On Mar 31, 7:22 am, "Dave" wrote:
"dansawyeror" wrote in message

...



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

the real important thing though is that the match is near perfect at either
1.135:1 or 1.01:1, so stop playing around and put some power to that sucker
and make some contacts! remember, 'too low an swr can kill you'! or at
least keep you off the air for as long as you are tweaking it.

My experience with a screwdriver antenna is that the bandwidth vs vswr
is not as good as he is showing. My bet is that the ground is acting
pretty much like a dummy load making the SWR look real good. He could
probably use a good ground system before he tries talking . If I were
using a screw driver antenna as a base antenna I would consider
seriously increasing the length of the radiator.

Jimmie