RF transmission testing inside a Faraday cage

RE: RF transmission testing inside a Faraday cage

rsjawale24 — Mon, 13 Jun 2022 09:50:41 GMT

Hi! An antenna designer here. Read some very interesting comments on this post. A fully metallic chamber is called as a reverberation chamber and is used to determine the efficiency of the antennas. Anechoic chamber is used for testing the radiation pattern, gain, etc of the antenna. The cost of building an anechoic chamber is very high. We recently got one in my lab and it cost us around 12800 USD and it works only till 18 GHz.
The size of the cone determines the absorption levels and the frequency of the chamber.
Free air testing is also possible for radiation pattern measurements given that there aren't many obstacles in the room so as to minimize the reflections from the objects.

RE: RF transmission testing inside a Faraday cage

14rhb — Sun, 12 Jun 2022 15:39:12 GMT

Great link beacon_dave - I didn't know the 3rd mouse pulled up all the graph adjustments in GNU Radio - so very worthwhile watching for me. The bit on 'good neighbours' was also very useful as he went about demonstrating how to reduce the unwanted noise in adjacent BW. Thank you.

RE: RF transmission testing inside a Faraday cage

14rhb — Sun, 12 Jun 2022 15:24:08 GMT

Good point: my idea with metalised surface would just bounce the EM wave down into the valley, I guess at extreme it would then eventually reflect back out?

For low cost approaches: Carbon from lumpwood charcoal perhaps - pulversed in an old blender and mixed with cement? Render it over plasterboard or plywood substrate?

Some commercial materials look like pieces of antistatic foam from IC boxes. Maybe larger sheets of antistatic matting could be used to line a timber framework? https://uk.farnell.com/c/static-control-site-safety-clean-room-products/esd-protection-products/anti-static-mats

RE: RF transmission testing inside a Faraday cage

ralphjy — Sun, 12 Jun 2022 15:09:35 GMT

It seems the trick is in using the absorptive material (he was using carbon and graphite). It would be a fun experiment but I couldn't justify the time and expense to do it. Maybe if I were designing antennas...

I was thinking absorptive foam (flat sheets) might be good enough for my purposes if I could find something inexpensive to try.

RE: RF transmission testing inside a Faraday cage

14rhb — Sun, 12 Jun 2022 13:32:13 GMT

It was at the higher end of homemade solutions and definately messy, so good call on giving it a miss. I think it would likely cost quite a lot of money in reality give the price of epoxy and even then the results are uncertain.

I've wondered if lower cost approaches would work. Typical RF anechoic material is a series of pyramids like:

photo source: from Google search => https://www.comsol.com/blogs/modeling-rf-anechoic-chamber-using-periodic-structures

I wonder if a vacuum former would be able to make something like this which could then be spray painted with metalic paint ? Might result in being able to make many panels reasonably cheaply ?

RE: RF transmission testing inside a Faraday cage

shabaz — Sat, 11 Jun 2022 19:27:38 GMT

I think you did the right thing, there's so much use to be had with the DVB-T dongles, amazing what can be done at such a low price.

The Lime SDR Mini version (and now presumably the 2.0) is the best value of all the Lime SDR models, but the documentation is really bad. I've found it quite crippling to do stuff (I don't have the Mini, but the original model, which was overkill, way more capability hardware-wise than I needed, but the documentation still sucks).

If I had the choice, nowadays I'd try ADALM-PLUTO over the Lime SDR, or if a business was paying for it, then I'd want to try the Ettus SDR that was Roadtested by some users a while back.

RE: RF transmission testing inside a Faraday cage

ralphjy — Sat, 11 Jun 2022 18:59:50 GMT

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.

RE: RF transmission testing inside a Faraday cage

ralphjy — Sat, 11 Jun 2022 17:45:45 GMT

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 ☺...

RE: RF transmission testing inside a Faraday cage

beacon_dave — Sat, 11 Jun 2022 13:47:38 GMT

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.

RE: RF transmission testing inside a Faraday cage

14rhb — Fri, 10 Jun 2022 19:23:20 GMT

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.

RE: RF transmission testing inside a Faraday cage

ralphjy — Thu, 09 Jun 2022 16:10:50 GMT

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...

RE: RF transmission testing inside a Faraday cage

colporteur — Thu, 09 Jun 2022 14:59:49 GMT

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.

RE: RF transmission testing inside a Faraday cage

ralphjy — Thu, 09 Jun 2022 14:37:58 GMT

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

RE: RF transmission testing inside a Faraday cage

shabaz — Thu, 09 Jun 2022 12:50:36 GMT

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

(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.

RE: RF transmission testing inside a Faraday cage

ralphjy — Thu, 09 Jun 2022 05:11:26 GMT

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

RE: RF transmission testing inside a Faraday cage

ralphjy — Thu, 09 Jun 2022 05:05:23 GMT

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...

RE: RF transmission testing inside a Faraday cage

ralphjy — Thu, 09 Jun 2022 04:56:15 GMT

Thanks Shabaz.

