All,

This is a real world "what is going on" question. The antenna is a
'screwdriver' on a 3 foot stand in the back yard. The earth ground is
normal loam, the original soil was forest. The feed cable is about 130
feet 50 ohm coax that measures to be in good shape. The ground system is
8 wire radials between 30 and 60 feet in length.

The measurement tool is an HP 8407a network analyzer and an HP 8601a
sweep generator. The frequencies of interest are from 3.5 to 4 MHz. As
the antenna is moved from resonance at 3.5 MHz to 4 MHz the return shows
a marked difference. It varies between -24 db and -50 db. The pattern is
irregular, however it is repeatable.

The antenna design is a coil that moves up or down across copper
fingers. As the coil moves those fingers contact more or less of the
coil and change the impedance.

What could cause the variation in return across a relatively narrow
band. A return of -24db is relatively small, an impedance very close to
50 Ohms and an SWR about 1.135. A return of -50 db is less then 1.01 SWR
and represents a small change in Z, however a change non the less.

I can think of three potential causes for this change:

1. variations in the coil change Q

2. variations is the finger to coil connection change R

3. variations in the ground radials.

Are there others?

Thanks - Dan