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Concerning the SID monitor calibration, is there any reason to worry that our daytime data are near the -3.5 range and can get as low as -4.5 at the sunrise/sunset dips? Do the monitors have a linear response throughout the range?

The monitors do not have a linear response to the way their gain is calibrated. The SID graph in the daytime is like a hump due to the non-linear effect of the Sun's angle and the layers of the ionosphere reflecting the RF signal to your antenna. The time of day that you adjust the monitor has a lot to do with it. For example: 1 volt changed in the morning will not be 1 volt by the noontime, it might be more (everyone's monitor will be different).

Assuming that your antenna is oriented correctly and no other RF sources of inference are to blame, then you should increase the gain until you are in the -1.25 to -1.5 volt range around noontime. If you can't make the adjustment at noon, then look at the previous day's graph and try to estimate how many volts you need to go up. Expect that you'll have to do this a couple of times as the effect is not linear and overcompensation might occur. (Avoid trying to calibrate your monitor at night -- the variability makes it too difficult!)

Proper gain calibration is critical and the good news is that once you get it right then there is nothing else to fuss with!

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