AM Radio reception fades in in and out

You are correct in that FM is line-of-sight. This is why travelling up and down through hills results in sporadic loss of FM (and cell phone) coverage due to the shadowing by the topography. As for AM, it tends to travel long distances by "Tropospheric ducting", via the atmospheric layer for which it is named.

Going into a concrete structure means that you are going into a Faraday Cage created by the metal re-bar that strengthens the structure. If the gaps in the re-bar are smaller than the wavelength of the signal, nothing passes through. If you happen to be travelling near high voltage power lines, the electric field radiating from them may be saturating your receiver and blocking AM reception. Although the power frequency is only at 60Hz, the high amplitude of the level (ie: >300kV) can be spawning multiple harmonics (120Hz, 180Hz, etc) until one of them is within the AM band and lands on your station. Your radio is only looking for a signal whose strength is measured in microvolts (or smaller).

62 Val said

Thanks for your response, Pauly. I would tend to agree that signal strength from the transmitter may have something to do with the receiver processing the signal, but I have a feeling that one of the amplifier tubes may be malfunctioning. I reckon I'll have to go to work and try to find a shop with a vacuum tube tester in the Seattle-Tacoma area. If my recollection serves me,
I believe that the FM signal is dependent on line of sight, as FM stands for Frequency Modulation, and AM is more likely to travel a greater distance, as AM stands for Amplitude Modulation, and its signal travels in more of a wave-pattern.

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