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The Pros and Cons of Yagi Antennas

2.4 GHz Yagi antenna
2.4 GHz Yagi antenna | Source

Introduction

Yagi antennas, more formally known as Yagi-Uda antennas after the two Japanese inventors, are sometimes called patch antennas. Yagi antennas are commonly used in TV reception, ham radio and as a bridge antenna to connect a site to a wifi access point.

Yagi antennas have a dipole as the main radiating or “driven” element. A “reflector” is usually located behind the driven element. The reflector usually adds 4 dB to 5 dB of forward gain. However, not all yagi antennas have a reflector. If any extra directors are added to the antenna, the antenna gets an average 1 dB of gain.

Parasitic elements which don’t radiate signal can be added to the main element to alter its performance. The amplitude and phase of the signal depends on the size and spacing of these parasitic elements and their location relative to the driven element.

What are the advantages and disadvantages of Yagi antennas? What are the pros and cons of yagi antennas? When should you use a Yagi antenna?

Yagis have more gain than similarly sized log periodic antennas like these.
Yagis have more gain than similarly sized log periodic antennas like these. | Source

Advantages of Yagi Antennas

Yagis are directional antennas. Most yagi-uda antennas have a 50° to 70° beam width. Because they focus all their input in one direction, they put out high gain relative to omnidirectional antennas. This makes them ideal for receiving lower strength signals.

Their high gain gives yagi antennas relatively good range. Yagi's have the most gain for their physical size compared to antennas like log periodics.

The yagi antenna’s design filters out almost all signal noise coming from the opposite direction. This makes yagis a good choice when you have a high demand application like telecommunications.

It is easier to aim a yagi antenna than some arrays. And their construction makes them easy to mount on vertical towers or any other type of structure.

Because yagi antennas are simpler than log periodic antennas, they usually cost less than a comparable LP antenna. While printed circuit board yagis are available, others can be built from simple rods properly positioned.

Disadvantages of Yagi Antennas

Bandwidth or frequency range is somewhat limited.

If you want a high gain level, the antenna will need to be very long. And even then, gain is limited to around 6-9 dB unless you have more than one yagi antenna assembled in an array.

Yagi antennas experience a degradation of their electrical characteristics (gain, feed point impedance, front to back ratios) as you move away from the frequency range the yagi in designed for. This is because yagis are based on resonant elements, and off-resonant operations introduce reactance that cause the SWR to increase.

Observations about Yagi Antennas

Yagi antennas are designed to be balanced but can be unbalanced if a balun is placed at the feed joint line where it connects to the drive element. Depending on the design of the driven element, they can be balanced or unbalanced. Note: cheap Yagi's are balanced, so no balun needed.

Adding elements to a yagi antenna increases its directionality. It essentially becomes narrower in its focus, but it then receives signals from that direction even better than before because the signal to ratio improves. In short, the interference levels are reduced, especially from the sides.

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