Thanks for the interesting screen shots and your well-written report.
It looks like you have done well putting together the SDR.
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In either case, pointing directly at the source (160 deg Magnetic) yielded no usable signal, while pointing at ~90-100 degrees often worked well.
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I think you are receiving a signal reflected off the hill that is east of your location. Here is a view looking ~SSE toward 7 & 9:
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The criteria for "usable" here is a high symbol quality (as well as strength) for a SiliconDust HDHR networked tuner. Symbol quality is the number 1 factor that determines if a recording will be watchable, or even occur.
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Yes, exactly correct. The symbol quality is the inverse of uncorrected errors; 100% symbol quality = zero uncorrected errors. The FEC (forward error correction) has a limited ability to correct errors. Once that limit is exceeded, the signal can not be decoded.
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I do not see a lot of noise overlapping the bands in which the digital data is being transmitted either.
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The OTA digital TV signal is an analog (AM 8VSB, 8-level vestigial sideband amplitude modulation) signal that carries the digital information.
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I'm forced to believe that the reception problem is some multipath-like effect where the digital data is arriving at slightly different times leading to errors and low symbol rates.
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I agree, that seems most likely. Multiple reflections of the signal are arriving at your antenna from different directions and at different times. The greater the time difference between the primary signal and a reflection (echo), the more difficult it is for the tuner to resolve.
Special test equipment is needed to display the echoes.
The antenna worked better inside because it didn't have to deal with as many reflections, and a UHF antenna worked well because some of the reflections were attenuated.
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atmospheric reflections may be the problem (it has become cloudy here and the symbol rate has gone up to 100%!
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If a temperature inversion forms between the transmitter and your location, the signal can become trapped in an upper layer, unable to bend down to your location by refraction. When the temperature falls, the signal is no longer trapped.
http://www.mike-willis.com/Tutorial/PF6.htm
scroll half way down to Example of a path profile diagram
http://www.dxfm.com/content/propagation.htm
scroll down to An illustration of a high-altitude tropospheric duct.