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08/21/2003: "More on phase jump"

Following up on the earlier entries about the diurnal step change I'm seeing in my Loran-C phase plots... here are two spectrum analyzer shots showing the AM broadcast band as received by the Loran antenna.

Here is a shot taken at 2000 local time and here is one taken at 2100 local time. When these were taken (August 11/12) the phase jump occurred between these two times. There are two significant differences between the two plots: first, the two strongest signals increased in strength at sunset, with the strongest going up by about 3dB. Second, two of the local stations went away completely. So, the total power seen by the receiver doubled at night, and the absence of a few signals possibly changed the number and nature of any IMD products being generated in the receiver front end.

As noted earlier, putting a 30dB attenuator in the antenna line greatly reduced the amount of the phase shift, though the most recent phase plots still show a diurnal effect that may or may not result from this.

I suspect that the high power being presented to the Austron receiver causes it to overload, and the changes at night alter the overloading, resulting in a phase shift.

If this is the case, the right way to fix the problem is to put a low-pass filter in front of the receiver to block the AM broadcast signals while leaving the Loran signal strength alone. My next project is to build one of those.