A radio filter is a configuration of electrical components designed to work together to create a circuit that can either reject or allow certain frequencies. There are three different types of filters including the high pass, low pass and band pass.
RF filters are used in both transmitter and receivers. In transmitters they work to only transmit the desired frequency and filter out unwanted noise. In receivers they work similarly by only receiving the desired frequency and filtering out any other transmitted frequency.
Have you ever wondered how you can set your receiver to a certain frequency and only hear that frequency? That is the power of radio frequency filters.
A tuned circuit is one example of a radio filter. A tuned circuit is a capacitor and an inductor wired in parallel. Either the capacitor or the inductor must be variable. Varying the inductance or capacitance is what is tuning the circuit to a desired frequency of tuning a radio to a desired frequency.
A tuned circuit works when the capacitor is charged, the inductor pulls the current around the loop and then the other side of the capacitor gets charged. The inductor then pulls the current back around the other way.
The rate at which the current goes back and forth around the circuit is the resonant frequency. If a frequency picked up by an antenna matches the resonant frequency, then it will pass and be heard by whomever is listening.
Interference and Filtering
Filtering is also used to filter out unwanted noise or other unwanted frequencies when transmitting or receiving. A big use for filtering is when you are transmitting and your neighbor calls and says he can hear your radio calls through his tv! How does that happen?
Extra noise and hearing unwanted frequencies can be super frustrating when trying to tune your system. Figuring out what it takes to fix the issue can be a very time consuming trial and error effort. If your neighbor is complaining of hearing you through their tv, be sure to be patient, polite and respectful and do everything you can to alleviate the situation.
If a HAM is operating within the boundaries of the rules set in place by the FCC, then the problem might be that the neighbor has purchased a cheaper tv where the manufacturer didn’t put in the extra money for good shielded cable. The answer to your problems might be suggesting a low pass or high pass filter for your neighbor.
The first step is to find a filter that will block the frequencies that you as the HAM operator are likely transmitting on. If you put this inline with your neighbors tv, it could very well clear up the issue. If not, there are other non filter related things to try but this should maybe be the first effort.
Filters are one of the most important components of radio. Without them you would hear anything and everything. It would be nearly impossible to transmit or receive signals on specific frequencies.
There are three types of filters for radio: low pass, high pass and band pass. Low pass allows lower signals to pass through the circuit and while blocking higher frequencies. High pass works the opposite way where higher frequencies are allowed to go through but lower frequencies are blocked.
Band pass allows you to select a specific band anywhere on the frequency spectrum. This is how a radio tuner works. The selected frequency is called the resonant frequency and as you adjust the know on your radio, you are actually changing the circuitry to accept the desired frequency.
Radio filters are also important for filtering unwanted noise or signal from other electronic devices. It might be a good idea to have a few on hand so when you do have problems with interference, you will be equipped and ready to handle such problems.