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
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