Stryped wrote:
"I know in a ground plane the angle of the radials can, but what about a
simple multiple element yagi?"

The feedpoint impedance of a resonant 1/2-wave dipole is about 67 ohms
resistive (page 120 of Kraus` 3rd edition of "Antennas"). The impedance
of a 1/4-wave antenna perpendicular to ground is about half the dipole`s
impedance (page 568 Kraus` 3rd edition).

Drooping radials place a ground plane in a transition between a 1/4-wave
perpendicular to ground and a 1/2-wave antenna. Impedance follows suit,
ranging from 30 ohms with no droop to 60+ ohms with complete droop. In
between, a match for 50-ohm coax can probably be foind.

An antenna element alone in free-space has a particular self-impedance.
Aide by side elements are coupled. Current flowing in one ekement
induces voltage in other elements nearby. This affects the feedpoint
impedance of the driven element because it is feeding an array. Another
name for coupling is mutual impedance. It is measured as the voltage
produced in one element bivided by current in the driven element. It`s
reciprocal. Either element can be driven and the other can be parasitic.
The mutual impedance is the same either way. The feedpoint impedance is
a complex sum of all the array impedances.. It is often tedious to
calculate. Kraus shows the mutual impedance story on page 452 of his 3rd
edition of "Antennas".

Cecil, W5DXP gave a good explanation of impedabce in a standing wave
antenna. He showed how a high impedance at the open-circuit end of the
antenna is converted to a low voltage and a high current 1/4-wave back
at the feedpoint. Their quotient is the low self-impedance at the
feedpoint.

Best regards, Richard Harrison, KB5WZI