Homer, I think you're close enough to consider moving it to the higher mount. You may have to re-tune due to the change in feed line however. If it is easier to tune where it is now, then maybe I'd hook up the other line before going to all that trouble.
The other day I told Booty Monster that my gamma lies 3" inches outside of a plane between the two radials, but I think that could be misunderstood. My gamma is positioned outside of a plane between the two radials like the images tried to show, but the gamma is actually the same distance from the center of the radiator as the radials at that point, just like your antenna shows. The distance on my S4, between the OD of these elements, is 5" inches measured at the dog bone. At the feed point end of the gamma the center of the FP is about 3/4" away from the OD of the radiator, and the radials are about 1 1/4" from the OD of the radiator to the center of the radials, so the angles here are a bit different, but not much. Again, I don't know how much difference this will make if your measurements are not precise at this point in the antenna, but it has to be considered.
I think you also have to consider the diameter to length ratio for your radials.
Does the total overall length matter, I don't really know, but we have to consider what Bob and SW tells us about that. I haven't tried to make anything longer on my antenna as yet, and if I understand Bob correctly, I may not see a similar difference at a distance, simply because my antenna only has three radials instead of 4. On the other hand when I told Bob I could see a noticeable difference between 3 and 4 radials on my Marconi, he suggested the difference I saw was likely due to changes in conditions, and not the number of radials. So, I still don't know for sure about this, but that could also mean what Bob saw was also due to conditions. Check the dimensions real close and if necessary try and get them a little closer. Then see if you can get a better handle on the tune, and then work if for a while and see if you can tell how it compares to whatever else you have up, your 5/8 wave or the AstroPlane.
Within reason, I generally don't consider tune to be the defining factor in antenna performance, as most do. But, Bob makes a good argument for the collinear affect with the Sigma/Vector design, so I think tuning, in this situation, does bear some consideration. As a result, I think from my own experience that this antenna is very sensitive to its construction, you may not be able to get by with even a little differences, here and there, and still expect it to work as intended for best performance at a distance.
I'll also mention that my modeling of the Sigma4 did not bear out that the longer version was better...as far as gain and angle were concerned. My models showed the opposite, in that a bit shorter overall length showed more improvement. I don't recall if I made the radials longer or not, but that could be the difference we here about, but Sirio did make their New Vector 4000 model shorter for some reason, didn't they.
Homer, have you considered that the small amount of insulator material you use in your gamma may not provide the capacitance range necessary to counteract the inductive reactance in this antenna? I would like to know what the inductance for your antenna is at or near resonance? I have a feeling that your feed point may be showing more inductive reactance than desired. If I'm right, base on my assumptions from prior experiences, you may note your antenna is a bit deficient in the receive function. If you see a noticable difference when you change your feed line length, then I would consider such a tell-tale sign that reactance is the problem.
I could be wrong, but only some good old fashion on-air testing and comparisons work could help show that.
Good luck and keep us posted,
