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I am part of a research team looking at the possible effects of EM fields on psychological function. The experiment calls for participants to perform a series of tests while inside a Faraday tent that blocks most of the radio spectrum. We'd like to have the participants perform the same tests inside the tent, but now while being exposed to roughly the same EM fields that are present outside of the enclosure. If we open the door or a panel to allow EM fields inside, then the participants will know. Instead we need the participants to not know whether they are being exposed to ambient EM fields.

Our current plan is to link 2 broad spectrum antennas outside the Faraday tent with a matched pair of antennas inside the tent. We'd have a relay switch in the cable to control the on/off function, and a preamp to slightly boost the signal. I'm not convinced this will work though.

What would you guys recommend? It doesn't have to be a perfect replication, just "good enough." We're most interested in reproducing the 25MHz-3GHz range.

Thanks for your suggestions!

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  • $\begingroup$ Just bring some cell phones into the room. $\endgroup$ – Phil Frost - W8II Mar 17 '16 at 20:51
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Something along those lines might work. I think you would need to use a spectrum analyzer hooked to a broad-spectrum antenna to analyze your test environment. You could record a sweep of the ambient spectrum outside the tent, another sweep of the interior of the tent with your "noise repeater" off, and another of the interior with your noise repeater on. Those results will tell you if your noise repeater works adequately or not. You might have a problem with the signal being reproduced more accurately at some frequencies than others. I would think that you would need the data from those scans to include in the writeup of your results.

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To add to the good answer from rclocher3,

1) as you will be looking for a very small effect, (possibly non-existent) it might help to be able to vary the levels in the tent, from zero to say 100 times ambient. Then you can plot a dose response curve, not just on/off. To do this you'd need quite a large, high gain amplifier, and a step attenuator, adjustable from 0 to 90 dB.

2) you must measure the power spectrum - the EM environment can vary by 1000x between places, depending on your proximity to broadcast towers, cell phone masts etc. You don't want that variation clouding the results. No need for a separate measuring antenna, just tap off a fraction of the signal you're injecting, into a spectrum analyser.

3) you might find that the external, source environment changes too much over time due to people with phones and wifi devices moving around the experiment.

It might just be better to generate controlled signals yourself, with a signal generator. Depending on how much you spend, you can generate quite realistic signals, but even the most basic signal generator would do for a simple pulsed or modulated signal at one frequency.

4) if you deliberately expose people to amplified RF signals you'll need to monitor compliance with safety standards. As a rough guideline, 1 watt into the tent is unlikely to exceed the safety levels, as this is what an average phone produces, against your body. With a 1 watt amplifier, you could prove compliance with only a calculation based on the amplifier specifications and the experimental setup. You might need more power than this to get useful fields at lower frequencies, but then you'll need some professional monitoring for safety.

5) you will need very broadband antennas, for example the TESEQ CBL6143. In fact you might consider renting an EMC facility for the test, they'd have all the antennas, amplifiers and the shielded room. But all the equipment can be rented loose, too.

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