[RFI] AM Radio Interference On 75 meters

[Jim Brown]

On 9/18/2012 9:54 AM, David Robbins wrote:
> 3960khz is not a harmonic of 1530khz so something else is also happening here.  listen carefully to the kfbk signal and see if there is some other signal mixing with it.  check for other harmonics and other signals that it might be mixing with to end up on 3960...

Yes.   I did some poking on the FCC website.  KFBK is 50kW DA2 -- that 
is, directional both day and night, different patterns day and night. 
The nighttime pattern has deep nulls east and west, putting all of 
California in their main lobes. WA6RKN is in Reno, which is 80-100 miles 
almost due east of the KFBK transmitter site (which is 32 miles N of 
Sacramento).  If their antenna system is functioning properly, there 
should not be much fundamental in the direction of Reno at night.  
Obviously that directivity exists only on 1530, not on the frequencies 
of harmonics or spurs.

That suggests a spur in their transmitter or a strong mix local to them. 
Or perhaps a local translator near Reno that's putting a harmonic around 
3960?  Something else?

When I first moved to the Santa Cruz mountains, I was hearing somewhat 
distorted Hawaiian music on 1860 kHz at night. I found the fundamental 
on 930 kHz, and eventually traced the source to a neighbor who was a 
broadcast engineer. He was pumping music around his house using an old 
AM exciter, and didn't believe I could be hearing the second harmonic.  
He showed up with his field strength meter and was quickly convinced.

I grew up in Huntington, WV, about a mile from  5kW on 930 kHz and 5kW 
on 1470 kHz. The resulting mix on 3870, as I recall, was loud, with 
clearly defined audio from both stations.  Thankfully the 1470 station 
was a daytimer.

73, Jim K9YC