microwave sensor independant of polarization

[pdp114]

Hello there,

I am looking for a microwave sensor for 140Ghz which can read this radiation without discriminating certain polarizations.
Currently people have been using Schottky diodes, but these have waveguide inputs that favour linear polarizations...

Does anyone know of providers that have diode detectors with circular waveguides?

I have been suggested bolometers, but these are too slow... (<1ms response needed)

Does anyone have any other sensor suggestions?

Thanks a lot!

 

[VEBill]

I don't know much about 140 GHz (who does? ), but there's an old technique of inserting a diagonal dielectric block at the aperture that will effective convert a linear antenna into circular polarization. If this trick could be applied at this frequency range, and calibrated, then it might be useful.

CP should ideally respond equally to linear signals at any angle. But nothing is perfectly perfect.

Good luck.

 

[biff44]

well, I would use a circular polarized antenna (probably in round waveguide), then convert it back to single mode waveguide, and then hang a schottky diode detector across the E field direction. Schottky diodes are pretty fast, if loaded with a resistive load on the bias port.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[pdp114]

Hi there,

Thank you for your answers!
140 GHz is indeed a rare frequency. For microwave experts it's rather small a wavelength, and for optics people: it's just too far in the infrared! We are using it to heat electrons in plasma (ECRH).

The problem I have is that I wish to measure incoming power regardless of polarization, I should have mentioned that before.
In most microwave applications you focus in either circular or linear polarizations to narrow down on your signal, but I simply need the detector to measure any polarization after several reflections from diffusive walls.

biff44: Say I use the scalar feedhorn (or the conical horn) in

http://www.millitech.com/AntennaQuasioptical.htm

, and I use the "POL" and "WAC" a circular to linear polarization adapter (same site).
However, I am not sure the adapter welcomes circularly polarized waves and couples them fully inside the rectangular-waveguide output (To then go into a Schottky diode, which, yes are absolutely amazing when it comes to time responses). I worry about the title it's a linear-to-circular polarizer, but a circular to linear 'adapter'. Do you think the adapter could reflect part of the incoming (elliptically polarized) power to extract the linear (rectangular waveguide mode) polarization? ... I need to go back to the textbooks for this one, and your views are highly appreciated!

Thanks again!
FYI: We are running some tests with Pyroelectric sensors (gas/fire/motion detectors) for power measurement: about 10 times faster than bolometers. You have to modulate these sensors, but they work pretty well.

 

[Higgler]

Use a dual polarized (circular?) waveguide horn. If your test equipment can measure phase, or compare the phase of the two polarizations) then the polarization of the incoming energy can be determined exactly mathematically. If no phase information, it would help to have two dual polarized horns so that polarization could be V,H,slant left, slant right. If it's circular polarized energy, you won't know which, Right or Left hand.
If multiple signals come from different locations into the horn, to fully characterize the energy, it gets more complicated and costly (Rotman lens dual polarized arrays)