# Diffraction of Waves through a circular aperture

1. Jun 2, 2009

### mrh1192

Hi

I was just wondering if anyone knew a forumla for calculating the power intensity of waves, when diffracted through a circular aperture?

I have searched all over the net, but cannot find one which solves for the power intensity of the wave directly.

2. Jun 2, 2009

### Andy Resnick

The far-field diffraction pattern is a well known result [J1(kr)/kr]- what are you having a problem with?

3. Jun 3, 2009

4. Jun 3, 2009

### mrh1192

Thanks guys

What I am trying to apply this to, is the calculation of the wave intensity of an electromagnetic wave when it passes through a Faraday's cage, which is unable to successfully shield from all radiation.

In testing this, I have placed a phone inside a metal box, and slowly increased the size of a single hole in the box, until the phone started to ring.

Now that I have obtained the diameter of the hole necessary, I was wondering if it was possible to calculate the intensity of the waves that were passing into the cage.

5. Jun 3, 2009

### Andy Resnick

That's very clever- mind if I use that for a lab someday?

The easiest thing to do is simply find out the sensitivity of the phone- what signal level does it need to operate?

6. Jun 3, 2009

### mrh1192

Sure, go ahead!

I assume that the phone will operate with any signal level, as long as it isn't zero.

Is there a simplified expression that solves directly for the intensity of the waves, that you know of?

7. Jun 3, 2009

### Andy Resnick

Not for your situation- there's too many effects.

8. Jun 3, 2009

### Nick89

I highly doubt this! I can in no way justify my claim, but I would find it very odd if an extremely weak signal could make the phone operate.

You should at least try it with different phone models to see if it makes a difference.

9. Jun 4, 2009

### Born2bwire

Yeah, why not just make a monopole antenna and measure the voltage off of it? All you need is a copper plate and a coax cable. If you want to calculate the leaked the radiation, one thing you can do is use a periodic mode-matching wave solver. You can estimate the cage as mesh of rectangular cylinders with a finite conductivity. You can then solve for the transmitted modes and add up the significant contributors to estimate the transmitted power. This will ignore leakage due to seams and imperfect connections and such though.

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