L'esperimento condotto dall'HAARP consiste nella trasmissione di un segnale di tipo elicoidale, con un fronte d'onda ad anello con un "pitch", una altezza variabile lungo la circonferenza. L'effetto è un segnale spiraleggiante, elicoidale, ottenuto appunto alimentando una struttura di antenne circolare con correnti alternate opportunamente ritardate. La variazione del fronte d'onda nello spazio tridimensionale potrebbe contenere le informazioni numeriche che si vogliono trasmettere, occupando una sola frequenza.
Twisted radio beams could untangle the airwaves
13 February 2009 by Colin Barras
The human race is not only exhausting tangible resources such as oil. The radiofrequency spectrum available for wireless communication is becoming the increasingly crowded, with virgin "veins" of frequency running short. However, Swedish physicists say that twisting radio beams into a helical shape as they are transmitted could help ease the congestion. Radio frequency encompasses electromagnetic waves between 3 kilohertz and 300 gigahertz, and as wireless communications technology advances much of that range is being used. Satellite TV, wireless computer networks and cellphones are among the growing technologies vying for space up to 30 gigahertz, with some technology even beginning to extend beyond 100 gigahertz leaving a dwindling supply of virgin terrain to exploit.
Physicist Thomas Leyser at the Swedish Institute of Space Physics in Uppsala, Sweden, thinks he has a novel solution. Along with an international team of physicists, he has demonstrated that it is possible to put a spin on radio beams during their transmission to produce a twisted beam. "Twisted laser beams have been researched since the 1990s, but it has only now become possible to create twisted beams at the much lower radio frequencies," he says.
That advance could prove important as it provides a new way to encode information into radio transmissions. Leyser says that "the information encoded in the twist is independent of the amplitude and frequency of the radio waves" - the features of a radio wave more normally used to encode data. "It is a feature of radio waves that has not been utilised before." Leyser and his co-workers created the first twisted radio beams at the HAARP facility of 48 radio antennas in Alaska, normally used to investigate the aurora borealis and other features of the atmosphere. "In order to transmit a radio beam, one needs an array of antennas," he explains.
The signal is twisted by firing antennas in sequence to describe a circle, instead of having all of them transmit the same signal at once. "What we did was to feed all the antennas in the array with slightly different currents," says Leyser. Each antenna received an alternating current slightly delayed from the adjacent antenna in the circle. The time delay ripples around the array so that the beam emerges to describe a helical wave front.
To confirm that the radio beam had this characteristic shape, the team studied the effects it had on the ionosphere above the array. "The twisted beams excited plasma turbulence in the ionosphere that was consistent with the ring-shaped beams and different from that excited by regular beams," Leyser says. The twists remain coherent across vast distances - light years, even - and can store information in the form of digital bits (1s and 0s), encoded into the pitch of the twist.
What's not yet clear is how much extra information can be transmitted using twisted beams. In theory, huge amounts of data could be sent, says Leyser. It is possible to use a much smaller array to produce twisted signals, he adds, although most consumer technologies such as cellphones use dipole antennas that cannot produce twisted beams. Larger tripole antennas that can twist their transmissions might be suitable for linking fixed points, though, for example between cellphone towers.
(Journal reference: Physical Review Letters, forthcoming publication)