RF transmission testing inside a Faraday cage

I worked for Litton in the 70s - working on inertial navigation systems and our EMI test facility reminded me of a cold war bunker.  I have memories similar to yours - the predominant sound was the ventilation fans...

Thanks for the link.  Never thought about Faraday cages with respect to observatories, but it makes sense.  I'll need to look into it.

Hi Ralph,

I hope all is well.
I've rarely used a faraday cage larger than a biscuit/cookie tin so I'm the wrong person to comment, but gut instinct suggests as you say, that it could be a very experimental/science project to see if it meets your needs.

You might see benefits or disadvantages, depending on the precise test. The Faraday cage will help eliminate external interference and prevent broadcasting unwanted signals to others.

On the other hand, there are these issues/things to work through:

(a) if it is a very small cage, antennas would be used in near field (it's a distance proportional to the square of antenna dimension), so it's not the same as a normal far-field condition. This is almost definitely not an issue at all, if the test is protocol testing, but if the desired test is to examine antenna performance, then it is a factor

(b) the metalwork may cause some antenna detuning, depending on the particular antenna design, again it may or may not be an issue depending on what's being tested

(c) there will be reflections off the inside walls

(d) there could be an overload risk with close antennas (I guess not an issue for WiFi/BLE since these are at low power levels)

For (a) to (c) above, it may be of zero concern for some tests like protocol tests, but for others like performance tests (e.g. antenna performance) it could be an issue. I've never tried it so it's definitely worth taking what I'm saying with a pinch of salt.

I believe it could be very difficult to get both absolute measurements, and relative/performance comparisons, because even a slight change in angle or position could cause a different measurement, due to (a) to (c).

Ordinarily, for measurements with an antenna, it would need a location where there are very few reflections, e.g. outdoor field. Even quite large firms won't own antenna test chambers themselves, I'm not sure of the cost or practicality to self-build one, they have RF absorbing materials and shapes internally. The outdoor field should in theory give very good results, provided it is a large field, but even a small garden kind of works from my limited tests.

If you're mainly concerned about external transmissions influencing results (since you mention noise), I believe the value of the cage is reduced, because the effect of external transmissions is eliminated anyway, if you can control the transmit frequency (which you can if you're using the test modes), if you can tune out the unwanted frequencies (which a spectrum analyzer will do, as will any receiver). (Incidentally, using some test modes continuously outdoors is likely not legal in some countries, so that's a benefit of the faraday cage, but if there is no cage, keeping transmissions short in the ISM band (like a 1-second burst for a test) at typical BLE power levels, just from a technical perspective I cannot imagine would practically cause any problem to anyone, but from a legal perspective I cannot say, since it depends on your country, laws, etc). 

So, in summary, I believe this would definitely be a science project : ) and you can probably eliminate some of the potential issues with clever test designs, but I reckon you may be better off just using your existing indoor environment (or using a garden/field) and changing some things like transmit frequency, or move some tests to coax environment if not every test is antenna related, and so on. Changing receive antenna to something like log-periodic might be a benefit too. It's all a bit subjective though, and some things are hard to predict. You're likely learning all sorts of on-the-job tips and tricks just by trying different things.

Hi Shabaz,

Thanks for the your insight.  I've gotten the sense that this is something that I should avoid trying at the moment for the sake of not going down a rathole.

I recognize to some degree the issues that you pointed out.  Within the context of my project I realize there might be some issues with the accuracy of my measurements (due to effects that you mention and others), but I'm hoping to develop a reasonably simple and repeatable setup to compare boards and antenna configurations.

Using a Faraday cage would be a good learning experience and fun to try (although a pain to construct), so I'll hopefully find some time to try it in the future.

Ralph

Your description doesn't lead to a quote but is sufficient to frame some requirements. I trust your urgent care needs are satisfied. It is never a place I visit I suspect I will die from whatever ails me because I avoid going to urgent care for treatment. Our wait times could be in the double digits hours.

The cage I viewed was approximately a 12X12 cube used to test cell phones. I can't recall if they used it to test antenna structures but I suspect it would. Other than a desk with test equipment and mounting points ( you were not allowed to pierce the walls, even with a pin), there was little in it. I often wondered about power distribution in the room but I wasn't present when it was installed to ask probing questions.

I generally try to avoid urgent care, but I did an e-visit and was told to go in.  Hopefully antibiotics and a sock filled with warm uncooked rice will fix my problem in about a week... 

Hi ralphjy a while back I did some RF measurements as part of a Molex antenna selection test. I've gathered a few more bits since then like SDR but I noted a homemade anechoic chamber in section 7.1 of my test report: /products/roadtest/rv/roadtest_reviews/400/molex_24ghz_5ghz_ant

The anechoic chamber not only sheilds external RF interfereing with your measurements but it also attenuates internal reflections that would also render your measurements inaccurate.If you can afford it I ponder if the low-cost metal sheds would make an idea starting place. However, RF has a way of sneaking in the very smallest of gaps (depending on wavelenght) and via feed through cables and pipework. Commercial anaechoic chambers as well as screened rooms have metal finger seals around the doors and filtering on the cables/pipes.

The mention of using a local park or field is another great choice if you can work off battery power and if a public space be ready to explain your setup...maybe also to the local law enforcement! A step up would be a boat where you can get away from RF signals and nosey questions plus have a bit of power available, and a fridge for essential test refreshments

I hope you'll have a lot of fun experimenting and learning with BLE and RF. Good luck.

The current YouTube series turns a bit into an intro to signal processing rather than just SDR. It looks like there are plans to refresh some of the series due to changes in GNU Radio since it was originally recorded.

A lot of the newer SDR modules are unobtainable at the moment due to ongoing FPGA supply chain issues.

The new LimeSDR Mini 2.0 looked interesting and sees a move from the original Intel MAX 10 to the Lattice ECP5.

https://limemicro.com/news/lime-microsystems-unveils-the-limesdr-mini-2-0-with-larger-and-more-capable-fpga/

The Analog Devices ADALM-PLUTO active learning module looked quite interesting as well using the Zynq Z-7010 but unfortunately they don't appear to have come up with the same on-line educational package as they have with their instrumentation platforms.

https://www.analog.com/en/education/courses-and-tutorials/active-learning-module.html

In the meantime I ended up with a cheaper R820-T2 based DVB-T tuner type device.

Hi 14rhb,  I took a look at the link in your roadtest and also looked at the first video where he talked about the construction of the RF absorber I Want to Build an RF Anechoic Chamber.

Interesting, but definitely too much work for my project.  And I think my wife would frown on my making a mess molding a bunch of cones - although she is pretty tolerant of my projects .

I took a look at RF absorption material, but even small sheets of it are too expensive to build a reasonable enclosure.  A large field would probably be the best option as you and shabaz have suggested.  I could probably clean out a small metal shed, but it's too bad that absorbing material is so expensive.

I will have fun experimenting and I'm sure that I'll learn something - I already have ...

It looks like Crowd Supply still has LimeSDR Mini 2.0 units available for delivery mid-October.  SDR is something that I'd like to try, but haven't researched much about how well supported the different units are.  I guess as you say, many may not be currently available.