These are RF band-pass filters/shorted quarter wave shorted stub traps. They are a DC short that will pass an FM radio frequency with little or no attenuation. //

Above is the test setup required to calibrate the trap to the correct frequency. A return loss bridge is used in conjunction with an RF spectrum analyzer and 50-ohm dummy load. The analyzer display is a VSWR picture of the performance of the trap at the frequency of interest.

These filters are normally used at the output of an FM (88 - 108 MHz) solid state broadcast exciter as a voltage spike protector. It is, in essence, a shorted quarter wave stub made of 50 ohm transmission line. They are needed on FM transmitters where the next stage of amplification is a tube. An arc in that tube could send a spike of hundreds or thousands of volts back to the exciter causing failure of the exciter's output stage.

Since the filter is DC shorted at the end, it presents a direct short to any DC voltages. It passes the FM frequency of interest because the cable is cut to an exact length. Each filter is cut and tested using a spectrum analyzer with tracking generator and return loss bridge. The return loss at the specified frequency is typically 45 dB, which a VSWR of 1.01:1.

Sometimes they are used at the output of a low power FM station or an FM translator to give lightning protection to the final amplifier. They will also help reject second harmonic radiation as well as offer some attenuation to carriers that are not on the frequency they are cut for. //