[Premium-Rx] Faraday Shield.....To Be, or Not to Be?? Need advice,
please!
refmon
monitor at referencevideo.com
Sat Jan 8 01:43:07 EST 2005
MessageHi
This no-leak-out point is quite important. Keep the room leakage down 60db or better and you'll get a major improvement in local ambient noise floor...assuming antenna cable is properly grounded at the penetration point. Once you achieve that, you get into the side-hobby I've become interested in: nearfield spot probing of computer and receiver/transmitter chassis...I am shocked at the easily solved leakages that left the factory on such spohisticated gear. Crazy observation: my Bearcat 895XLT scanner (stock plastic case) has better emmissions than a stock Cubic R3030A when nearfield probing displays and keypads...not a scientific comparison....just sort of ironic.
Also, I forgot to mention that a shielded room is a Faraday Cage, and as such will work just fine non-grounded. However, to avoid the neighborhood smelling a barbeque at an odd time (you frying thy butt) , do ground it and ground it very well. Not only is this a safety issue, but it then provides more legitimate signal grounds. In fact, with 10 foot ground rods (consult ARRL, etc for installation methods), a Zero-Potential Ground Plane may be established; it is practical to ground equipment to any point on that plane. Keep looking for info-even try "TEMPEST" as a keyword...just watch out for the know-it-alls.
regards
John Collins
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----- Original Message -----
From: Gary Geissinger
To: premium-rx at ml.skirrow.org
Sent: Friday, January 07, 2005 1:48 PM
Subject: RE: [Premium-Rx] Faraday Shield.....To Be, or Not to Be?? Need advice,please!
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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