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I've got an EMC testing setup of a 1 meter shielded tent, and a very high gain EMC antenna.

I'm seeing a phenomena that I can't quite explain.

I have a known product that was tested in an accredited 3 meter chamber.

I have a known comb generator that was also tested in a 3 meter chamber.

(dbuV/m are approximate):

In the 3 meter chamber, the comb generator at 100MHz is 40 dbuV/m (that's right at the Part 15 limit).

In the 3 meter chamber, the known emitter at 100MHz is 43 dbuV/m (so it failed by a few dbuV/m)


So now I take the known comb generator, and put it in my 1 meter shielded tent.

In the 1 meter shielded tent, the comb generator at 100MHz is 50 dbuV/m (there are unknown cable losses, scenario not calibrated, etc... but bear with me).

Now I take the known emitter, put it in the 1 meter shielded tent, and at 100MHz the signal is 20 dbuV/m.


From the calibrated chamber readings, we know these should be the same value or very close to each other.

What sort of phenomena could I possibly be seeing? Could the comb generator, as an intentional transmitter, be reflecting in my tent and somehow increasing the perceived signal strength?

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    \$\begingroup\$ It depends on the path geometry from antenna-to-wall-to E-field sensor. In reflective tent with a broad sweep you should see the comb generator have Ricean null patterns just before the peak amplitudes. A loose wall reflection orientation will make a broader range of frequency nulls then adds than an orthogonal Faraday cages like the ones I've tested. 1/4 wave at 150 Mhz is 0.5m which becomes a 1/2 wave to reflected centre. similarily 1/8th wave to the wall at 100MHz becomes a 1/4 wave null at the antenna. \$\endgroup\$ – Sunnyskyguy EE75 Apr 19 at 0:48
  • \$\begingroup\$ can you /use 2 strings to pull the sources and antenna to slightly different positions? \$\endgroup\$ – analogsystemsrf Apr 19 at 2:37
  • \$\begingroup\$ @analogsystemsf - - yes i could move all the elements around. What would i be looking for? I've got a third known emitter lying around to toss in the mix. I thought the comb generator would be a good baseline, but I feel like I'm seeing some other rf effect in this tent \$\endgroup\$ – Leroy105 Apr 19 at 3:31
  • \$\begingroup\$ @Sunnyskyguy EE75 - - what is a Ricean null look like? The comb generator sweep in the tent looks like just like the 3m chamber reading. But the amplitude of the readings are just whacked. I don't know if it's reflections, some near field vs. far field antenna phenoma, or something else. \$\endgroup\$ – Leroy105 Apr 19 at 3:36
  • \$\begingroup\$ @Leroy105 - I'm not sure this is possible, but have you tried to run the measurements outside the shielded tent? The idea is to try to avoid reflections as much as possible. With a 3m wavelength you're probably better off measuring outdoors, or at least having your antenna pointing outdoors. Another thing to keep in mind is that all the measurements you've taken (including the chamber one) are near field so things may not be as predictable as you may think. Is the DUT/antenna distance the same for both chamber and tent measurements? \$\endgroup\$ – joribama Apr 19 at 4:51
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Here is a random arrangement I made with a generator and output measured in a transmission line with reflections to simulate an RF tent with reflections.

The peaks and valleys are due the waves adding and cancelling on certain frequencies that correspond to different wavelengths.

enter image description here Ricean Fading

  • the dominant wave can be a phasor sum of two or more dominant signals, e.g. the line-of-sight, plus a ground reflection.
  • This combined signal is then mostly treated as a deterministic (fully predictable) process, and that the dominant wave can also be subject to shadow attenuation.
  • This is a popular assumption in the modelling of radio channels with building, ground and wall reflections

Recommendations.

Although a Faraday tent may be good for blocking signals going out ( egress) or in (ingress) , as you can see, standing waves due to reflections can add or cancel at specific wavelengths in your regon of measurements. A reflective tent lacks the RF absorbing materials common to expensive anechoic chambers. (ferrite sprayed cones)

  • best test is outdoors in a valley with low background noise.
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    \$\begingroup\$ This is not intended to be an accurate model of your test setup, which I have not seen. It is just a random one with 50 Ohm transmission lines and delays and mismatched terminations and lengths shown that happens to demonstrate peaks at 100 MHz. \$\endgroup\$ – Sunnyskyguy EE75 Apr 19 at 5:27

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