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	FM Exciter RF Filter
	Built by M. W. Persons and Associates, Inc. Made in Minnesota

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Cable

This an an RF band-pass filter that is a DC short.  The photo shows one near a 12-inch ruler for size perspective.  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 hundreds or thousands of volts of DC 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 to length 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.  

Filters for 88 to 108 MHz using high quality Belden 8259/RG-58A/U cable, with an Amphenol BNC connector and Amphenol BNC T adapter, are $65.00.  They are rated at 250 watts.  

Cables can be made with Belden 9251/RG-8A/U non-contaminating high quality 50 ohm cable for $95.00.  These come with a type N Tee adapter and are rated at 1000 watts.  

Cables for other frequencies are available.  Pricing upon request.  

page last edited 06/01/2010