Heh... Found something that is even better than the RSSI reading.
It's a tiny spectrum analyzer that covers 10KHz to 900+ MHz.https://www.tinysa.org/wiki/
On the display, a digital TV signal looks like an 8MHz wide "plateau". The first thing I noticed is that the low end of the channel in question had more amplitude. As I turned the antenna, the plateau balanced out, and the signal strength also seemed to increase just a tiny bit. (Not sure of the "nuts and bolts" of this, since my methods are very primitive, but it did work.)
You have to be careful to not use it near any transmitters, lest you overload it or burn out the receiver front end, but otherwise it does exactly what I needed.
Essentially, what it told me is that my antenna was off axis, to receive the one major station that I wanted, but by zeroing that one in for the best signal, I lost several others. (This is puzzling, because there are only like 3 degrees between any of the Atlanta stations, and the antenna is not that
selective. Can you say "ragged edge"?)
So it looks like I need to add another ten feet of pipe (easy), or maybe buy an antenna with more gain. (Right now, at ~50 miles from the station, the antenna just peeks over the ridge of my roof. The signal I zeroed in on is at -80 to -85 dB. I can do better than that.)
Some people may suggest using a preamp. And that's also a possibility. But the use of a preamp pre-supposes that you actually have enough signal to work with, in the first place. If not, you'll be boosting what little signal there is, but you'll also be boosting any competing interference or other background noise. Again... "ragged edge". Going to raise the antenna.
[This message has been edited by Raydar (edited 11-16-2021).]