Ken,
It would be helpful to look at the power required at all the probe positions.
Sometimes you can deduce where reflections are occuring from the probe values,
ie: are the nodes/nulls row or column dependent.
FWIW, I never use the constant field method, instead I use the constant power
method. The reason is that you will have nulls occur at some probe positions,
which will make it impossible to level at the desired field especially if you
have a marginal setup (amp power, antenna gain or beamwidth, etc). Using the
constant power method, you will get an idea of field uniformity, and can
determine which probe positions will be discarded (probably the ones in a
null).
After the calibration file is created (levels adjusted to bring 12 points into
the required test level) you must determine if the system is still linear and
meets the harmonic requirement.
>From my experience, 100w is marginal for 10v/m (18v/m calibration) at 3m
distance and 80MHz. If you are having problems above 650 MHz, this is most
likely due to cable losses.
Have you tried changing the antenna height?
Bob Richards, NCT
khcmacgr...@aol.com wrote:
Hello,
I'm trying to qualify my 3 meter chamber for field uniformity at 10 V/m and
could use some help. I was able to get 18V/m at all 16 points of the grid
(only need 12) with the antenna in the vertical position, but couldn't do it
when the antenna was switched to the horizontal position. I moved the floor
patch all over the chamber, moved the antenna and probe to different areas,
but still haven't found just the right combination. Here are the some of the
details:
Problem frequency: It shifts but typically 80MHz and above 650 MHz It seems
we can correct one but not both.
Chamber dimension: 16' x 24' x 10'
Lining: Mix of ferrite tile and 24" cones
Placement of lining: 14' x 10' ferrite tile patch centered on each side wall
10' x 10' ferrite tile patch on wall behind
isotropic probe (EUT end)
8' x 14' ferrite tile patch on ceiling
24" cones on wall behind antenna
Mostly 24" cones, but some ferrite tile in all
remaining wall surfaces.
Floor Patch: Thirty 2' x 2' ferrite tiles shuffled in countless configurations.
Test Distance: 2.5 meters (It failed at 3 meters so I moved the antenna in to
2.5. It passed with the antenna in the vertical position, but failed in
horizontal position.)
Amplifier: 100W amplifier outside the chamber
Floor patch: Thirty 2' x 2' ferrite tiles
Thanks!
Ken
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