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

 

Engineer's Log of Mark W. Persons
 Filter required at WJON Radio
While trying to do the FCC annual required NRSC occupied bandwidth and RF harmonic measurements on WJON Radio in St. Cloud, MN, a problem was discovered.  WJON is seen on 1240 KHz in the center of this spectrum analyzer display.  A new station in town, KYES, 50 KW at 1180 KHz is clearly seen three divisions to the left of WJON.  That signal gets into the WJON transmitter and is mixed to produce an unwanted emission on 1300 KHz, three divisions to the right of center.  As you can see, it is 5 dB above the red line "mask" allowed by the FCC. 
November 20, 2009:  This is the signal after a new filter was installed and tuned.  The filter gave enough attenuation of 1180 KHz to result in a 15 dB drop in the unwanted mixing product.  The station is now FCC legal and passed the occupied bandwidth test nicely.  On the left side of the screen is KASM Radio at 1150 KHz in Albany, MN.  The mix product from them is more than 80 dB below the WJON carrier.  
Here is your intrepid engineer, Mark Persons, happy when the job turned out well.  He is standing next to the original WJON AM antenna coupling network.  Just below his elbow is a new box with the filter, which is tied to the 50 ohm side of the antenna coupling network.  On the floor are a Delta RG-4 Receiver-Generator and a Delta OIB-3 Operating Impedance Bridge used to setup the network.

Station engineer Mark Young built the new aluminum filter box with the help of a friend who does aluminum welding.    

A photo of the filter network.  Designed by the great people at Kintronic Labs, this filter is series resonant at 1180 KHz to "short" 1180 KHz to ground.  That is done with the coil and capacitor on the right side.  The variable capacitor, on the left, makes the filter parallel resonant at 1240 KHz so it will pass WJON with a minimum of problems.    

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 11/21/2009