Thanks again Ed. From everyone of your posts I learn something new.
The MIL-STD-461E requirement for absorbed is a 10 dB return loss at 250
MHz.
Assume you would test the chamber return loss with a tuned dipole having
free space return loss 10dB. i.e. some physically realizable antenna with
a return loss of 40 dB at the test frequency. I suppose, with an inductivly
loaded dipole, you could test the return loss of a 3 m chamber down to 30
MHz. There were some questions raised about possible reflections in the 3 m
chamber due to imperfections in the installation of the pyramidal foam. I
tried sweeping from 1 to 10 GHz with the log spiral antenna, coupling to a
non-standard antenna, and performing an inverse FFT on the network analyzer
data to generate a time domain plot. I had very little success in actually
seeing reflections. For best resolution the ideal would have been to sweep
from 30 MHz to 20 GHz with two wide band antennas, but the company did not
want to spend the money for any new antennas. What I am thinking is that
careful return loss measurements may have shown if any reflections were
present.
I have 24" tall pyramidal foam, and that meets the requirement. As
frequency decreases, the foam essentially disappears. By 10 MHz, it has
almost no effect.
I think we were using 12" pyramdal foam, even on the floor, with inverted
foam to provide a walking area.
The pyramidal foam is expensive, about $50 / sq ft. If you want more
return loss, you need taller pyramids; those mythical governmental labs
have had foam up to 72" tall (and the wall absorbers tend to droop a bit
g).
With a 3m chamber, anything greater than 12" is not really practical.
A newer technique is to use ferrite tiles, especially on the floor. They
are less than a half-inch thick, and perform much better at low
frequencies. And the cost is about $100 / sq ft. I like to think of my
walls and ceiling as covered with $5 bills, and the floor carpeted with
$10's.
Your anechoic chamber is never really perfect; however, it becomes "good
enough" when you run out of money.
With the dark blue pyramids and black tiles, a chamber looks like a bat
cave. One vendor decided that the new millenia needed white paint on the
foam; another vendor touts pyramids that have a 90-degree axial rotation
part way up the taper, and yet another truncates the pointy tips, telling
us that works better. It's just like the antenna game.
I have heard of the ferrite floor tiles, and are probably a much better
solution than inverted pyamids fitted into the floor mounted pyramids.
No, 461 doesn't like log periodics either, saying:
"Other linearly polarized antennas such as log periodic antennas are not
to be used. It is recognized that these types of antennas have sometimes
been used in the past; however, they will not necessarily produce the same
results as the double ridged horn because of field variations across the
antenna apertures and far field/near field issues. Uniform use of the
double ridge horn is required for standardization purposes to obtain
consistent results among different test facilities."
The MIL-STD defines a 104 cm rod from 10 kHz to 30 MHz, then a biconical
from 30 MHz to 200 MHz, and finally, horns above there. Since pyramidal
horns are only good for about an octave, a smart Navy guy added
exponentially flared ridges to the horns, and came up with multi-octave
horns. A typical horn for 200 MHz to 1 GHz has an aperture of about 1
meter, then another horn tries to go from 1 GHz to 18 GHz. That's a bit
too far for me, as the antenna factor really climbs above about 14 GHz, so
I switch to a common, non-ridged horn for 12 GHz to 18 GHz. For 18 GHz to
26 GHz and 26 GHz to 40 GHz, I use standard-gain flared horns. With a
pre-selected spectrum analyzer, really good coax, and a couple of
low-noise pre-amps, that lets me get comfortably below the most stringent
RE102 limits.
I think they were considering horns and low noise amps to get above 10 GHz.
I did a lot of analysis to figure out what was required, but never got to
finish it, on account of being laid-off! Nobody ever seems to want to spend
the money to get it right.
OK, just for trivia's sake. If the antenna base was cylindrical, painted
grey crinkle, had a 6-position range switch and a brown bakelite top
insulator, it was an Empire VA-105.
Describes it perfectly
But, if it was almost a cube, painted battleship grey, had a black front
panel and an 8-position range switch, it was a Stoddart 92138-1 (that
number is a hazy memory). Both were passive antennas. The Empire was used
with the NF-105 receiver,
That was the one I used, now you mention it I remember the model number as
the NF-105
while the Stoddart antenna was associated with the NM-22A (that's why the
range switches were different, to match the ranges on their associated
receivers).
--
Ed
WB6WSN
El Cajon, CA USA
73,
Frank
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