"Frank" wrote in message
news:1V8be.56318$yV3.14588@clgrps12...

"Ed Price" wrote in message
news:H_Vae.2007$pk5.904@fed1read02...

Everbody loves to argue about antennas; their calibration, application &
accuracy! In the EMC area (my side of the elephant), we are frequently
looking for emissions with a maximum limit so low (imposed by the
standard) that we have to be inside a shielded enclosure. Since the cost
of a chamber increases as the square (or maybe the cube) of its volume,
only extraordinarily well-funded (uhh, governmental) labs can afford
really huge chambers. Thus, most EMC testing happens in more modest
volumes (my chamber is 36' x 24' x 9').

Last place I worked with EMC facilities they only had a 3 m cube chamber.
The dimensions you quoted are huge compared to my experience. (I think
ETC, in Airdrie Alberta, had a similar chamber to yours; also General
Dynamics in Calgary had two similar chambers. Also Nortel has some EMC
capabiltiy.) The insides were covered in microwave absorber, and there was
some question as to how effective the absorber was at 30 MHz. It must
have done something, since before the absorber was installed it was
interesting to see the effects on a transmitter keyed inside a shielded
enclosure.

The MIL-STD-461E requirement for absorbed is a 10 dB return loss at 250 MHz.
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. 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).

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.

Here's what MIL-STD-461E says about conical logarithmic spiral antennas:

"Previous versions of this standard specified conical log spiral
antennas. These antennas were convenient since they did not need to be
rotated to measure both polarizations of the radiated field. The double
ridged horn is considered to be better for standardization for several
reasons.

Very interesting Ed, will forward your comments to my last company. Doubt
they will do anything tho, as they never want to spend any money. Assume
the recomended type of antenna is a linearly polarized log periodic.

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 remember the Singer (Was it Singer-Metrics), and using it to measure
radiated spurious in a cow pasture at 50 m from a 1kW TMC linear
(Canadian Marconi, Montreal). The test monopole had a cylindrical base
with a rotary switch.

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. 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, 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