wrote in message
oups.com...
I know in a ground plane the angle of the radials can, but what about a
simple, multiple element yagi?

The spacing of the elements and thei angles they're mounted out (a yagi
consists of a regular old dipole with a reflector and a bunch of directors
added).

How does one measure what the impedance is or will be?

The equations for designing a Yagi are rather involved, but you certainly
could use them if you're dead set on it. Otherwise, you use an antenna
modeling program such as NEC to predict the impedance, and an antenna
analyzer to measure the impedance of an actual antenna. (Internally, an
antenna analyzer is measuring the SWR [and relative phase] of a small RF
test signal applied; it can convert this back into an impedance.)