Strong Signals at N8UR

AM radio reception isn't a problem at my house... we have several local stations, and though none of them run more than 10kw, they put a mighty powerful signal into my antennas. This is a 24 hour plot of the power delivered by my AMRAD active LF antenna to any unfortunate receiver it's connected to:

The sudden increase in power occurs when the stations shift from daytime to nighttime operation. Two of the three loudest ones shift to directional antenna patterns that double the total energy that I see. Actually, the situation is even worse than this, because this plot (and the spec analyzer images below) were taken after a power splitter -- the single-port power is 3dB greater than this.

By the way, this image was taken using an HP 3586C selective voltmeter which can measure wideband power between 20Hz and 32MHz with a resolution of 0.01dB.

This strong (and changing) power at the antenna terminals has a real-world impact. Here's is a phase plot of a LORAN frequency measurement receiver showing more than 100 nanoseconds of phase shift when the power level at its antenna changes:

Apparently, the change in power (more than 1 MHz away from the tuned frequency) changes the operating conditions of the receiver's front end, and causes this phase sihft. Adding a low pass filter to reduce the AM broadcast band signal level by more than 50dB solved the problem.

Here is a spectrum analyzer shot of the signals that create virtually all of this power, in daytime operation:

And here is the nighttime view. Note that some stations go away (they're daytime-only) but two of the three major spikes get bigger:

These signals are strong enough to cause some real havoc. I am using a AMRAD low pass filter to protect my low frequency receivers, and need to build a similar high pass filter for my HF gear.