So with COVID-19 I have time on my hands at home and did a bunch of comparative measurements. Used Nuts About Nets spectrum analyzer to compare--the only variation in the configuration was the final antenna.
Note that the dish is mounted on a short pole (due to wind loading issues, couldn't go higher) but the ground level slopes off and the dish is effectively maybe 10 feet above ground level.
Also note that this would have to be called deep-fringe conditions--the nearest real station is 75 miles away and some of the stuff I'm shooting for is 125 miles away. Due to atmospherics, signal strength varies significantly during the day, so I always did my measurements within the same hour and comparable weather conditions.
Baseline was as shown in the photos above--very thin copper "whiskers". They are long because (1) I'm trying to get some VHF channels and (2) from what I read, making them longer doesn't significantly hurt the UHF.
Modified the whiskers to make solid triangular pieces. Within the variability of the signal/atmospheric effects, no measurable difference.
Put a reflector screen behind the active elements. No appreciable difference
Modified the whiskers and made "stacked" dipoles. Maybe 1 dB difference.
Took a Channel Master 4228 HD antenna and used that as the active antenna (facing in toward the dish). Depending on its location in the dish focal plane, got up to 5 dB of gain. The 4228 HD can be modded to improve its gain by another 3 dB (there are a couple of sites that describe how to do this)--I didn't make those mods, but I'll believe the claims.
But---the advantage of the parabolic, collecting a huge amount of signal, is fighting the disadvantage of "no altitude". And the measurements showed that it's a losing battle.
I took the Channel Master and raised it about 15 feed higher than the parabolic--and got more signal at every channel from the naked 8-bay bowtie at that altitude. Sigh.
Then I did a comparison of the Channel Master versus a pair of stacked broadband Yagis (AntennaCraft) and got surprising news: at almost every frequency, the Yagis beat the Channel Master. Obviously in VHF (actual channels 7, 8, and 11) there was no contest...but even in high UHF the Yagis were providing more signal, or were within a dB or two of the Channel Master.
So in this deep-fringe reception area, the lessons are pretty clear (even if somewhat counter-intuitive):
- Altitude trumps everything else (duh)
- If you need VHF channels, a Yagi or two is in your future
- The 10-foot parabolics don't offer that much of a gain advantage at terrestrial TV frequencies (this was a surprise to me)
- Modifying the active elements of the antenna didn't make as much of a difference as I'd expect
- I'm glad I ordered the Channel Master via Amazon, as they allowed me to return it
Although this has turned out to be a wild-goose chase (I'm back with the Yagis I started with), hopefully my experience will persuade some of you to not go down this path unless you have a really strong tower to put your 10-foot, 100 pound parabolic on.