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08/10/2003: "AM Broadcast overload?"

One possible cause of the diurnal phase shift is overload from nearby AM broadcast stations. The new antenna results in some strong signals coming down the line -- about -9dBm for the strongest station. All the local stations either change power or antenna patterns (or both) at sunset, so it's possible that a change in the broadband power level seen by the Austron LORAN receiver could cause a phase shift. It's also possible that the directional antenna arrays could be a source of re-radiation, although the nearest station is about 2.5 miles away.

As a quick experiment, I inserted 30dB of attenuation in the antenna line. The Austron still locks up nicely on LORSTA Dana, though the gain figure (Status 3) is showing about 30dB higher. The noise value (Status 2) is about the same as without the attenuator -- in the 2 to 5 range. I'll log data for a day or so to see if this attenuation changes (or eliminates) the phase shift. If so, the longer term solution is to build a low pass filter that will knock down the AM stations while keeping the LORAN signals at nearly the same level.