Demonstrating antenna diversity, Part 3: The Yagi antenna

Part 1 of this series introduced two fundamental antenna concepts while part 2 discussed the PIFA antenna. Within the universe of available antennas, there’s one which has been a long-time favorite among do-it-yourself (DIY) enthusiasts as well as for commercial and professional situations: the Yagi antenna (or more correctly, the Yagi-Uda antenna) (Figure 1).

Figure 1. The basic Yagi antenna is a three-element antenna widely used in commercial, residential, and military application, it is often see on rooftops or masts. (Image: EuroCaster/Denmark)

Why are Yagi antennas popular? There are many reasons:

• You can easily calculate out the necessary dimensions for the antenna, even though the underlying electromagnetic analysis is complicated.
• Any adjustments to these dimensions to accommodate practical issues such thickness of the elements and end-fringe effects are well known and can be factored into the sizing analysis.
• It is easy to build and compatible with DIY projects; the boom can be metal or even wood.
• It is fairly easy to make a rugged version for outdoor using standard pipes, lumber (if using a wood boom) and fittings available at a home-supply store.
• They have a modest size for the performance they offer.
• They have a simple, single feed point with a mostly resistive impedance, easing matching to the transmission line if needed.
• They offer multiple degrees of freedom and are easy to modify in-house or even in the field to adjust gain, directivity, bandwidth, side lobes, and other factors within reasonable limits.
• The basic design offers good gain (8 dB to 10 dB typical) and high front-to-back ratio (10 dB to 20 dB typical), and modest bandwidth (10% to 20% of center center), and there are ways to improve or modify these numbers.
• They can be designed and built to operate over a wide range of frequencies, and many commercial units available for the 30 MHz to 3 GHz range.
• They are easily scaled in their construction to provide versions that can handle higher power, into the hundreds of watts and more.

Yagi background
The Yagi antenna was a developed in 1926 in Japan, when Prof. Shintaro Uda presented the theory of this antenna in a Japanese-language journal. It received wide attention when an English-language translation was published soon after by Prof. Hidetsugu Yagi in the Proceedings of the Imperial Academy (“Projector of the Sharpest Beam of Electric Waves”) and thus Yagi’s name is most closely associated with it.

In the days before cable TV and Internet streaming, the only option to receive a TV signal was as an over the air (OTA) RF signal using a physical antenna. If you were more than around 10 miles to 20 miles from the TV transmitter (depending on the power of the transmitter and your site) you needed an antenna with higher gain than a simple folded dipole could offer, and the Yagi was the most-common choice for this role.

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