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Antennas

May 16, 2016

The term, "antenna," is understood in different ways by different people. The biologists among us would likely think of arthropod antennas that are used to sense airborne molecules, such as pheromones. Engineers, however, will think of radio antennas used to couple electromagnetic waves to electronic circuitry. This article is about the radio engineer's antenna.

As a child of the space age, I enjoyed looking at photographs of antennas. Most space antennas were huge, a fact that appealed to one who enjoyed reading books about dinosaurs. Quite unlike the straight rod automobile antenna, the space age antennas had unusual shapes that appealed to their mystery. The iconic antenna of the early space age was the Jodrell Bank radio telescope antenna, now called the Lovell Telescope in honor of its creator, Bernard Lovell (1913 - 2012).

Lovell Telescope at Jodrell BankThe original Jodrell Bank radio telescope.

This telescope, configured as a radar system, tracked the booster rocket of Sputnik 1, the first artificial Earth satellite, in 1957.

(Photo by Matt Buck, via Wikimedia Commons.)

The principal purpose of an antenna is to efficiently convert the electromagnetic radiation present in free space to an electrical current. We're aided by the fact that the impedance of free space is about 375 ohms, which, as Goldilocks would say, is neither too high nor too low. Its value is exactly equal to the product of the permeability of free space and the speed of light; that is 119.9169832π.

The original Jodrell Bank antenna has a diameter of 250 feet, and the reason for its huge size and the size of its radio telescope cousins is the desire to collect as much radiation as possible from faint extraterrestrial radio sources in a very narrow beam. When receiving terrestrial signals of higher power, the size of the antenna can be made smaller; smaller, that is, up to a point.

Antennas will operate efficiently only if their dimension is at least a large fraction of the wavelength of the electromagnetic radiation they are designed to detect. That's why the antennas of AM radio stations are huge towers, more than a hundred meters high. In contrast, wireless routers have antennas just a few centimeters long. That's because Wi-Fi signals of 2.45 and 5.2 GHz have wavelengths of just 12 cm and 6 cm, respectively.

For signals in the VHF band, such as FM radio stations, the most popular antenna type is the Yagi-Uda antenna, generally called just a Yagi antenna in the United States, since Hidetsugu Yagi published the concept in English, while Shintaro Uda only published in Japanese. Yagi also had a 1932 US patent for this antenna.[1]

The Yagi-Uda antenna is directional. In radio parlance, it offers gain over the isotropic; that is, it enhances sensitivity in one direction, at the same time rejecting noise at angles away from that direction. It accomplishes this using phase interference.

Schematic diagram of a Yagi-Uda antennaSchematic diagram of a Yagi-Uda antenna.

(Drawn using Inkscape).

In the figure above, the reflector element is spaced a quarter of a wavelength from the driven element. This leads to constructive interference for waves in the forward direction, and a lesser signal at other angles. The director elements likewise re-radiate energy they receive in a way that enhances the signal of the driven element in the forward direction and diminishes the radiation in other directions. An important variation of the Yagi-Uda antenna is the log-periodic antenna, which is essentially a combination of Yagi-Uda antennas of different frequency connected together.[2]

The driven element of the Yagi-Uda antenna is a dipole, a simple antenna type that operates without the reflector and director elements. A lone dipole receives signals over a wide range of angles (see figure). The transmitting antenna, used by Heinrich Hertz in his 1887 demonstration of radio transmission and detection, was a dipole.

Relative reception patterns of a dipole and Yagi-Uda antennaRelative reception patterns of a dipole and Yagi-Uda antenna.

A greater number of elements will sharpen the Yagi-Uda pattern.

(Drawn using Inkscape.)

The reflector principle can be applied to a simple antenna type called a bowtie antenna, named after its similarity to a bowtie. This antenna type is common today for the reception of digital television. In areas of high signal strength of the broadcast signals, such as metropolitan areas, these can be mounted on an interior wall.

bowtie antennaA bowtie antenna.

The grill reflector and multiple dipoles give directivity, and the shaped dipoles give wide bandwidth.

(Modified Wikimedia Commons image.)

My favorite antenna type is the axial mode (end fire) helical antenna, invented in 1946 by John Kraus (1910-2004). Kraus described the dimensions of his prototype antenna in his classic textbook, "Antennas."[3] The helix was seven turns of copper wire with a 12 centimeter circumference, this being the wavelength of his signal source (2.5 GHz). Helical antennas are used in many high frequency applications, such as satellite communication (see figure).

Helical antenna for satellite communicationA satellite communications antenna pictured in 1984.

