[Premium-Rx] Faraday Shield.....To Be, or Not to Be?? Need advice, please!

Fri Jan 7 16:48:19 EST 2005

A comment about MIL-STD-285 testing.

It is a good test sequence; it places the receiver in the screen room
and the RF source outside.

In my experience, however, I am more concerned about my computers and TV
getting into my antenna; i.e., you don't want the stuff inside the room
to "leak out".  For that reason the NSA 65-6 procedure is better;  it
places the emitter in the chamber.

If all is fair, you can do the test either way.  Unfortunately that
isn't always the case.

Gary WA0SPM

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	-----Original Message-----
	From: premium-rx-bounces at ml.skirrow.org
[mailto:premium-rx-bounces at ml.skirrow.org] On Behalf Of Carcia, Francis
A HS
	Sent: Friday, January 07, 2005 1:37 PM
	To: 'monitor'; Aidehua at aol.com; premium-rx at ml.skirrow.org
	Subject: RE: [Premium-Rx] Faraday Shield.....To Be, or Not to
Be?? Need advice, please!

	MIL STD 285 is the one we just had to pass. 

		-----Original Message-----
		From: monitor [mailto:monitor at referencevideo.com]
		Sent: Friday, January 07, 2005 3:06 PM
		To: Aidehua at aol.com; premium-rx at ml.skirrow.org
		Subject: Re: [Premium-Rx] Faraday Shield.....To Be, or
Not to Be?? Need advice, please!

		Hi Ed,

		Yes, you absolutely can screen your room, and it's
fairly simple.  Be sure to keep the workmanship precise, as even a tiny
hole, gap, or tear will not only leak, but may actually act as a slot
antenna with unpredictable results (all the results stink, though).

		First, regardless of what anyone offers as info, read,
research, etc every item you can find.  Be sure to look into Mil
Standards;  they exist for grounding & bonding, shielded rooms, etc.
This stuff is getting scarce, as the mil guys are killing access to all
sorts of stuff.  I have it all if needed...just have to get into my
packed-to-move PC's.    Try a google search for keywords such as:
bonding, grounding, military standard, etc...combine those keywords to
call up the more specific stuff.  Be sure you understand what you are
working with before attacking the project.   All, and I mean all, parts
and supplies came through ebay at steal prices.

		I have build, used and maintained a screened room and it
worked beatifully AND served as a fantastic learning platform.  I used
aluminum window screen under the floor, behind the walls, and above the
ceiling.  Windows, antenna panel, etc penetrations had the screening
folded into the hole and stapled in place in prep for panel and shielded
window bonding.  Seams are triple-folded and stapled about every inch.
Power is a 50 amp 2 phase (220VAC) feed with braided shield between the
pickoff box and the shack box...the shield is broken and overlapped with
insulation so as to isolate ground loop on the shield.  All power wiring
is kept inside a bonded steel box where a 1-- amp EMI filter does
initial filtering... just this filtering silenced the power feed, which
carried all sorts of noise and RF signals.  A shielded umbilical then
feeds a 8KVA 220/110 VAC isolation transformer with integral power
regulator...ebay unit apparantly retired from Washington State Police
facility.  All power in the room is distributed through this filter and
iso transformer, including lights, airconditioning, etc.  Use only
incandescent lights, no low voltage.  Depending on your air conditioner,
you may need a power filter on the AC cord.  You'll also have to bond
the metal airconditioner housing to the screened wall.

		Your antennas, control lines, and phone must enter/exit
through a metal (aluminum) panel that is bonded to the screened walls.
F-to-F "N" barrel bulkheads  are on ebay all the time.  You can
buy/build simple emi filters for control and phone lines.  If you stick
to this discipline, you won't have to deal with RFI coming in on your
antenna lines.

		The steel door idea mentioned in one of the other emails
is a good one...of course, I build my own shielded door, which works,
but was a big project in itself.  You can find the required beryllium
copper fingerstock on ebay at crazy low prices.

		By the way, after all this, the 60-70db figure mentioned
elsewhere is about all you'll get...so, I then applied a second layer of
screening, also known as fence wire/dog screen, to attack the magnetic
portion of the fields;  this raised my shielding level to about 87db,
which indicates I have a magnetic leak somewhere, I think.

		To do this right is a lot of precise work, but is well
worth it.  There is nothing quite like sitting at the bench making a
measurement or test and having, in essence, no ambient RF.  BTW, now
that you'd have no ambient RF, such issues as PC EMI and
receiver-generated EMI are easy to find...read up on "near field
probes", "near field measurements".  You can buy or build probes for
scope/spectrum analyzer that can be used to zero in on chassis and
digital display leaks, 

		Ed, you can contact me privately for further discussion.

		regards

		John Collins

			----- Original Message ----- 
			From: Aidehua at aol.com 
			To: premium-rx at ml.skirrow.org 
			Sent: Wednesday, January 05, 2005 1:59 PM
			Subject: [Premium-Rx] Faraday Shield.....To Be,
or Not to Be?? Need advice,please!

			I am building a radio room for my Premium-Rxs
and transceivers from scratch, and was wondering the pro-and-cons of
constructing a totally shielded (screen room) approach, with bypassed
120, and multiple short grounding paths to the earth (all bonded, of
course!).  In other words, a Faraday shielded room for ultraquiet
conditions and maximum RF suppression.  I am turning to the experts on
this List, as I'd like the appropriate guidance.  Your input is
appreciated!  Happy New Year!!

			73,
			Ed NI6S  

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