1

I'm working in an office with a Wi-Fi router, desktop, and a few cell phones making RF noise.

I have an 8 feet long by a 3-feet wide table, and I have an Aaronia BICOLOG 30100 + spectrum analyzer + comb generator (at calibrated EMC limits).

I'm getting too much ambient noise from my surroundings.

I was thinking of a giant cardboard box coated in aluminium foil inside and outside of the box, as a makeshift Faraday cage since I can calibrate my antenna readings off the comb generator.

I can put my DUT and antenna inside the cardboard box at the same time, and probably be within a few dbu/V of an actual reading since the comb generator is calibrated from a real chamber.

Has anyone come up with something better for a homemade Faraday cage?

Our office doesn't have space for a shielded chamber at the moment!

Swanand
  • 3,245
  • 5
  • 28
  • 45
Leroy105
  • 1,837
  • 3
  • 21
  • 34
  • Anyone try one of these shiedled tents ever? https://www.select-fabricators.com/rf-emi-shielding/rf-emi-test-shielding/portable-enclosures/ – Leroy105 Feb 19 '19 at 00:41
  • Consider a hardwire to your wifi router. – Optionparty Feb 19 '19 at 00:53
  • Also see the chart on [Shielded tents and rooms and enclosures](https://hollandshielding.com/Faraday-cages-EMI-RFI-shielded-tents-rooms-and-shielded-enclosures). – jonk Feb 19 '19 at 00:55
  • @Optionparty, I've got 60dB of pre-amp running to the BICOLOG -- I think that antenna is picking up my building suite mates Wi-Fi at this point. I'd need to move to Montana. – Leroy105 Feb 19 '19 at 01:01
  • @jonk -- I think you can imagine what this is being used for.... I feel like the tent may be the only solution. I think my missus is not going to be pleased with the size tent I'd buy, if I'm for a tent... – Leroy105 Feb 19 '19 at 01:02
  • @jonk -- EMI fabric off Alibaba is pretty darn cheap. I could see just making a tent out of plastic tubing and importing the fabric. But I really don't want to torture myself like and find out my fabric is no good or some other gotcha. Tin foil on a box, would be marvelous. – Leroy105 Feb 19 '19 at 01:05
  • @Leroy105 One way to find out. Make it and then see what you get. But keep in mind that chart. Also go look at [EM Shielding -- wiki](https://en.wikipedia.org/wiki/Electromagnetic_shielding). You might get some ideas from there. – jonk Feb 19 '19 at 01:13
  • 1
    I have purchased "Mission Darkness" "TitanRF Faraday Fabric" in the past. It is Nickel plated Copper, with RF shielding rated at 80-100dB. Not a recommendation, just a good rating. – Optionparty Feb 20 '19 at 03:35
  • @Optionparty Noted. Thanks! I'm considering a tent myself more seriously. That is a very good rating to most cheap tents – Leroy105 Feb 20 '19 at 14:06
  • Use "clamp on" RF chokes for all cabling entering/leaving your tent. – Optionparty Feb 21 '19 at 15:42

3 Answers3

2

Reading the yearly "this is our conference on EMI for this year", named Interference Control, I recall an article on surviving EMP. The article discussed using ---- yes ---- aluminum foil inside rooms, with TWO LAYERS of foil, overlapped 50%.

The 50% overlap provides a HUGE capacitance between the two layers.

If aluminum-oxide is the dielectric, along with some air, then we can compute the capacitance between 0.25meter by 2meter (1 foot by 8 feet) overlap of two layers of foil, each layer composed of 2 foot by 8 foot pieces running up the wall, across the ceiling, and ON THE FLOOR.

