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

[Carcia, Francis A HS]

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 <mailto:Aidehua at aol.com>  
To: premium-rx at ml.skirrow.org <mailto: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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