[BC] 9th FM harmonic causing interference

[Chuck Lakaytis]

This is a problem that crops up with these harmonics.  Bandpass filters, 
second harmonic filters, etc do no protect against reentrant harmonics 
at those high frequencies.  That is why I have specified a low pass 
filter on my latest install that is near a cell site.
> Yup, several years ago. 9th landed right on one of their channels,  
> causing them to shut it down. I wasn't inclined to believe that 9x94  
> would find it's way through PA, 2nd harmonic filter and antenna with  
> any substantial energy, even if it were being generated. But it was  
> and it did.
>
> Cellphone tech and I visited transmitter one evening with his $30K SA,  
> and sure enough, there it was. A *little* tweaking of grid tuning,  
> neutralization broucht the level down by about 25db, if I'm  
> remembering the facts correctly.
>
> Had this transmitter been out of town, it would have never been  
> noticed, but it was about a mile from cell site. Anyway, they got  
> their channel back and I got a bit more educated.
>
> Dave Dunsmoor
>
>
>   

--
Chuck Lakaytis
Director of Engineering, Alaska Public Broadcasting
135 Cordova Street, Anchorage, AK 99501
office   907-277-6300