[RFI] Fw: RF tight rooms - somewhat OT

Sun May 12 19:25:50 EDT 2013

Prior to my retirement in 2002 my primary objective was getting cell phone 
and radio coverage into almost every square foot of corporate buildings 
which were designed to limit the EMI, RF, and computer noise egress and 
ingress. Metal decks, metal wall panels and metal film or foil backing on 
ceiling and wall panels along with metal oxide or metalized exterior glass 
were in the building specifications and that's how the buildings were built. 
Along about the time glass and doors were hung, I'd be called to fix the 
cell phone and on-site two way radio coverage in-building. See 
http://www.emr.com/ for some of the BDA systems which evolved to solve my 
problems; multi-channel gain block Bi-Directional Amplifiers for Motorola 
trunking systems (think one HT on one channel very close to the BDA and 
another HT on a different channel at the maximum distance from the BDA 
during a time when four channels are being used).

I have no relationship of any kind with EMR Corp.  I used EMR products some 
of which were designed specifically for State Farm Mutual Insurance 
Companies.

73 ES DX,
Gary -- AB9M

-----Original Message----- 
From: Dale
Sent: Sunday, May 12, 2013 3:21 PM
To: GARY HUBER ; RFI
Subject: Re: [RFI] Fw: RF tight rooms - somewhat OT

Gary and All:

I feel compelled to comment at this point because I had more than 13 years 
of engineering experience in the RF shielding industry.  (I was employed by 
Lindgren RF Enclosures, Inc., based in Glendale Heights, IL.  Lindgren is 
now ETS-Lindgren, absed out of Texas, but still running the offices in IL 
and other cities around the world.  I have no current affiliation with 
ETS-Lindgren, but I do work as an independant RFI consultant.)

First, the folks at lessemf.com are indeed catering to those persons who, 
for whatever reason, feel some sort of threat from the RF fields that may be 
impinging upon them.  (I have to wonder how they ever get along without cell 
phones or pagers, or even computers.  After all, they are RF emitters.) 
However, some of their RF shielding products are, indeed, genuine RF shields 
when properly applied and used.  Remember, the only complete RF shield is 
that which employs the principles of a Faraday cage.  In short, the shield 
must be complete in all aspects as it surrounds the area to be protected. 
Any penetrations of the shield must be designed appropriately to either 
block RF ingress/egress or must absorb an impinging RF.  Incomplete shields 
may be thought of as being "field disruptors", in that they tend to deflect 
impinging RF.

Metallized fabric has been around for many years.  I don't know if I totally 
accept the claims for some of it to offer 50 dB SE ("shielding 
effectiveness") up thru 3 GHz as claimed, but it does work.  For example, 
transportable large shields made of such fabric have been used for RF 
testing of large military vehicles in the field when mods were required to 
those vehicles.  They are practical as shields, but did you note the prices 
per linear foot?

On the other hand, the information at the Mayes web page is pretty much 
right-on.  Techniques shown there are quite acceptable for low to moderate 
performance RF shield rooms.  In this case, I am talking about shielded 
enclosures having SE ratings in the range of 40 to 70 dB or so.  The 
greatest problems occur with doors and/or windows, as they become large 
apertures and must be designed to be as good as the solid walls, floors, and 
ceilings.  (Yes, you MUST shield all 6 sides of a room to be effective 
across a wide range like 100 kHz to 10 GHz.)  If greater SE values are 
needed, then it is time to talk to one of the specialty manufacturers of 
shielded rooms.  Case in point: the next time you go for an MRI scan, the 
chances are pretty good that you will be in a fully shielded room when you 
go into the machine.  Yes, even if the MRI machine is a mobile unit, mounted 
in a trailer.  (If you guessed that the trailer IS a shield room, collect a 
cookie or lollipop for making a good guess as you leave the machine.)  While 
not all MRI machines require a shield room, many do.  Typical specs call for 
100 dB SE in the range of 5 MHz to 100 MHz.

In case anyone is wondering, yes, there are standardized, accepted ways to 
measure the SE of RF shielding.  Prior to 1995, a very common document was 
MIL-STD-285.  That was eventually cancelled when the US Government accepted 
IEEE-STD-299-1997 as its replacement.  The IEEE 299 document was re-written 
by a large group of professionals from the shielding industry, as well as 
representatives from branches of the military and the EU countries.  Over 
most of the test frequency ranges, the standard calls for an RF source 
antenna to be located 1m from any surface of the enclosure (room) being 
tested and the detector antenna to also be 1m from the opposite side of the 
same surface.  In most cases, the source is outside the room and the 
detector (usually a spectrum analyzer) is inside the room.  Because the 
standard defines the measurement conditions and techniques to be used, and 
requires calibration of the field generated prior to making measurements, 
the results, given in dB of SE, are repeatable and accurate (within +/- 3 
dB, the usual measurement error).

The most recent version of the IEEE standard is IEEE-STD-299-2006.  There 
exists an ANSI standard derived from the IEEE document, and Europe does have 
a similar document in the IEC 61000 series of standards.  I was the Chair of 
the IEEE working group that wrote both the 1997 and 2006 editions of the 
standard.

I hope this clarifies some of the issues about RF shielding and measurement 
of shield performance.  As for the tougher issues of emissions from 
computers and appliances disrupting ham communications, the sad truth is 
that just about every installation will be different.  Probably the most 
significant driver is the separation distance between whatever the RFI 
source is and the "victim" device.  In the case of your radio, the radio is 
NOT usually the victim - your antenna is.  That is why it is important to 
get as much distance as possible between the antennas and gadgets in your 
house.  That is a rough situation when the antennas are mounted on or 
adjacent to the house.

73, Dale
WA9ENA

Pres. & Senior EMC Engineer
E-N-A Systems, LLC

-----Original Message-----
>From: GARY HUBER <glhuber at msn.com>
>Sent: May 12, 2013 1:35 PM
>To: RFI <rfi at contesting.com>
>Subject: [RFI] Fw:  RF tight rooms - somewhat OT
>
>see
>http://www.ramayes.com/Data%20Files/US%20Foils/EMI-Shield%20Installation%20Instructions.pdf
>
>I have NO relationship, financial or otherwise with the company RA Mayes or
>any of the products or services referenced in the link above.
>
>73 ES DX,
>Gary -- AB9M
>
>-----Original Message----- 
>From: mtnredhed
>Sent: Sunday, May 12, 2013 1:00 PM
>To: rfi at contesting.com
>Subject: Re: [RFI] RF tight rooms - somewhat OT
>
>I doubt he needs RFI tight, but he does need RF attenuated.  There is a
>company that caters to those that believe RF is doing bad things to their
>bodies (not endorsing that viewpoint by any means, just noting their target
>market).  Its called www.lessemf.com.  I've ordered the odd item from them
>in my own battles with local RFI (some cloth and paint to save an item from
>the bucket), and they seem to be legit.  The bulk materials (paint and
>fabric) appear to be real.  I got a patch of fabric that looks like drapery
>liner (slightly stiff), but not "wire mesh".  One wrap around my cell
>phone, and it's off the network.  Same thing with my VHF HT.  Haven't tried
>it with AM BCB.
>
>Some of their site is flat out looney tunes and good for a chuckle except
>that you have to realize that there are people who actually believe this
>stuff.
>
>jim
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