Use of band pass filters in amateur radio

Now that many of us use commercial radios, band pass filters would be there or you expect them to be there within them. In software defined radios, there are several  band pass filters built-in. While homebrewing radios, band pass filters help us to prevent radio frequency interference which might occur due to harmonic radiation. You may be aware that a 7 MHz tuned circuit will have harmonics on 14 MHz, 21 MHz, 28 MHz and so on, though the amplitude of the signals will be lower as you go higher up in the harmonics. I still remember that I could hear the second harmonic of my local medium wave radio station as I was staying only a few kilometers from the radio station, a few decades back. That used to occur even with professional broadcasting systems in those days.

If you connect a low-pass filter which allows only signals below a certain frequency to pass through, harmonic radiation can be minimized. The low-pass filter can be connected between the transmitter and the radio and will help us to prevent radio frequency interference on other bands. Suppression of harmonic radiation is a regulatory requirement as well. In contrast to low-pass filter, high-pass filter allows only signals above a certain frequency to pass through. This can prevent radiation of inadvertent signals below the operating frequency. High-pass filters in receivers prevent interference from low frequency sources like power line noise. Front end high-pass filters can improve the performance of receivers by cutting off signals below the operating frequency which might be strong enough to overload the front end of the receiver.

One classical example is suppression of VHF signals while receiving UHF signals from Amateur Radio on International Space Station. When the receiving radio is on during your transmission on VHF uplink, as you are supposed to listen the downlink during satellite operations, the front end of the radio will be overloaded by the strong transmitted VHF output unless you have a good front end filter installed. All full duplex radios have good front end filters to enable listening on downlink while you are transmitting on uplink.