Having enjoyed the knowledge of experts in this forum, I'm hoping my experience might help others.
I left cable more than five years ago and started using an internal antenna with a CM-7000. I'm in Oakland, CA about 11 miles east of Sutro tower so signal strength has not been an issue and I receive clear reception of local broadcasts. Several years ago I switched to an Elgato EyeTV connected to an Apple Mac mini which served as a DVR, and at times the reception would cause a few drop-outs, but nothing intolerable. A year ago I decided to add a smallish external antenna, an EZ-TV from Denny's Antennas, on the roof about 12' above ground. As expected almost every station had good reception with no dropouts. I used a compass and several up-and-down ladder trips to aim the antenna so the trickiest stations 5, 7, 9, and 44, were coming in well. Everything was good until March when channel 5 and 9 started experiencing almost constant dropouts. Checking the EyeTV's signal strength readout showed good signal strength with a 40-50% drop several times a minute on channel 5 only. What I haven't yet mentioned is that 30' from the antenna, in direct line with Sutro tower are some really tall redwood trees. I was starting to suspect an interaction which had evolved to the point of affecting channel 5's reception. I also considered the EyeTV was starting to fail after 3+ years of constant use.
Assuming that the Mac mini (now 7 years old) and EyeTV probably needed replacing, I started to price some new hardware. After discovering Elgato no longer sold USB tuners in the USA, I decided to buy a new TiVo Roamio and abandon the computer as DVR system. After connecting and setting up the TiVo I tested all the channels and discovered weak reception on channel 9, which brought me back to the antenna/tree dilemma. The first and easiest test was to try adjusting the antenna up on the roof. Since I could control the TiVo with my iPhone, I started a FaceTime between my iPad, pointed at the TV and my iPhone up on the roof with me. I turned on TiVo's signal quality overlay and started re-aiming the antenna. To my complete astonishment a movement of less than 1/8" caused the signal to increase 30%. With the slightest aiming change I could send the signal fluctuating between 42% and 72%!
For some reason I assumed aiming the antenna meant getting it within a couple inches for maximum signal strength not a couple millimeters. Oh well, live and learn. The new TiVo was a great choice and after re-aiming the antenna is working well. It will be interesting to see what happens when as the weather changes over Fall and Winter.