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I do like the WSPRLite transmitters if they can be configured to produce a steady signal for testing. Other than for "hey it kinda works" experiments, WSPR is too inaccurate for any meaningful absolute measurements. This might change if the WSPR network reported the absolute signal level rather than S/N. Don't understand why they don't since the information is available. Oh well.
The problem, of course, is we can never really get a power source infinitely small so even its body will be a bit of electrical "mass" (or counterpoise if you like) to push against, but the data seems to be suggesting it doesn't have to be electrically large. Put another way, there's nothing about the EFHW that is fighting our ability to energize it so long as we handle the impedance transformation... and that's the larger point here.
By the same reasoning, neither is the quarter-wave ground-plane antenna, but they can be made more functional with an additional set of radials or equiv... just like the J antenna... and that's only if they need a connection to a conductive mast. Independently, both are whole and proper antennas and the rollup J antenna, yes with a feedline choke, personifies the near ideal antenna.
"Why keep building and promoting these pieces of crap?"
Well because they aren't bad as tests and research easily confirm. Yeah they have their idiosyncrasies like any antenna topology, but work fine in the hands of the skilled.
Of course my larger point is to not mix Layers 7 and 1 into the broader demonization of Winlink. There are responsible ways to use broad bandwidth digital modes... especially when it can finish the transfer faster and then stay out of the way for a good while. Think of it as TDMA in super slo-mo... something stunningly ignored by some in the anti Winlink parade. I'd rather not see one application's alleged abuse of the digital landscape needlessly demonize the mode used during the data transfers. It's quite unfair to the modem designers be it SCS or open-source soundcard techniques. I have no dog in the hunt for Winlink. They can fight their own fights. Consider me a "mode warrior."
The original Hoverman design had limited frequency range. That ~5 inch length is indeed the primary refinement of the more recent Hoverman designs (7") and it extends the frequency range of the overall design quite a bit upwards. With the abandonment of the upper television channels this is less important now. To answer your question more directly... While 16ths of an inch precision might matter in a perfect NEC model, I'm not convinced manufacturing tolerances for any given build are going to be accurate enough to make the length of that last element more critical than, say, 1/2 inch precision. That was certainly the case with my example above and it tested very very well in chamber measurements.