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Technical Tips from Mark W. Persons

Unwanted Spurious Radiation on an FM Station


This is a spectrum analyzer view of an FM radio station on 94.1 MHz.  I looked at the station after being alerted by the manager that the station was being heard on no less than seven spots on the radio dial.  Sure enough, there was unwanted radiation on 93.5 MHz, 93.7 MHz, 93.9 MHz, 94.3 MHz, 94.5 MHz, and 94.7 MHz. 

The IFR A-7550 Spectrum Analyzer told the gruesome story.  Now...what to do about it!!!


The first place I go looking for unwanted spurious radiation is the FM exciter.  In this case, it was a 16-year-old Harris MX-15.  It seems just like yesterday when I stalled it new.  This photo shows the exciter's 15-watt RF Power Amplifier, Harris number 992-6204-001. 

Electrolytic capacitors C8 and C18 needed to be replaced.  The original capacitors were Sprague 30D TE1305 20 MFD/50 VDC.  The bad pair are shown to the left of the module on the service bench.  They are silver in color and I have seen this type of capacitor fail before.  The replacement capacitors are in the module and difficult to see.  They are blue in color and to the right side just above and below center on the module.  


Success!!!  The station is now heard on just one spot on the FM dial.  The station manager and I laughed about whether this was the right thing to do or not because they need all the listeners they can get.    

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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