wideband Narda NBM are equipped with antennas that capture the FM transmitter. As long as you don’t stand in between the emission path acting as a human shield, or standing behind it like a reflector, it is most likely that both devices will indicate the same result. So far so good.

Scenario 2: You are standing on a rooftop terrace. There are several mobile communications antennas on the next-door rooftop, and you want to check that their emissions are within the prescribed limits. Off in the distance you can also see the silhouette of the local FM transmitter tower. If the SRM is fitted, say with a 420 MHz to 6 GHz antenna, it will not detect the FM transmitter in the 100 MHz band, because this is outside its reception bandwidth. In contrast, if the NBM is equipped with a 100 kHz to 6 GHz probe, it will definitely measure the FM transmitter as well. More likely than not, the SRM and NBM will then display different results. Both results are correct! But they are measuring different field sources according to their frequency ranges. If you get different results like this, take a look around you. The answer is often easy to see. Of course, there are antennas available for the SRM that will capture the FM radio as well, if that’s what you want to measure.

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