________________________________
From: [email protected] [mailto:[email protected]] On Behalf Of
[email protected]
Sent: Tuesday, January 13, 2009 1:50 PM
To: [email protected]
Subject: Anechoic Chamber: Pass-through vs. Bulkhead
EMC Gurus,
We have a Space Saver chamber that we bought used and installed
ourselves.
When we installed it, we ran our antenna (signal) cabling through a
pass-through along with our fiber optics. Today, someone mentioned that we
ought to have at least our antenna cable connected through a bulkhead so that
the cable shielding was terminated on the chamber. When we went to
disassemble the chamber when we bought it, there were no bulkheads for
anything. Are the bulkheads necessary? Does anyone else use a pass-through
for the antenna cable? It doesn't seem to have had an effect on measurements,
but I still have a lot to learn so hopefully someone can share some wisdom.
Thanks,
Chris Anderson
The traditional technique with a shielded enclosure was to have a specially
designated penetration port. This was usually a thick plate of aluminum or
tin-plated steel, about 16" x 16" x 1/4", mounted to the side of the chamber
like a picture on a wall. The plate covered a 12" x 12" hole cut into the
chamber wall and was bolted around its perimeter to chamber wall. There's
nothing magical about those dimensions, they are simply convenient.
This provided an access panel that could be customized for the special needs
of an EUT. Did the EUT need cooling water, visual observation, optical
stimulus, compressed air, a waveguide port, big multiconductor I/O cabling;
you could machine and equip a panel as needed without ripping into the chamber
itself. You could keep several special port covers for recurring unique jobs.
The penetration port was a way of organizing your use of the chamber. Thus,
there is no real need to use one. You are free to run every penetration as you
want, but the many possible problems with this (control of grounds,
preservation of shielding effectiveness, crosstalk, safety) will probably make
you wish you had used designated penetration ports. (I have 3 penetration
ports on my chamber, one primarily for emission testing, another primarily for
immunity testing, and the third as an I/O port to a second smaller shielded
chamber typically used to contain the EUT support equipment. I once built a
penetration port for the transfer of jet turbine gas out of a chamber.)
More to your specific question, John Woodgate has pointed out the issue of
safety (RF can travel along the ungrounded shield, possibly toasting you or
your equipment or radiating into the environment, although this is more of an
issue in military testing than in commercial testing). The ungrounded coax
cable could just as well allow external RF (radio stations or noise from your
own test and support equipment) to get into the chamber and be taken for
emissions from the EUT.
Ken Javor talked about my favorite technique of grounding the coax without
breaking the continuity by use of a packing gland or "stuffing tube", but he
didn't say why you might want to go to this trouble. Let me add a few words
here. The easiest way to ground your coax as it penetrates the chamber wall
(hopefully at a penetration port) is to use a female-female bulkhead
feedthrough fitting. These are cheap and easily replaced when worn or damaged.
The down side is that they are always a slight impedance discontinuity, and
will create loss at extremely high frequencies and with high power, present a
small concern of voltage breakdown or localized heating.
If you use the packing gland method, you do not break the coax structure, and
eliminate a pair of male-female connections. I use this on an ultra low-loss
coax that I use for emission testing up to 18 GHz. The same techniques would
also be a good choice for conveying high-power RF into the chamber (but those
of you who are pushing 3 kW into your chamber have probably already heard of
this technique).
Bottom line is; ground that coax, no matter how you do it (well, don't use a
pigtail strap with alligator clips). It's simply good engineering practice.
Ed Price
[email protected] <blocked::mailto:[email protected]> WB6WSN
NARTE Certified EMC Engineer
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780
Military & Avionics EMC Is Our Specialty
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