I'm trying to compare the transmission performance of a number of BLE boards in their normal use configuration, so I don't want to bypass the antenna. On boards with an external antenna connection, doing a direct connection as you suggest would provide a good baseline.

I am looking at Direct Test Mode as it would allow me to automate tests, but it will require external control via UART. I hope that I might get that working for a couple of boards.

RE: RF transmission testing inside a Faraday cage

ralphjy — Thu, 09 Jun 2022 04:37:08 GMT

Another channel subscribed to...

RE: RF transmission testing inside a Faraday cage

ralphjy — Thu, 09 Jun 2022 04:36:42 GMT

Thanks for the link. I enjoyed the first section of the video (didn't have time for rest of it). Reminds me that I really would like to get into SDR someday soon...

RE: RF transmission testing inside a Faraday cage

ralphjy — Thu, 09 Jun 2022 04:27:17 GMT

Thanks for all your input and sorry for my unresponsiveness - I had an unplanned trip to urgent care which consumed the better part of my day. Urgent care is definitely a misnomer lol...

I realized that I was a little ambiguous about the specific type of information that I was looking for. For an antenna based transmission system, what is the minimum separation between the antenna and the shield that would not compromise the RF transmission. Question is oversimplified as it will be affected by the type of antennas used. I guess I was trying to figure out if there was any value in building a small cage for reducing measurement noise. This is probably a science project 😄.

RE: RF transmission testing inside a Faraday cage

scottiebabe — Wed, 08 Jun 2022 21:56:07 GMT

Spoken from the King 👑 of SDR, you know it has to be good! lol

RE: RF transmission testing inside a Faraday cage

beacon_dave — Wed, 08 Jun 2022 19:22:08 GMT

🙂

"...I know some people who use discarded microwave ovens..."

SDR with HackRF One, Lesson 11 - Replay - 720p

https://youtu.be/DbuQsq4QjR4?list=PL75kaTo_bJqmw0wJYw3Jw5_4MWBd-32IG&t=418

RE: RF transmission testing inside a Faraday cage

scottiebabe — Wed, 08 Jun 2022 18:46:11 GMT

For rudimentary functional tests, I have heard of individuals using old microwave ovens as 2.4GHz faraday cages lol

RE: RF transmission testing inside a Faraday cage

colporteur — Wed, 08 Jun 2022 18:43:50 GMT

I worked for a Canadian national telecommunication company that supported a cellular network. There was a Faraday cage facility on one of the floors for testing RF devices. I visited it once or twice and found it difficult to work in the isolation. Not only was RF eliminated but sound also. It reminded me of the freezer door closing and being trapped inside. I understand it was expensive to build.

RE: RF transmission testing inside a Faraday cage

shabaz — Wed, 08 Jun 2022 16:37:03 GMT

Hi Ralph,

For transmission testing, just a spectrum analyser type of tool can be used (with appropriate attenuation), and a coax would be connected from the 50 ohm output (i.e. not solder any antenna matching circuit). This provides the information on the power level without any antenna connected. No large enclosure is needed, because the entire transmitted signal goes to the attenuator/spectrum analyzer (the small transmitter board itself may have a shield, but it won't affect the measurement because the spectrum analyzer will only be sensitive to what arrives via the coax).

For antenna testing, a literally field measurement could be done; not super-accurate, but better than nothing, useful for comparing antennas (rather than precise absolute measurements), since an antenna chamber is (guessing) expensive even for most firms (most organizations pay external consultancies to do their antenna work for them, as far as I'm aware).
For the outdoor field method, any field could be used, e.g. large garden or park. For an example of the type of measurement possible, I attempted in a small garden using 2.4 GHz, here (near the end - see the section 'measuring radiation pattern': /products/roadtest/rv/roadtest_reviews/442/molex_24ghz_5ghz_ant_4

A large park should be way better though.
I used a log periodic antenna to perform the measurement, and an SDR (not as accurate as a proper spectrum analyzer, but lower-cost). The log periodic antenna is low-cost from ebay.

Also, often WiFi/BLE chips have test modes, to simplify measurements, e.g. to just transmit at a spot frequency rather than across multiple channels, or to continuously transmit test patterns or data.

At a place where I once worked, we would use the outside field for testing (we had a license for that), for a different frequency band far lower than WiFi/BLE. It was handy for the occasional test. However all real testing was done with a spectrum analyzer or power meter, i.e. all done at 50 ohm, connected through coax.

One more approach (if SDR/spectrum analyzer is not available) useful for a more end-to-end test, is to simulate the transmission path loss only, by chaining loads of attenuators together, until you have a signal level realistic for the receiver, and then measure (say) received and demodulated signal quality, or bit error for digital transmissions, or any other metric, such as successful packets received (e.g. CRC being correct could be used). For such a test, again coax is used with 50 ohm connection from transmitter to receiver, but insert in a load of attenuation (realistically you may need 100 dB or more, and they usually come in various values up to 30 dB, so you'd need a chain of them).