OAM (Orbital Angular Momentum), aka "Radio Vorticity" aka "Twisted Radio Beams":
OAM is a fairly well-known capability for Lasers and has been observed in Space.
But Tamburini's 1Mar2010 article is the FIRST report of a public On-Air Test using
Radio signals (2.4 GHz Wi-Fi Band).
Generating a OAM signal can be accomplished with a circular array of antennas, each
fed with the same signal, but adjacent antennas have a phase difference = 2*Pi*l/n,
where n=number of antennas and l=integer (e.g. 1, 2, 3, 4, 5, etc), so that the
phase rotation around the entire circular array is 2*Pi*l.
The Venice Experiment injected OAM by using a Horizontally Polarized Parabolic
Antenna cut from the outer edge to the center and separated by 6.25-cm at the outer
rim, thereby generating the increasing phase difference for l=1. Yagi's would be l=0.
They only built one Parabolic to generate l=1. If they had used two additional
Parabolic Antennas with the Gap twice and three times as big, they could have
transmitted THREE orthogonal OAM signals on the same frequency, thereby
TRIPLING total capacity.
At the receiver, two identical, Horizontally Polarized Yagi Antennas were summed
together, except one of them went through an inversion, so the output was the
DIFFERENCE SIGNAL. When receiving from OAM Parabolic, the output of this
" was the OAM Signal. A parallel, Yagi Antenna link was
transmitting at the same time on the same frequency. Transmit power was
adjusted so that the received signal level matched the interferometer output,
thereby adjusting for the much higher Gain in the Parabolic than the Yagi and
overcoming whatever losses were in the Interferometer. I THINK that the
interferometer had to be reconfigured to switch between the Yagi signal (l=0)
and the Helicoidal Parabolic signal (l=1)....or maybe they used another Yagi
at the receiver????
They had to carefully move the receive interferometer around until they found the fairly
small "sweet spot" where the OAM signal could be detected.
I would like to see how much "leverage" the OAM signal has versus conventional,
since it appears to be a minor "tweak" to the primary signal, so I would expect the
sensitivity would be much less....and how much loss was there in the interferometer....
And I don't know what they had in mind when they claimed an additional 5X increase
in capacity using "multiplexing" techniques.....
Current OAM antennas are clearly limited to point-to-point links, vice broadcast.
I followed several references....below are a "must read".
Fabrizio Tamburini et.al. - Encoding Many Channels in Same Freq thru Radio Vorticity -
First Experimental Test - 1107.2348 - 17pgs 1Mar2012.pdf
[Better Quality Graphics]
Bo Thide + Fibrizio Tamburini et.al. - Radio Beam Vorticity and Orbital Angular Momentum -
1101.6015 - 3pgs 31Jan2011.pdf
[A bit more info re Venice Experiment.]
H Then + B Thide et.al. - Detecting Orbital Angular Momentum in Radio Signals - 0805.2735 -
5pgs 18May2008.pdf [THEORY, Tripole & Multi-Element Arrays]
B Thide et.al. - Utilization of Photon Orbital Angular Momentum in Low-Freq Radio Domain -
0705.1208 - 4pgs 1May2009.pdf
[Multi-Element Antenna Array]
Bo Thide - Nonlinear Physics of the Ionosphere and LOIS/LOFAR - 0707.4506 - 6pgs 31Jul2007.pdf
[Multi-Element Antenna Array]
These were conveniently found via Columbia Univ's Archive Search Engine:
arXiv.org Search for Downloads - Tamburini_F
arXiv.org Search for Downloads - Thide_B
Some additional references:
R. Compton, Jr - "The tripole antenna: An adaptive array with full polarization flexibility"
[IEEE is a subscription service....or visit a well equipped Univ. Library]
H.T. Hayvaci et.al., Univ. of Chicago, Presentation: Tripole antennas for Wi-Fi: