Microwaves going through a hole - attenuation?

Question

We have a microbalance that has a "hangdown wire" supporting a sample pan in a steel chamber. We need to heat this with microwaves, most probably at the common 2.4GHz of microwave ovens.

This wire will probably be a quartz fiber. However, it has to enter the chamber through a hole and must not touch the sides of the hole. External to the chamber there are some expensive and sensitive electronics, not to mention people.

For a given diameter of hole, what attenuation of the signal can I expect?

Is there any way of blocking the microwaves entirely given the mechanical constraints? eg multiple baffles.

Naturally we will be starting at very low power levels of a few Watts before pumping in kW pulses.

Can you instead of just a hole make it a tube with that inner diameter? — winny, Mar 12 '24 at 13:57
@winny Yes, but will that help or just act as an inefficient waveguide? — Dirk Bruere, Mar 12 '24 at 13:59
@DirkBruere Aren't there already holes in microwave oven doors? Have you tested your sensitive equipment near one to provide some initial experimental results to consider? — periblepsis, Mar 12 '24 at 14:04
It would attenuate the RF even more than a hole of the same inner diameter. — winny, Mar 12 '24 at 14:19
See more here: https://electronics.stackexchange.com/questions/413012/effect-of-wall-location-for-guided-wave-attenuation — winny, Mar 12 '24 at 14:33

score 5 · Accepted Answer · answered Mar 12 '24 at 17:37

The best thing to use will be a 'waveguide beyond cutoff' attenuator. This absolutely guarrantees a minimum level of screening, easily calculated, and easily increased to any value in a simple structure and compact size.

It is a tube of diameter much less than the wavelength of the radiation. An evanescent wave goes along the tube, being attenuated by 27 dB for every tube diameter. The attenuation provided by a tube 3 times as long as its diameter will be coupling_in_loss + 81 dB + coupling_out_loss. With a small hole in mesh, the only attenuation you get is the coupling_in_loss + coupling_out_loss.

Do bear in mind that, as you suggested, you must use something insulating like quartz for the fibre. Any attempt to use a conductive wire will result in you having built a coaxial cable through the wall, whether the wire passes through a hole or a tube.

This type of isolator was extensively used in a company I worked for, building low noise signal generators, which passed high speed data into a screened enclosure, using a LED, and tube soldered through the wall, and a photodiode at the far end.

That paper is a nice gem! Thank you for mentioning it. – Kuba hasn't forgotten Monica Mar 12 '24 at 19:35 — Kuba hasn't forgotten Monica, Mar 12 '24 at 19:35

score 4 · Answer 2 · answered Mar 12 '24 at 15:04

You should be looking at the mesh requirements for a Faraday Cage for the appropriarte microwave frequencies. The diameter of the Quartz wire has not been given. If you examine a microwave oven door you may be able to see a mesh size that will be suitable for your purposes.

Wavelength at 2.4GHz is ~12.5cm. Rough rule of thumb is a recommendation of a hole in a Faraday Cage should be less than 1/10 of the wavelength i.e. 1.25cm. Personally I would go with a much smaller mesh, as closs to the Quartz wire diameter as you can manage. If you are going to be using power with people in the area I would recommend the device's leakage be measured, engage an RF engineer and have an appropriate testing, safety and maintenance processes documented, etc. Under no circumstances should power be applied with a door open. Typically this will require two door sensors so failure of a single sensor won't enable power to be applied.

It looks like we will be doing some low power tests eg putting a stub in a waveguide, then cutting holes in the waveguide and measuring leakage. We can certainly use holes smaller than 5mm diameter — Dirk Bruere, Mar 12 '24 at 15:09

Microwaves going through a hole - attenuation?

2 Answers2