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Radio Broadcast Technical Consulting and Sales
 10032 Island Drive, Brainerd, MN  56401

 

Technical Tips from Mark W. Persons

AM RF Harmonic Filter/Trap


Sometimes I run into an AM station where the radiated RF harmonics 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.  This photo shows the filter/trap on the 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. 


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 the service bench.  When installed at the client station, the circuit was connected at a 50-ohm point near the main/auxiliary transmitter switch.

We build and tune these in the shop here for stations that do not have the test equipment to get the tuning right. 

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

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page last edited 01/31/2010