I have had a second Merlin owner offer to fix me up with some exact measurements. I'm looking forward to that from either or both.
So far, things as they are, I have seen the conditions hand the performance edge by very small differences to either antenna.
I will say that over the past few days the Merlin has begun to show a trend of performance advantage reflecting its design intent. Until I double check my dimensions against the originals I anticipate coming to me soon (fingers crossed) I will not give a definitive statement to that point. On the other hand, if asked what I think at this point in the situation where someone was considering buying a full length 5/8 GP or a Merlin to run in a height restricted area, I would advise they consider an antenna such as this Merlin - Top One, Astroplane - that is designed to provide more consistent performance under the conditions.
Why I am leaning this way at the moment is:
Although there are no, again I repeat, zero S-units (bars) of difference on my meter except for when I reported earlier in the above thread for the 5/8 GP, the Merlin does report to my ears a detectable audio increase most often and most consistently. Regardless of the debate that will potentially ensue regarding how the human ear can not detect audio gains except with a given amount of measurable db increase, I will not rescind the statement. I, Homer, clearly can detect the difference. Some (or all) of this may be due to the fact that the noise floor is clearly lower for the Merlin with its feedpoint at 36' as opposed to the 5/8 GP fed at 19.5' with tip heights the same.
In as close as 10 - 15 miles the performance of either seems similar, but whichever has a quieter noise floor at the moment (usually the Merlin) becomes the antenna of choice. When I work local stations, mobile or base, beyond that distance out to 30 miles (I have not heard any farther out when I was in the shack) the Merlin has been better over 90% of the time.
Am I ready for a conclusion? As I said, not yet. I will be out of town over the next 5 days and will be unable to report on anything, nor make changes until afterwards, so the tweaks and tunes I will be making to both antennas will have to wait until after. It will only be then that I will say how I finally believe this washes out for the Same Tip Height Test (STHT).
I will likely be adding a twist to the .625 antenna before I am done. Claims to the effect that the I10k was bested by the Merlin has me considering adding a Top Hat of the dimensions that the I10k uses and run a test between the .625/.64 vertical configuration where both of the antennas potentially use the capacitance hat technology to adjust toward optimum current maximum on the vertical radiator.