RF transmission testing inside a Faraday cage

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 .

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

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