Selective and Wideband Measuring Devices

Do you get the same results with a selective measuring device and a wideband measuring device?

The answer to this question is complicated as it depends on when and how the device is used. The scenarios below illustrate why results may differ depending on whether you use a selective or wideband measuring device:

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 do something stupid like acting as a human shield and standing in front of one of the devices, or standing behind it like a reflector, it is most likely that both devices will indicate the same result. So far so good.

Scenario2: 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 immissions 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.

For more information about the selective and wideband measuring devices, please contact your local Air-Met Scientific office.