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On Point: The Yagi Antenna
Thursday, 25 May 2017 13:01

If you happened to look up during a drive down a suburban street in the US anytime during the 60s or 70s, you’ll no doubt have noticed a forest of TV antennas. When over-the-air TV was the only option, people went to great lengths to haul in signals, with antennas of sometimes massive proportions flying over rooftops.

Outdoor antennas all but disappeared over the last third of the 20th century as cable providers became dominant, cast to the curb as unsightly relics of a sad and bygone era of limited choices and poor reception. But now cheapskates cable-cutters like yours truly are starting to regrow that once-thick forest, this time lofting antennas to receive digital programming over the air. Many of the new antennas make outrageous claims about performance or tout that they’re designed specifically for HDTV. It’s all marketing nonsense, of course, because then as now, almost every TV antenna is just some form of the classic Yagi design. The physics of this antenna are fascinating, as is the story of how the antenna was invented.

“Uda Who?”

Shintaro Uda. Source: IEEE Cincinnati Section

What would come to be known as the Yagi antenna got its start in the early 1920s in the lab of Professor Shintaro Uda in the Tohoku Imperial University in Sendai Japan. Dr. Uda was working in the VHF band and was looking for ways to make antennas more directional. While experimenting with a resonant loop antenna, he discovered that placing a static loop near the antenna tended to shape the signal away from an omnidirectional pattern, almost as if the loop was acting as a reflector.

Together with his colleague Hidetsugu Yagi, Uda experimented with different configurations. They eventually replaced the loop antenna with a simple dipole, and added additional elements, which they called directors, on a long boom to further shape the beam. Using eight directors on a 15-meter wooden boom mounted to the roof of their laboratory, Uda and Yagi were able to communicate over a distance of 135 km at 68 MHz, no mean feat at the time.

Hidetsugu Yagi and “his” antenna. Source: Physics World.com

Having dubbed their invention the “wave projector directional antenna,” it was inevitable that the antenna would be named after someone. How it came to be credited solely to Dr. Yagi is a tale of some treachery on Yagi’s part with a dash of naiveté on Uda’s. Dr. Uda published the first Japanese language papers on the antenna, but for reasons unknown, Dr. Yagi applied for both Japanese and American patents for the antenna with no mention of Uda. The Japanese patent was assigned to the Marconi Company in England, while the American patent went to RCA. With no mention of Uda, and with Dr. Yagi touring the English-speaking world to discuss “his” antenna at various radio engineering conferences, the antenna gradually became simply the “Yagi antenna” or the “Yagi array.”

Ironically, thanks to inter-service rivalries and a silo mentality in Imperial Japan, it was only the capture of a British radar set during the Battle of Singapore in 1942 that introduced the homegrown invention to the Japanese military. The Japanese intelligence officers didn’t even consider “Yagi” to be a Japanese name — they supposed it was just a code word made up by the British.

Parasites

The chief characteristics of the Yagi-Uda antenna are high directionality and high gain. Given the fact that the length of each element needs to be close to some fraction of the wavelength of the signal, it’s most practical for the higher frequencies, mostly above 30 MHz. That’s not to say that it can’t be used for the longer wavelengths, though — plenty of hams work the 20 m and 40 m bands through a big Yagi.

A Yagi for the ham bands. Note the driven element with feedline, seven directors, and a single reflector. Source: Antenna-Theory.com

As in Dr. Uda’s original design, a Yagi consists of a single driven element parallel to and coplanar with at least two parasitic elements. A minimal design is a single reflector element located “behind” the driven element (relative to the direction of the radio signal) and a single director element in front of the driven element. A practical antenna is likely to have multiple directors, the more of which there are the tighter the directionality and the higher the gain, at least up to a point.

This gives Yagis their characteristic appearance – a horizontal boom with multiple elements arranged perpendicularly. There are some variations, of course — some Yagis have multiple reflectors, or have a corner reflector arrangement. And some antennas, particularly TV antennas, have the parasitic elements swept back at an angle rather than perpendicular to the boom. Additionally, the elements can be arranged horizontally or vertically, depending on the polarization desired.

Phasing

To understand the Yagi’s design, recall that a plain old dipole antenna in free space has a radiation pattern that is the strongest broadside to the antenna. That results in two big lobes off the front and the back of the antenna, with little signal off the ends. The driven element of a Yagi is just a half-wave dipole, or sometimes a folded dipole to increase the impedance. The parasitic elements shape and direct the beam using constructive and destructive interference.

As Dr. Uda discovered, the parasitic elements can either be inductively or capacitively coupled to the driven element. Inductive elements are slightly longer than half-wave, while capacitive elements are slightly shorter. The directors are all shorter than half-wave and are therefore capacitively coupled, while the reflector is longer and inductively coupled. The difference from the ideal half-wave is small — usually only 10% to 15%.

Constructive and destructive interference in a Yagi antenna. The green wave represents the sum of the red and blue waves. Source: RadarTutorial.eu

Both the reflector and the directors work by reradiating power from the driven element. The spacing of the parasitic elements relative to the driven element determines the phase of the reradiated signal. The reflector, being inductively reactive, reradiates power 180° out of phase with the driven element. The spacing is set so that this causes destructive interference off the back of the antenna, while at the same time being nearly in-phase with the driven signal off the front of the antenna. This results in constructive interference, boosting the power off the front. Similarly, the capacitively coupled directors are spaced so that they reradiate power more-or-less in-phase in the forward direction, while radiating out-of-phase to the rear.

The result is greatly amplified signal toward the directors, and almost none behind the reflector. And recall that antenna theory states that any antenna that transmits can also receive, and with the same characteristics. It doesn’t matter whether the driven element in a Yagi is driven by a 100-watt power transmitter connected to the feedline, or by a few microwatts picked up from a distant TV tower. The directionality and gain will be the same. And Yagis can have remarkable gain – up to 20 dBm when correctly designed.

As useful as the Yagi antenna is, it’s far from perfect. Because of the critical size and spacing of the parasitic elements, Yagis have a relatively narrow bandwidth. Also, the directionality of the antenna can be an inconvenience, requiring that the antenna be rotated to point more or less exactly at the transmitter or receiver.

But if you need to pull in a single distant signal, that directionality is just what you need. The Yagi is a workhorse antenna, and given the impact it has had it’s probably right and good that many have taken to referring to it as the Yagi-Uda antenna.

 


Filed under: Engineering, Featured, History, wireless hacks