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FM Interference
by Mark Persons 
Minnesota Broadcaster Newsletter

August 1999

FM Ducting Interference

Summer weather conditions bring about quite a bit of VHF ducting.  Many FM broadcasters call me to complain about some of the interference.  Ducting is caused by heavy layers of moisture in the atmosphere.  This prevents FM signals from leaving the earth in a straight line.  Instead, the signals are curved back to the earth hundreds and sometimes thousands of miles away.  

For broadcasters, this means interference from other broadcasters on their same frequency and from adjacent channels.  Here are some recent examples with the call letters left off:

1.  100 KW station with 800 feet of height could not be heard in an important community 35 miles from the transmitter site for half a day.

2.  100 KW Station at 1000 feet had very noticeable interference in their city of license 30 miles away for several days.

3.   A non-commercial translator, 80 miles from the main station, rebroadcast a station from 250 miles away for most of four days

Fortunately, this phenomenon lasts for just a few hours or a few days in the summer.  Then conditions return to normal until another massive weather system moves in.  You will only rarely hear of this in the wintertime. 


See you further down the road.  I'll leave the soldering iron on for you. 

Editor's note:  Mark Persons, ham W0MH, is Certified by the Society of Broadcast Engineers as a Professional Broadcast Engineer with over 30 years experience.   He has written numerous articles for this publication over the years.  His website is:

     Questions?  Email Mark Persons:       

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