Re: analysis and hypothesis question?
- From: "Frank's" <frank599@xxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 31 Mar 2007 03:10:44 GMT
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
Dan, 24 dB return loss is about as good as you can possibly get
for any type of design: matching network, filter, you name it.
Anything better is just pure luck. In any design you always
get some very deep return loss notches. They could just as well
be minor errors in network analyzer calibration, and how
accurately your standards are characterized.
73,
Frank
.
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