Jack, K9CUN wrote:
"I have referred to my various engineering texts on antennas and
transmission lines and can not find any discussion of antenna "vigor"."

I did not copy my statement. Vigor is defined in my "American College
Dictionary as: "1. active strength or force---".

A rod in free space becomes excited and accepts energy, which it must
re-radiate, when it is swept by a passing wave of its resonant
frequency. Its first resonance is near a 1/2-wavelength. At frequencies
slightly off-resonance, little current flows in the rod due to the
opposition of its reactance.

You may have seen a mechanical analogy in the vibrating reed frequency
meter.

Best regards, Richard Harrison, KB5WZI