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Teflon Slab ... Circular polarization conversion

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tanuki

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swedish microwave polarizator

I've cracked open a dead DSS-LNBF to study.

How does a "slab" of Teflon (or other dielectric) in the feed horn circular waveguide convert circular polarization to linear?
 

sergio mariotti

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circular polarization conversion

The concept is easier to understand than explain by text.
I'll try.
1) The ensemble of radiatiatin wave energy is always the sum of two "orthogonal" polarization. We may speach of H&V or LHCP&RHCP.
2) A polarizer is a reciprocal device. If fed by Circular then a Linear come out and vice-versa.
3) A circular polarization is the vectorial sum of two linear one. For example just add the V with the H delayed by 90°.
4) Many practical modes exist to do this. The dielectric slab, since is slower than the air, will delay the linear polarization that flow into it (for example the V), the ortogonal one (for example H) don't flow into the slab, so it will be no delayed. So at the output You'll have a circular polarization.
5) apply point 2) so if you feed with a circular polarization , you'll have a linear one that is what you want.
6) Of course it works into a limitated bandwidth, tipically 10 to 20%.
7) depending on what do you want, you may collect only one polarization or both , in the latter case you need an OMT after the polarizator.
 

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tanuki

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circular polarity conversion

Thanks for your reply.

Ah ... so the dielectric slab needs to delay the perpendicular component by a 1/4 wave length (ideally).

As the circular wave travels along the slabbed wave guide the polarization becomes elliptical, until progressively, at some point, it is linear. The piece wise equivalent of light in a birefringent media.

The slab has dovetailed ends. Is this a tapered impedance matching?

The is no OMT structure (that I recognize) the waveguide (D= 15.82mm) continues 8.4 mm past the probes (the end is a metal cap ... i.e. shorted end).

I have noticed that the circular polarization probes (R/L) are at +45 and -45 degree angles to the slab, while the linear probes (H/V) are 0 and 90 degrees.

I'm confused by the +/- 45 degree probe angles. Can you help?
 
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tanuki

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circular to linear polarization conversion

+/- 45 degrees .... of course it's the vector sum of the resulting phase aligned waves.

Thanks again.
 

sergio mariotti

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teflon slab

tanuki said:
Thanks for your reply.

Ah ... so the dielectric slab needs to delay the perpendicular component by a 1/4 wave length (ideally).

As the circular wave travels along the slabbed wave guide the polarization becomes elliptical, until progressively, at some point, it is linear. The piece wise equivalent of light in a birefringent media.

The slab has dovetailed ends. Is this a tapered impedance matching?

Yes, of course. You have correctly answered yourself.
tanuki said:
The is no OMT structure (that I recognize) the waveguide (D= 15.82mm) continues 8.4 mm past the probes (the end is a metal cap ... i.e. shorted end).
Two, geometrically orthogonal launcher act as OMT. From your picture i see 2 launcher strip on PCB.
tanuki said:
I have noticed that the circular polarization probes (R/L) are at +45 and -45 degree angles to the slab, while the linear probes (H/V) are 0 and 90 degrees.

I'm confused by the +/- 45 degree probe angles. Can you help?

Once again you answered yourself... on the next reply.
In any case, to respect the simmetry, the PTFE slab cannot be aligned with any launcher on the PCB. So what be the angle that respect the simmetry? Only 45° !
 
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