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@msn.com>
Sent: May 12, 2013 1:35 PM
To: RFI <rfi@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@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
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|