What is that capacitance? Assume air (thickness 1mm thick, even if you use staples every inch to pin the two layers to the cardboard). 0.25 meter by 2 meters

C = Eo * Er * Area/Distance

C = 9e-12 Farad/meter * Air=1 * (0.25 m * 2 m) / 0.001 meter

C = 9e-12 * 0.5 / 0.001 = 9e-12 * 500 = 4,500 pF = 4.5 nanoFarad

Z(C) at 1MHz = 1/omega*C = 220,000,000 / (2*pi*1MHz) = 220,00,000 / 6.3 Million

Z(C,1MHz) = 40 ohms between sheets, if the spacing is 1mm.

If, under the staples, the spacing is down to 100 Angstroms (or whatever thickness is aluminum oxide), then that dominates and the impedance at 1MHz becomes 1mm/100Ang smaller or 1e-3 / 1e-8 = 100,000X smaller, or about a milliohm between overlapping sheets (50% overlap). Notice the staples are key.

How will you make a door?

Or have the foil on OUTSIDE of the large cardboard box, and just tape the door over with more foil.

analogsystemsrf
  • 33,703
  • 2
  • 18
  • 46
  • Ha. I am going to look for that article. I was considering staples as well. I'm not quite sure why the 50% overlap would be better from a physics perspective. But I sure can try the box for $10. I've got a control reading with the comb generator. I'm trying to do actual microcontroller emissions in the box. I can eyeball intentional emissions at 3 meters pretty accurately with the BicoLOG and any ambient noise. – Leroy105 Feb 19 '19 at 03:22
  • Why the enormous overlap? You use the very thin oxide layer to achieve VERY HIGH capacitance (over 1uF per 0.25 by 2meter overlap). Just cover the box in X direction, on all sides. Then rotate the foil 90 degrees on all sides. And staple. You will **NOT** have ohmic contact, because of the instantaneously-forming oxide. But the Z between sheets will be very low (well under an ohm, if my math is correct). – analogsystemsrf Feb 19 '19 at 03:29
  • 1
    @Leroy105 The thing about aluminium is that it naturally oxidises on contact with air, producing a very thin layer of aluminium oxide. Aluminuim oxide is an insulator. So two pieces of foil touching each other may actually be insulated from each other. A big overlap creates a capacitor, and that may be enough to look like a conductor at RF frequencies. – Simon B Feb 19 '19 at 13:29
2

I ended up buying a Tekbox shielded tent, a 4' x 2' x 2' model (~$2000 USD). I'll update the answer if it provides enough shielding to be used in a suburban office or not.

UPDATE:

I got the tent set up. It's almost good enough. With my Bicolog and the spectrum analyzer pre-amp (~20db), I get a little bit of ambient noise at 700MHz to 800MHz (I'm assuming that's cellphone band ambients).

It looks like it is nearly 40db of attenuation. If you get one of these make sure to seal the door completely, I had a small leak and had almost no effective attenuation from 700MHz to 900MHz.

I'm going to do some more micro-controller measurements for emissions and bring out the comb generator and see how accurate we can get for readings.

(Yellow trace is the ambient reading outside the tent, purple trace is reading of ambient inside the tent).

enter image description here

enter image description here

Leroy105
  • 1,837
  • 3
  • 21
  • 34
1

A Faraday cage is good for receiver testing with pad attenuation but not good for antenna radiation pattern testing due to the standing waves made inside the chamber which creates 2x peaks and nulls. This requires an anechoic chamber.

anecdote - ( fond memories of Faraday Cage at Burroughs EMC test centre in Paoli Pa. performing immunity tests with 1kW RF Amp and standing wave nulls on E field sensor. When I heard the 1kW Amp hum, I said this is (bad)then fixed it with a limiter on the sweep gen in a feedback loop.)

Best bet is to go offsite where they don't have good cell phone coverage with your battery-powered equipment where you dominate the background noise.

I've used the Aaronia Spectrum Analyzer up to 10GHz and it works well with waterfall charts and you might be able rig up a turntable synchronized to a slow time sweep on the SA.

Tony Stewart EE75
  • 1
  • 3
  • 54
  • 182