do plastic enclosures act like microwave resonators at high frequencies?

[sarepairman2]

?

[vk6zgo]

No

[T3sl4co1l]

Yes.

[Bud]

Thanks guys, i learned a lot from this thread.

[Richard Crowley]

Yes.

Assuming non-conductive "plastic", that seems counter-intuitive.  Can you elaborate?

[rs20]

Guessing: Yes, because the relative permittivity of plastic is different to that of air, so reflections are possible. See the Wiki article on dielectric resonators for an example of how permittivity can be deliberately exploited to make resonators.

[T3sl4co1l]

To be precise, we're both correct.  What's lacking is degree.

Consider a hall of windows: each one is a little reflective, so you get a "hall of mirrors" effect between them, but because the reflections are smaller and smaller each time, you don't see very "deep" into the "hall".

The effective refractive index for common plastics at RF might be in the 1.5-2 range, so it's not insignificant, but it also depends on wall thickness (how close to 1/4 wavelength (or more) is it?), since we're in a "thin film" sort of regime.  Bubbles are quite transparent, but still look iridescent because they are some small number of wavelengths thick; we're talking even thinner here.

We're also talking near fields, so there will be evanescent waves conducting along the surface of, and within, the dielectric.

In any case, we're talking characteristic frequencies on the order of the characteristic lengths of the box: long waves simply can't be trapped inside a too-small box, period; long waves penetrate thin walls very easily; and even for very short waves (where the wall thickness is enough to reflect them), the effect won't be strong-as-in-metal-walls strong, just a modest peak/valley, probably not much more than 10 or 20dB.

So -- no, as in, most stuff won't rattle around.  But yes, as in, some will, if not by a whole lot.

Tim

[vk6zgo]

An enclosure is a very different thing to a solid piece of material which is optimised for the particular function.

IMHO,the permittivity of the volume of air inside the enclosure would be dominant,with the very small volume of plastic in the enclosure walls having a minimal effect

There would almost certainly be no conventional resonant cavity effect,as the plastic,even if it ceases to look like a good insulator at high enough frequencies,will look like a very lossy conductor.

[hendorog]

To be precise, we're both correct.  What's lacking is degree.

Should be possible to find out :- Download Sonnet lite or Feko (both free but with painful key registration) and test it.

[T3sl4co1l]

An enclosure is a very different thing to a solid piece of material which is optimised for the particular function.

IMHO,the permittivity of the volume of air inside the enclosure would be dominant,with the very small volume of plastic in the enclosure walls having a minimal effect

There would almost certainly be no conventional resonant cavity effect,as the plastic,even if it ceases to look like a good insulator at high enough frequencies,will look like a very lossy conductor.

Dielectrics can be just as reflective, in fact moreso*, than conductors!  My "hall of windows" example is relevant, indeed exact -- considering light is EM, just not coherently resonating, as we are looking for at RF.

*Check out "supermirrors".

A typical example might be ABS, having a wall thickness of 3 mm, dielectric constant of 2-3.5, loss tangent 0.5-1.9% (unstated frequency), in a 12 cm box.

The wall thickness is effectively sqrt(2) to sqrt(3.5) times thicker due to refraction, or 4.24 to 5.6 mm, which has a 1/4 wave reflection at 17 to 22.4 mm wavelength, or 13.4 to 17.6 GHz.  This would be about 75% reflective.  At double this frequency (i.e., half wave), it would likewise be extra transparent.  At inbetween frequencies, it will be a little over half transparent, and at very low frequencies (< 2 GHz or so), very transparent.

The box being about 12 cm across, suggests 5-7 wavelengths could be contained within, for resonant modes along that axis.  There will probably be one or two peaks and/or valleys where the wall reflection and cavity resonances coincide, and the energy inside the box will be several times greater (like I said, probably about 10dB, or less), assuming some means to couple energy into the box of course.

Regarding loss tangent: since it's not mentioned what frequency it was measured at, it's impossible to tell what might happen.  There may be molecular relaxation frequencies near the 10s GHz range, which would not only kick out lots of losses, but change the phase shift and dispersion along the way.  The reflectivity is unlikely to be any higher, so that the cavity mode peak might be only 3dB instead of 6 or 10dB, for lossy materials.  Take the lossless estimate as a ceiling figure, I'd say.



So -- if you're talking about more modest frequency ranges, you're absolutely right, it's simply not big enough or thick enough to contain resonances in the < GHz range -- but taking the EM spectrum as a whole (at least up to THz), some at least mildly interesting behavior will definitely be present.  Whether the OP was interested in the complete account, or just a more practical range**, I don't know...

**Amateurs I don't think are playing much in the 10s of GHz... but with bandwidth ever on the rise, it's getting more and more practical to consider these sorts of frequencies.  Not to mention already old standards like sat comms that operate in those ranges.

Tim

[sarepairman2]

well this question came about because I was wondering for compliance testing, lets say you have made a mock up of a product out of plastic that you glue together and it gets sent to a anechoic chamber. Then you make molds for hundreds of thousands of dollars. can you be sure that the molds will pass (since they are a solid piece rather then glued together sheets)

but the physics are none the less interesting because i can read a forum post but a text book on microwave physics is probobly not gonna happen anytime soon

[T3sl4co1l]

Ah, that's also a commercial concern because you can get SLA (or other 3D printed) proto enclosures, but they usually use different materials than the final product, which is going to be molded ABS or PC or something like that.

For that case, you'd want to check that:
- If you don't have anything over 2GHz or so, it probably doesn't even know the difference
- If you do, try to get a material with a similar dielectric constant.
- If you can't, roughly speaking, you can fudge the wall thickness by sqrt(k1/k0), where k1 and k0 are the dielectric 'k'onstants of the proto and final materials.  (If final k is higher, use walls thicker by that amount.)

Oh, and avoid lossy materials like PVC, unless you want that as a feature.

This would be fairly relevant for items like stick-on planar antennas for the 2.45GHz band (and GPS, which is.. kinda nearby), Wifi and BT being the most common services.

For on-board or free mounted antennas, try to avoid (2n+1)/4 multiples of wavelengths from walls, which will be reflective modes (again, if not much at 2GHz).  Butted right up against the enclosure might also not be a good idea (proximity effect?), but a little distance (1/10 wavelength maybe?) is probably good.  Half wavelength multiples should also be good.

For short distance comms like BT, it might not matter at all, or it might even be desirable to screw up the efficiency / range.  Again... bug/feature...

Tim