Here's a related question:

WHY do parasitic elements work the way they do?

Let's consider a two-element yagi with a driven element and a parasitic
"reflector", ie a parasitic element longer than a half wavelength.
(We could make the same arguments in reverse for a "director".)

The driven element radiates an electromagnetic field, some of which
impinges on the reflector. This causes a current to flow in the
reflector, and a voltage to appear across it. Since it is longer than a
half wavelength, it acts inductive, and the current LAGS behind the
voltage.

The reflector then radiates its own electromagnetic field in all
directions, some of which heads back toward the driven element.
(For simplicity, we ignore the mutual impedance effects and the new
current which is induced in the driven element.)

If the fields from the reflector and driven element are to be in phase
in the direction from the reflector towards the driven element, then the
radiated field from the reflector must be advanced in phase by how much
it lost traveling from the driven element to the reflector, plus another
same amount as it travels back. So the phase of the field radiated by
the reflector LEADS the phase of the driven element significantly.

Now the question is (assuming this is all right so far):
How do we explain the phase of the field radiated from the reflector, in
terms of the phase of the current and voltage in the reflector?


Bob W8ERD