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BBC: 'Twisted' waves could boost capacity of wi-fi and TV

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Proud Mary

Fri Mar 02 2012, 11:18PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

'Twisted' waves could boost capacity of wi-fi and TV

Pinky's Brain

Sat Mar 03 2012, 10:46PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Hmm, never realised you could simply shape a reflector to generate the desired phase front of the wave to generate/receive waves with orbital angular momentum.

Steve Conner

Sun Mar 04 2012, 07:56AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Can somebody explain how this is different to just running two antennas, one for each polarisation, each carrying a different signal? That would double the bandwidth, but if you turn the receiver sideways the channels get mixed up.

It's also a risky ploy because reflections can change the polarisation.

The twisted satellite dish in the picture looks like it would generate a circularly polarised beam, as a helical antenna does. That in itself is nothing special, but maybe they are beaming both polarisations onto it from two transmitters.

Avalanche

Sun Mar 04 2012, 10:29AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I recognised the 'Bo Thide' name from the author of a book on electromagnetic field theory, which I actually downloaded from a link on 4hv a couple of years ago. Whatever this guy thinks he's doing will no doubt fit the theory in his book somewhere (attached). I don't know if it's getting into the realms of pseudoscience or not, although IIRC the original thread did eventually get closed!
]bo_thide_-_electromagnetic_field_theory.pdf[/ file]

Proud Mary

Sun Mar 04 2012, 10:30AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve Conner wrote ...

Can somebody explain how this is different to just running two antennas, one for each polarisation, each carrying a different signal? That would double the bandwidth, but if you turn the receiver sideways the channels get mixed up.

It's also a risky ploy because reflections can change the polarisation.

According to Tamburini et al:

Unlike already existing radio communication protocols that use the spatial phase distribution generated by a set of antennae to artificially increase the transmission bandwidth, the immediate advantage provided by a protocol based on the physical OAM states as independent communication channels is that of using the peculiar spatial phase distribution of each of these states as a reference pattern to generate, modulate and detect them in a better way.

Pinky's Brain

Sun Mar 04 2012, 02:32PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Steve Conner wrote ...

The twisted satellite dish in the picture looks like it would generate a circularly polarised beam, as a helical antenna does. That in itself is nothing special, but maybe they are beaming both polarisations onto it from two transmitters.

AFAICS circular polarization is actually a separate issue ... circular polarization results from spin angular momentum but here they use orbital angular momentum. Either way, you can generate more orthogonal channels than with linear polarization (2).

The only problem I see is that if something simple like antenna shape can create/detect angular momentum, then simple environmental reflections will be able to change the angular momentum of a wave as well ... it only seems relevant to line of sight (and waveguide/fiber) applications.

IntraWinding

Tue Mar 13 2012, 01:29AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

This news also got me interested in the optical (laser) equivalent.

Even works on electrons.

WaveRider

Mon Apr 02 2012, 03:00PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

This is a quite different phenomenon from circular polarization. It is another form of "spatial diversity" that is angularly distributed instead of translationally distributed (x,y,z position of antennas..like MIMO).

It is a phenomenon common to any type of wave motion (even sound). I think labeling it with the "quantum orbital angular momentum" is somewhat misleading, but it shows where the idea came from....

The key to understanding it is to think about the phase of the wavefronts of a linearly polarized spherical wave at the reflector focus as they reflect from that "corkscrewy" parabolic reflector... what type of plane wave propagates away from the reflector into space? It will still be (mostly) linearly polarized but the "constant phase surface" will have a helicoidal shape. Then you will see how it is different from circular polarization.

Cheers!

Steve Conner

Mon Apr 02 2012, 03:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

WaveRider wrote ...

It is another form of "spatial diversity" that is angularly distributed instead of translationally distributed (x,y,z position of antennas..like MIMO).

Thanks! That's a great explanation. I get it now.

I'm still skeptical about how practical it might be if the receiver is allowed to move around, though.

radiotech

Tue Apr 03 2012, 12:21AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Wouldn't angular momentum infer that there would be a mechanical twisting effect in the
opposite direction acting on the dish mount?

Might make a neat space elevator drive.

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