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Stanford scientists test wireless antenna in Arctic
STANFORD - In research, you never know where the next question might lead. For Stanford student Melissa Bowen, a classroom query about the Earth's magnetic field led to the Arctic Circle and tests exploring a wireless antenna for satellite communication systems.
During one of her undergraduate courses, Bowen had studied the earth's magnetic field and wondered how they fluctuated over the course of a day. Her professor, Robert Helliwell, sent her to Antony Fraser-Smith, a senior research associate who had measured magnetic fields across the globe. Bowen was intrigued with Fraser-Smith's explanations and decided to work for a quarter with him in Stanford's Space, Telecommunications and Radioscience laboratory (STARlab).
When it came time for Fraser-Smith to test the feasibility of developing an innovative communication system for the U.S. Air Force and NASA last March, he invited Bowen, a native of Sitka, Alaska, to join a research expedition in the far northern reaches of her home state.
The experiment used a rocket to launch two electron guns high into the atmosphere, Fraser-Smith said. A compass control system aimed the guns, aligning the stream of electrons with the earth's magnetic field.
"Usually in an antenna you have wire that guides how the electrons flow and creates the antenna pattern," Bowen said. "With this experiment we wanted to see if we could make an antenna that didn't use wire - but just the stream of electrons - and control it with an electron gun."
It is possible that electron guns can be used to generate radio waves at frequencies low enough to penetrate seawater, so the Navy is interested in their development to improve submarine communications, Fraser-Smith said. Since low-frequency signals are less susceptible to interference from bursts of solar activity, the Air Force is interested as well, he said.
"It's a more secure way of communicating," he said.
Fraser-Smith set the frequency of the electron guns in the very low-frequency (VLF) range, from 4470 to 17,940 Hertz. Today's satellites would have to have antennas many thousands of feet long to generate such low frequencies, he said.
To prepare for the experiment, the Air Force flew rocket parts to Poker Flat Rocket Range near Fairbanks, Alaska, for assembly. A Stanford crew, consisting of Bowen, Fraser-Smith, scientific and engineering associate Paul McGill, and senior research associate Cal Teague, used a chartered plane to reach Ven etie and Fort Yukon - two remote towns above the Arctic Circle - where they set up ground antennas to receive the incoming signals.
Every night, Bowen and the rest of the group at Fort Yukon would don clothing issued by Eielson Air Force Base - parkas, mukluks, giant mittens and down-filled pants - and drive their rented pick-up truck to the end of a road to set up two 20-foot ground antennas. The antennas consisted of two triangular loops that crossed at 90 degrees to allow them to resolve the direction of incoming signals, which were boosted with amplifiers.
Then the researchers waited in below-freezing weather, sometimes for more than three hours, for a signal that the rocket was on its way.
"Communications were intricate," said Bowen, whose group communicated via walkie-talkie with a Fort Yukon radar station, which relayed their messages to the launch site by phone.
Countdown started 15 minutes before launch. Scientists at the antenna sites checked in periodically.
"For our experiment we wanted it magnetically quiet in the atmosphere," Bowen said.
In addition, researchers at Poker Flat had a special camera that filmed the glow from the electron guns on the rocket as they spewed out electrons, Fraser-Smith said. Clouds and auroras, or northern lights, could hinder observations.
The rocket had to be launched during "windows of opportunity" from 10:30 p.m. to 2:30 a.m., when light, weather and astronomical conditions were optimal. If the team missed a window, the next one was the following day.
For several nights, technicians cancelled the launch because of clouds, auroras, botched communications or - in one case - dead batteries in recording equipment.
On nights the launch was cancelled, the crew read, napped, watched movies on the VCR in their cabin, or watched auroras. During the day, they took walks.
Finally, at week's end, the rocket flew. When it reached 100 kilometers, the two guns began firing a steady stream of electrons from open ports. The rocket peaked almost 300 kilometers into the ionosphere, and the guns continued to fire until the rocket fell to earth on the Yukon Flats.
"The purpose of this experiment was to test the gun," Fraser- Smith said. "It was actually questionable whether you could even fire these new high-current electron guns in space. Now the gun has been fired - it worked well - and it generates signals near the rocket, as we'd expect. If the experiment had been scaled up a little more, we're confident that it would generate signals on the ground."
Earlier calculations showed a minimum of three guns was necessary for a fully successful experiment, he said. The batteries used to power the electron guns were so heavy that the rocket could only fit two.
"I'm very happy with the results of our experiment," Fraser- Smith said. "We set our antennas up. We got very good sensitivity. And everything was calibrated, so we know exactly the noise background that we were observing. We know exactly when the rocket fired. We know the power of the electron beam, and we know pretty much all the conditions in the atmosphere."
The experiment, which cost $2 to 3 million, was conducted jointly with Peter Banks, the project's principal investigator who is on administrative leave from Stanford while serving as the Dean of Engineering for the University of Michigan; Jim Ernstmeyer of the U.S. Air Force; and Prof. John Raitt of Utah State University. Fraser-Smith directs the project at Stanford.
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