Actions
Forum Thread Details
  • Replies 26 replies
  • Subscribers 356 subscribers
  • Views 8930 views
  • Users 0 members are here
Related

RF transmission testing inside a Faraday cage

ralphjy

Since there is RF expertise in the community, so I thought I'd try to get some insight into an area where I don't have much experience.

I'm currently working a project testing BLE transmitters and my current workroom location can experience a fair amount of RF noise in the 2.4GHz ISM band (primarily due to existing WiFi and Bluetooth/BLE).  My primary mitigation when possible is to increase distance from or turn off noise sources.  I started thinking about whether I could test within an RF shield.  My previous experiences with testing within RF enclosures was over 40 years ago and we used room scale enclosures to house the equipment and operators.

I was wondering if anyone has tried small scale enclosures to shield only the EUT and measurement setup.  What would be a good minimum size at this frequency?

TIA for any feedback.

Ralph

  • in reply to beacon_dave

    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.

  • in reply to ralphjy

    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:

    Anechoic material series of pyramids, reflects RF down into the valles.

    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 ?

  • in reply to 14rhb

    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.

  • in reply to ralphjy

    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

  • in reply to beacon_dave

    Great link - 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.

  • in reply to 14rhb

    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.