If one defines* "half wave resonance" to mean a resonant condition where there is a single
current maximum in the electrical center with high voltages at the ends (similar to a half wave
antenna), then these antennas are "half wave resonant", and will have the same radiation
pattern and properties regardless of where they are fed: center, off-center, or at one end.
* Whether this is a useful definition, or one that makes any sense at all, is left for a
separate discussion. But I think it helps to understand how the antenna operates without
getting overly bogged down in semantics.
A classic example of this type of antenna is the Cushcraft R7: in that case the "radials" on
the bottom are actually a capacitance hat. The antenna is fed off-center (essentially between
the capacitance hat and the vertical portion) where the impedance is lower than at the very
end, so more suitable for a step-up transformer. The original R3 version used a motorized
variable capacitor to adjust a parallel-tuned circuit at the base, with true end-feed, but the
tuning range was somewhat limited. (The R3 only covered 10 / 15 / 20m.)
1. Does this kind of end-fed antenna require a "counterpoise" or radial wire, or not? It seems that N0LX doesn't use a counterpoise, as far as I can tell from the photos. In that case, the case of the rig will be hot with RF, right? The electrons pushed into the antenna have to come from somewhere. I suppose this isn't a problem at QRP levels.
Such an antenna WILL have a counterpoise, whether we provide an intentional one, or
leave it off and let the coax shield serve by default. Because of the high impedance there
is less current flowing at the feedpoint, so the counterpoise does not need to be as
elaborate as would be the case for a quarter wave vertical.
Yes, you can still have problems with RF in the shack at QRP levels, but how much of a
problem it is depends on many factors, including the coax length. Typically I've had more
issues with end-fed antennas when the tuner is connected through a length of coax rather
than having the tuner right beside the rig, though in many cases the ill effects are not
2. Would a capacity hat help with this kind of antenna? If so, do you need one at each end, or only at the top of the antenna?
A capacity will have the same effect as for the same antenna fed in the center: it allows you
to shorten the antenna and/or use smaller loading coils. The principle is the same as for the
short dipole you have been considering.
3. Electrically, how is this antenna different from a typical loaded "vertical" antenna that needs to be worked against a ground? Do the dual loading coils in the "shortened EFHW" design somehow make it ground independent, unlike a vertical?
I would compare it instead to a shortened vertical dipole: it's the same antenna with the feedpoint
shifted. (Shifting the feedpoint location will change the radiation pattern of an antenna with multiple
current maxima, but not with "half wave resonant" antennas.)
Compared to a "quarter wave resonant" antenna fed against ground, the efficiency is improved because
any series loss resistances are small relative to the feedpoint impedance. (Note that the radiation
is still the same as for the equivalent dipole, in spite of the higher feedpoint impedance.)
That doesn't mean it is "ground independent", or "has no ground losses", as there are still losses due
to the vertically polarized wave propagating over ground with poor conductivity. And the high impedance
ends also have some fields that extend into the dirt. But the component of ground loss resistance due
to ground return currents that appears in series with the feedpoint impedance has less impact on overall
For a specific maximum antenna height, it isn't entirely clear which would give the highest radiated power,
as the "half wave resonant" version requires larger loading coils (effectively squeezing twice the
electrical length into the same physical length) and therefore higher coil losses. The answer will depend
on the extent of the radial field: for a temporary portable antenna where it isn't convenient to lay a
good ground radial system, the "half wave resonant" antenna may work better.