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Technical Tips from Mark W. Persons
AM RF Harmonic Filter/Trap

Sometimes AM radio stations radiate RF harmonics (2, 3, 4 and more multiples of the station's operating frequency) that do not meet FCC specifications.  This problem is usually caught when the annual occupied bandwidth and RF harmonic measurements are taken.  RF harmonics are required to be 73 dB or more below a 1000 watt AM carrier and 80 dB or more below a 5 KW carrier. 

This simple series-resonant filter/trap is tuned to 2680 KHz, which is twice the 1340 KHz operating frequency of the station.  The photo shows a filter/trap on a bench being readied for service.  Note the wire that connects between the right end of the coil to about 1/3rd of the way in.  That wire shorts the end of the coil to get just the right inductance to resonate the circuit at 2680 KHz in this case. 

Here is a view of the filter/trap being tested using a Delta OIB-3 RF Operating Impedance Bridge and a Delta RG-4 Receiver-Generator on a service bench.  When installed at the client station, the circuit was connected at a 50-ohm point near the main/auxiliary transmitter switch.

Actual component values are determined by the station's operating frequency, t harmonic frequency that needs to be attenuated and power level.

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   ham W0MH      

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