Hey Marconi, I just got to reading that, now I'm wondering how long you needed to make the top tube when you tried the AP without top hat radials? How was it's performance compared to stock configuration?
As per the following Antenna Work Sheet reports, I made three iterations to get my AP resonant at 27.205. Started with a full 1/4 wavelength of 79.5".
Report #1, using the 1/2" x 45" tip element from my Sigma4, the report shows I was short and high in frequency up near 27.605 mhz.
Report #2 shows I then went to 89.75" length, and the VA1 showed I was about 8 channels low at 27.105 mhz. That was fine considering the antenna is so broadbanded, but it wasn't where I wanted it.
Report #3 shows I changed the length to 87.5" using the same 5/8" element on the Top One and hose clamped the Sigma4 tip element to its side. The VA1 then showed a very nice dip at 27.205, but the data recorded after that also was skewed...as you will note on the report. This sometimes happened, I think, when someone in the area of my station keyed up some big-time RF. You'll see that the RF did not seem to affect the SWR inline meter scan that I did soon after the VA1 scan. Also I noted later, when I rehashed the reports, that I made an error in the final adjustment or I recorded the info wrong. I made an error that looks like about 1", so I'm not too sure what the actual top element length really was. It had to be close to 87.5" however to make my Top One center at 27.205 with a straight full length 1/4 wave top element.
Based on the report dates and other reports of the time, I had this full 1/4 wave TO configuration up only a few days. I do have two reports #13  dated 08/1/09 for the AP vs. Sigma4 showing the Sigma4 besting the AP by .9 Sunit on average, but the reports around that time were random reports only and were not even close to showing the reliable differences I was expecting...even if it's not possible to make reliable signal reports.
Based on the fact that I took the thing down and fixed the TO back to its original top hat configuration suggest to me...that I didn't like the results of making the top a full 1/4 wavelength. I didn't even make notes. Soon after that I started remodeling my house and I stopped all testing. These Signal and Work Sheet reports are all posted in my albums.
http://www.worldwidedx.com/members/marconi-albums-marconi-s-signal-reports-may-august-2009.html
View attachment AstroPlane Antenna Work Sheet 073109.pdf
Close enough.
Same results I got many decades ago when, as a teen, I compared every antenna I could get my hands on and found the Hy-gain Penetrator.64, Taylor GLR-4.64, Wilson AlphaV5/8 & Avanti Sigma5/8 outperformed all others, including the Starduster & Astroplane, when mounted atop my 45' mast over the then 12' high roof, 57' to the mast top.
I never even thought about adding another section to the 5 section mast to 'even things up a bit'.
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I just figured you buy an antenna, install it on the highest mast available and let it do it's thing, ...and may the best performer win.
You're right 007, when we're just installing antennas, but I like to think my comparison work is devoted to testing or comparing rather than the simple process of our CB buddy "Fried Chicken Livers" installing his new antenna his mommy got him for Christmas. I'm not telling anyone to follow my instructions, I'm just relating my limited experiences doing "what if" experiments. I became inst rested long ago when a buddy of mine had a Starduster and he talked to guys in the distance I could not copy. Even back in the late 70's guys complained that the SD and the AP, both very short antennas were junk, while Johnny talked all over with his Washington radio and SD antenna. Other guys heard this too and went out and bought these same antennas and installed them, but they put them on a 10' or maybe an 18' foot mast and they failed to be able to do what Johnny could do. So, they all talked bad about his little antenna. I discovered later that Johnny live in a very high two story garage apartment that was about 30' to the peak. He had two 10' sections of Schedule 40 conduit screwed together with a 50' foot pushup pole inside about 4' feet. His antenna was well above 60' and that made the difference folks were seeing. Johnny was notorious in the area and everyone figured he had a big amp, but Johnny only had a 80 watt modulator that talked like it was a 1000 watts. Of course if he had installed a big old Vector 4000, Penetrator, or an I-10K up there he likely would have done even better, but then he had to worry about the wind with such a flimsy install.