BM, I can't be for sure, but I tend to think the importance and length of radials on certain antennas depends on how much the antenna needs radials, and how close they are to a current source.
The 1/4 wave really needs radials, so 1/4 wave radials tends to work best.
For me a 1/2 wave hardly needs radials at all, and they are typically positioned far away from a current source so maybe this is why we hear stories that radials don't seem to show a difference.
The 5/8 wave is about in between the 1/4 and the 1/2 wave in this regard, so generally less radial affects will do, with maybe some exceptions. My experience with my I-10K proved to be an exception. I removed the two end pieces of its radials and the antenna's match went to pot.
Why the difference between this Imax type and the I-10K, I can't say for sure, but I do have an idea. It has to do with the way the Imax tuner balances the feed point currents around the capacitor.
<gotproof> This idea is not mine, and is better explained in the Yates link I posted on this thread, as the genesis for this project. I think there is a picture in his article describing this balance idea.
