An expert explains how to choose radio frequency filters

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radio frequency filters

Military expert Serhiy Flesh (Beskrestnov) tells how to use radio frequency filters and what kind of RF filters are needed.

There are several types of them:

  • A filter that passes only the desired frequency range and attenuates everything else. Such a filter is called a bandpass filter.
  • On the contrary, it suppresses a certain area and lets everything else through. It is called a notch filter or NOTCH filter.
  • Filters that don’t pass all frequencies below a certain level, but pass everything above it. These are called High-pass Filters (HPF).
  • These filters do not pass all frequencies above a certain frequency, but pass everything below it. These are called Low-pass Filters (LPFs).

Where to use

“Always near cities, the FM 67-108 MHz band is a powerful source of interference. There are a lot of radio stations in this frequency band and they are all powerful. Such a gift interferes terribly with any radio receiver (especially those with poor characteristics). Therefore, we can put a small filter between the antenna and the receiver, which will “remove” the interference from FM,” writes Flash.

He gives another example: “All FPV pilots know that the signals from a 900 MHz control signal transmitter interfere with the video receiver in the 1.1-1.2 GHz band. This happens because the input circuits of video receivers do not have good quality characteristics, plus the two antennas are usually located side by side. That’s why a filter is placed between the video antenna and the receiver, which passes all frequencies, including video, and cuts off the control frequencies at 900.”

In case you need to receive 118-136 MHz airplane frequencies, and you are not interested in anything else, you need a bandpass filter for the airplane section, and you need to put it between the antenna and the receiver. This will be a 100% guarantee that you will receive aviation well, and no interference will interfere with you.

“You only need to receive signals from Orlan in the 902-928 MHz band. The entire 900 MHz band is clogged with interference from car alarms and mobile communications. In such a situation, you can put a filter at 902-928 MHz and forget about the problems,” the expert notes.

The receiving frequency of the repeater is 136.200 MHz, and there is already a powerful repeater nearby, and not just one. And they all interfere with reception. For this, there is a special filter that can pass the desired frequency and cut out the rest.

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Reduced signal strength

There are a lot of ready-made filters available for a wide variety of applications. But there is a nuance. Any filters attenuate not only interference signals, but also the ones you need.

“Some are quite a bit, and some are up to 3 dB. But usually, the issue of interference is so urgent that we make even such sacrifices,” says the specialist.

And of course, it’s important to know how much dB the unwanted frequencies are attenuated. Sometimes the attenuation is not enough and you have to put two filters in series. Usually, a simple filter gives about 50 dB of suppression.

“Attention! Remember that if we put a filter on reception, we do not care about its power. But if the filter is placed on transmission, then “its power” should be no less than the power of the transmitter,” adds Sergiy Flesh.

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