John wrote:
"To achieve resonance, non-folded dipoles/monopoles must be cut slightly
less than 1/2 or 1/4 wave due to "end effect", so I`ve read. EZNEC
agrees.

Is this true for the folded dipole/monopole?"

My ARRL Antenna Book (19th edition, page 8) says:

"Since the antenna section (of 300-ohm twin-lead) does not operate as a
transmission line, but simply as two wires in parallel, the velocity
factor of twin-lead can be ignored in computing the antenna length."

I wish the author had said:

"---the transmission line velocity factor of twin-lead can be
ignored---."

Wave velocity on the antenna wire is very important.

End effect is sometimes defined as the capacitive effect at the ends of
a 1/2-wave antenna.

Length in free-space wavelengths is reduced in an antenna by a "fat"
radiator. The 1/2-wave resonance point (first resonance in an ordinary
open-circuit dipole), for a thin radiator, produces a drivepoint
resistance of 72 ohms. This can be cut in half by using conductors with
a very large periphery. Radiators of large periphery slow the wave
velocity along the surface. This reduces the physical length required
for electrical resonance.

Feedpoint resistance of the center-fed 1/2-wave dipole results from
far-end reflections.In the open-circuit dipole, the high impedance at
the ends of the radiators is transformed by the 1/4-wave return to the
feedpoint into a low impedance. The short-circuit at the far ends of a
1/2-wave folded dirole is reflected to the drivepoint as a high
impedance.

Constructed of the same size wire, a folded dipole has 2 wires
effectively in parallel as radiators and their size is enhanced by
spacing so that wave velocity is reduced more than in the open-circuit
dipole.

The slower the antenna wave velocity, the shorter the length to produce
resonance. I would expect more "end effect" in the usual folded dipole /
monopole.

Best regards, Richard Harrison, KB5WZI