This is a typical helical antenna.

(US Air Force photograph by SSgt Louis Comeger, via Wikimedia Commons.)

There's an interesting anecdote from Kraus' university career. When constructing a large radio telescope from multiple helical antennas,[4] Kraus needed a small building to hold the receiving equipment. Knowing that the university bureaucracy would preclude speedy construction of even such a small structure, perhaps even preventing it, he and a graduate student built it themselves. When the building was eventually discovered by the university administration, they wondered how he was able to do this. Said Kraus, "...When I requisitioned material, I said it was for a receiver enclosure, which it was. It was a really appropriate description. And we got the job done."[5]

I wrote about fractals in a recent article (The Fractal Author, February 22, 2016). After Benoit Mandelbrot's popularized this geometrical object in his 1982 book, The Fractal Geometry of Nature, it was just a matter of time before this geometry was tried as a fractal antenna (see figure). This antenna type seems to have a small advantage over others in small devices, such as RFID tags and cellphones.

Figure 7E of US Patent No. 6,452,553, 'Fractal antennas and fractal resonators,' by Nathan Cohen, September 17, 2002.Figure 7E of US Patent No. 6,452,553, "Fractal antennas and fractal resonators," by Nathan Cohen, September 17, 2002.

Engineers will notice the resemblance to a twin-lead "T" antenna.

(Via Google Patents.)[2]

Just in the news was the detection of gravitational radiation by the Laser Interferometer Gravitational-Wave Observatory (LIGO).[7] After reading about the complexity of this detection, it's surprising that it might be possible to detect such radiation using an antenna. In this case, the detection would be of primordial gravitational waves generated just a small fraction of a second after the Big Bang.[9]

These waves should make themselves known through a “swirl” in the polarization of the cosmic microwave background radiation, something thought to have been detected in 2014 by the South Pole–based BICEP2 experiment, but the detection was found to be an artifact of dust in our Milky Way Galaxy.[9]

The trick to preventing an error such as that in the BICEP2 experiment is to make measurements at multiple frequencies. The BICEP2 detectors were tuned for just 150 GHz, but measurement at multiple frequencies would allow subtraction of the interfering galactic emission. The proposed antennas for this new measurement are superconducting sinuous antennas made from niobium. The sinuous antenna is a fractal antenna, and the fractal nature of its structure allows detection of radiation over a wide band.[9]

Fig. 7A of US Patent No. 4,658,262, 'Dual polarized sinuous antennas,' by Raymond H. DuHamel, April 14, 1987A sinuous antenna.

Fig. 7A of US Patent No. 4,658,262, "Dual polarized sinuous antennas," by Raymond H. DuHamel, April 14, 1987.

(Via Google Patents.)

The sinuous antennas developed for the cosmic microwave background observations are augmented by a silicon lens. The smallest feature size of the antenna is about a micrometer, and the entire antenna, with lens, is about 5 millimeters across.[9] The experiment will be conducted at the Atacama Desert site that hosts the Atacama Large Millimeter Array. This site, at 5,000 foot altitude in a dry climate, is ideal for detection of high frequency signals usually attenuated by water vapor in the atmosphere.

References:

  1. Hidetsugu Yagi, "Variable Directional Electric Wave Generating Device," US Patent No. 1,860,123, May 24, 1932.
  2. Raymond H. Du Hamel and Fred R. Ore , "Logarithmically periodic rod antenna," US Patent No. 3,079,602, February 26, 1963.
  3. J.D. Kraus, "Antennas," McGraw-Hill (New York, 1988), 265 pp., via Amazon).
  4. John Kraus, "Big Ear," Cygnus-Quasar Books (Powell, Ohio, 1976).
  5. Robert W. Wagner, "Interview of John Daniel Kraus," Ohio State University, University Archives Oral History Program, Ohio State University Oral History Project, 2005
  6. Benoit B. Mandelbrot, "The Fractal Geometry of Nature," W. H. Freeman and Company, 1982, ISBN-13: 978-0716711865 (via Amazon).
  7. Nathan Cohen, "Fractal antennas and fractal resonators," US Patent No. 6,452,553 , September 17, 2002.
  8. Gravitational Waves Detected 100 Years After Einstein's Prediction, California Institute of Technology Press Release, February 11, 2016 .
  9. Rachel Courtland, "Swirly Antennas Will Hunt for the Twists of Ancient Gravitational Waves," IEEE Spectrum, March 23, 2016.
  10. Raymond H. DuHamel, "Dual polarized sinuous antennas," US Patent No. 4,658,262, April 14, 1987.

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