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Oh I was kinda sleepy when I responded, but when I did his math, 599/freq = about .608 or about 5% shorter than the .64 he mentioned, which would tune a 27.2 dipole to about 28.6, not an insignificant amount of error.

i just measured the vertical at 22 1/2 ft and 23 ft , from where the ring attached to it , not including any of the loop/coil for matching . at that location the only other antenna i had used was the wire starduster i made . so it's not a fair comparison IMO . but with the SD's feed-point at 17 ft and the 5/8 or .64 20 ft across the yard and at a feed-point of about 8 feet the 5/8 did much better . it got even better when i got it up to a 16 ft feed point .

but there could have been something wrong with my SD's build . you mentioned the SD's patent over at the quack shack and my version had 3 obvious differences to me , maybe more to you or someone else . according to the patent my ground radial angle was too wide , i didn't have a conductive mast at the feed-point running down the center of the ground elements , and my coax wasn't running up the center of the conductive mast .

the SD did have a donut shaped area about 20-25 miles out where it just didn't want to hear well and didn't tx real well either even with my little 2 pill or the 500v on . at about 30ish miles it did well again though . the 5/8 at 8 ft didn't have any hint of a dead zone and of course was mo' better up above the roof .

I wanted to find a nice and easy formula to find the wavelength in air if I entered the frequency in MHz so I did the math:
light = 30000000000cps / 2.54cpi = 11811 - the number I was looking for.
So,
11811/27.185 (center of band) = 434.5"
434.5 x .64 = 278"
434.5 x .625 = 271.5"
If the Imax comes in at 270.5" then it is 1 inch short for a .625 (5/8) which I bet is made up for by the fiberglass medium slowing the velocity factor a bit.
I believe the imax is a 5/8 not a .64

Another anecdotal comparison was done last weekend. I helped erect a Sirio Gainmaster in place of a good working Imax on top of a 36' telescoping mast from the ground up, in a backyard, 80' from the house, and using dacron guy line, no metal.
Coax was LMR-400.

The Gainmaster was actually lower in transmit signal strength to a couple stations about 35 miles away.
I wonder if the lower angle of the Gainmaster didn't get his signal up and over local hills as well as did the Imax with it's higher toa.

Maybe the Gainmaster doesn't like being right at one wavelength above ground

Basically they performed almost exactly the same nearby, a few stations in the distance saw improvement and a few others in a different direction saw a decline.

Overall his receive is improved but only a little bit and not in all directions.

Surprisingly, for his application, the Imax might be the better performer.