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Analog Angle Blog

RF Shielding: All or Nothing at All?

The growth in smart homes, with some many functions and nodes connected via Wi-Fi, has raised serious concerns about security and drive-by hackers. A friend suggested, half-jokingly, that perhaps homes of the future will have a mesh layer in the walls to block RF, functioning as a Faraday cage (while RF leakages due to windows and doors might be a problem, there are ways to deal with that, at a cost).

With this approach, connectivity would be via a cable (TV, Internet, and more) brought into the house, and in-home cellular access would be via a local nanocell connected to this cable. I was in a recently renovated gallery at a high-end museum, and when you crossed from the main corridor into this gallery, cell-signal strength disappeared, so I suspect they “caged” the room to prevent cell-phone ringing and talking. Or perhaps it was an unintentional side-effect of mesh placed in the walls for reinforcement if the concrete?

Will this be the high-end feature of the future? I certainly don't know, but it will be interesting to keep an eye on this.

Coincidentally, I saw an article several months ago about a hotel that was offering optionally shielded rooms as a guest amenity (The Hotel that Lets You Kill the Internet).What was especially intriguing was that, according to the article, “A silver switch next to the beds in the Villa Stéphanie spa resort in Baden-Baden [Germany] activates a copper grid in the walls to block all wireless internet signals.” The article claims that the walls also had a special coating which blocked 96 percent of the signals. (Quick question, without resorting to the web or an app on your smartphone: how many dB of attenuation is that? The answer is at the end!)

That started me wondering: is the RF block entirely passive, so the user switch just grounds or ungrounds the in-wall shielding? What's the effectiveness of an ungrounded cage compared to a grounded one at RF from, say, 500 MHz to several GHz (assuming the mesh pitch is fine enough)? Or, if the switch does not ground the mesh, how does this switchable passive system work? Or is it really some sort of active signal-blocking, which would be more costly, more complicated, and have regulatory complications? And how do you switch off the shielding effect of those coated walls?

Shielding is a major engineering concern in many deigns, either to block unwanted access or to minimize EMI/RFI issues. Yet while some engineers struggle to keep RF in (or out), other are faced with a problem at the opposite end of the spectrum (so to speak), that of maintaining transparency to RF while providing environmental protection to the antenna structure.

Around the time I saw the hotel story, I also came across a detailed technical article in Microwave Journal , “Efficient Design and Analysis of Airborne Radomes,” which showed how difficult providing both RF transparency and physical strength really is. Naïve as I was, I had foolishly assumed that this was not a big deal, but I was wrong, as both the material used and the placement of the panel polygons (their angles and support structure) are critical factors.

McMurdo Ground Station radome (Image courtesy of Seth White on NASA site)

McMurdo Ground Station radome (Image courtesy of Seth White on NASA site)

The problem is aggravated by the reality that these radomes are, by their nature, exposed to winds and storms and so are subject to significant structural loading on their panels, which must be both strong and RF-transparent—an often contradictory set of objectives. Reading this article gave me a lot of respect for those who must design this “simple” structure.

Have you been involved in large-area RF shielding, beyond the standard engineering-test anechoic chamber? Would you consider putting your house in a Faraday cage? Have you ever had to design a radome-like enclosure, even a very small one, to provide physical protection but with RF transparency?

[Answer: about 14 dB attenuation— by itself, that's only moderate amount, IMO]

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3 comments on “RF Shielding: All or Nothing at All?

  1. zeeglen
    October 13, 2015

    Wow, the marketeers really missed the boat on this one!

    96% power shielding is of course -14 dB, but a REAL marketeer would have instead expressed this in voltage ratio using 20 log and thus would be able to claim -28 dB.

  2. Paul Bryson
    October 14, 2015

    A faraday cage does not need to be grounded. I would assume that the faraday cage around the room is a simple passive conductive surface.   Probably, the switch connects/disconnects an active repeater or passive antenna. There is not much info in the article – the switch may just turn on the outlet that the wifi router is plugged into.

  3. Bill_Jaffa
    October 23, 2015

    Here's an interesting article about a new conductive wallpaper, Faraday cages, and how these cages are sometimes set up temporarily to prevent leaking at news events (“leakage” here is both figurative and literal):

    http://www.defenseone.com/technology/2015/10/new-material-promises-nsa-proof-wallpaper/123066/?oref=d-river

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