For years a 5 kw 1260 khz AM station, approx 1/4 mile distance has invaded 1 channel of our church audio system. The leaky input is from a choir mic suspended from a 45 foot high truss. I always attributed the problem to a funky belt pack power module up on the truss. We just changed mics, exchanging the power module mic with a new phantom powered Audix omni.
The background AM radio chatter continues at about 40 dB below full signal. The mic line from the truss to a stage box at floor level is approximately 80'. The stage box feeds a mixer at the back of the sanctuary through an 80', 9 shielded pair snake. All connections are xlr.
I tested a Sescom inline rf filter at stage box input and mixer input to no avail.
Previously I had gone through typical line and input swapping to no avail. I can feed a mic rom the stage box to the mixer with no problem. I can place a preamp at the stage box and no problem from the mic on truss. The right side mic on the truss (shorter line) has never had a problem. Only when the length of the snake is added to the length of the left side downline from the truss does the problem occur.
Could this be because the combined line lengths are approaching the 1/4 wave length of 1260 kHz and becoming a good antenna?
Any suggestions?
Thank you.
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