Cecil, W5DXP wrote:
"Passive non-resonant elements have very little effect."

True in a parasitic array such as a Yagi.

In some antenna arrays all the elements are fed by a transmission line.
Directional arrays are also made by using elements which are not
connected to a transmission line. These disconnected elements have
current induced into them by the driven element and are called parasitic
elements.

The effect of a parasitic element depends on the magnitude and phase of
the current flowing in it. To get substantial current, the element must
be near the driven element or other element carrying large current, and
it must be almost resonant. A non-resonant element has a large reactance
limiting its current flow and effectiveness.

A resonant length for a conductor in space is 1/2-wavelength.

A resonant length for a grounded conductor is 1/4-wavelength.

The 3-element Yagi is a common parasitic antenna. It has a driven
element, a reflector element, and a director element. The length of all
three elements is nearly 1/2-wavelength in most cases. To get large
currents, near-resonance is essential.

The trick is to get the radiation from all elements to reinforce in the
desired direction and cancel in other directions.

Phase of current in the reflector is often delayed by making it slightly
longer than a 1/2-wavelength. Phase of current in the director is often
slightly advanced by making it slightly shorter than a 1/2-wavelength.

Spacing between elements is close to get a large excitation of the
parasitic elements. This makes a compact antenna. Close spacing means
mutual impedance (coupling) between the elements, and this affects the
drivepoint impedance. Close spacing increases directive gain but lowers
the array`s radiation resistance. See Terman`s Fig 23-36 on page 904 of
his 1955 "Electronic and Radio Engineering".

Large non-resonant reflectors are also used. Backward radiation can be
eliminated with a plane conducting sheet. It`s impenetrable and must
reflect. The earth often acts as such a reflector. The parasitic
1/2-wave reflector is a "degenerated" (Kraus` word) case of the plane
reflector. I recall that either Kraus or Terman suggested that the best
way to handle non-resonant reflectors was on an image basis.

Best regards, Richard Harrison, KB5WZI