High-pass T filters:
Three-element filters can comprise a 'T' or 'π' topology and in either geometries, a low-pass, band-pass, high-pass, or band-stop characteristic is feasible. The components can be selected symmetric or not, depending upon the needed frequency characteristics. The high-pass T filter in the example has extremely low impedance at high frequencies, and extremely high impedance at low frequencies. Here it means that it can be inserted in a transmission line, resultant in the high frequencies being passed and low frequencies being reflected. Similarly, for the demonstrated low-pass π filter, the circuit can be connected to a transmission line, transmitting low frequencies and that reflecting high frequencies. By using m-derived filter sections along with correct termination impedances, the input impedance can be constant reasonably in the pass band.