How a Yagi Antenna Works: Reflector, Directors and Gain
The Yagi is the most familiar directional antenna in amateur radio, from the three-element beam on a tower to the small handheld one that points at a satellite. If you have ever wondered how a Yagi antenna works, the answer is surprisingly elegant: only one element is connected to the radio, and the others, which have no wires attached to them, shape the radiation by re-radiating what they receive. This guide explains the parts of a Yagi, the way the elements combine to push the signal in one direction, what controls gain and front-to-back ratio, why the bandwidth is narrow, and how multiband designs use traps. The descriptions follow the standard technical reference on the Yagi–Uda antenna.
What a Yagi Is
According to the reference article on the Yagi–Uda antenna, it is a directional antenna made of two or more parallel resonant elements in an end-fire array. The elements are most often metal rods that act as half-wave dipoles. A single driven element is connected to the transmitter or receiver through a transmission line. The other elements are passive radiators with no electrical connection, and they usually include one reflector and any number of directors. The antenna is also called a beam antenna or a parasitic array. It was invented in 1926 by Shintaro Uda of Tohoku Imperial University in Japan, with a lesser role played by his boss, Hidetsugu Yagi, which is why you will see both names. The name Yagi is the better known of the two.
The Yagi is widely used as a directional antenna on the HF, VHF and UHF bands. The same reference describes moderate to high gain of up to about 20 dBi, depending on the number of elements, and a front-to-back ratio of up to about 20 dB. It radiates linearly polarised waves and is normally mounted for horizontal or vertical polarisation. It is relatively light, inexpensive and simple to build, which explains why it is used on rooftops for television, on towers by amateurs and by broadcasting stations for long-distance shortwave links.
The Three Types of Element
The Driven Element
The driven element is usually a half-wave dipole or a folded dipole. It is the only part connected to the feed line, and the only part that is excited directly. Most often it is fed at the centre, so its two halves must be insulated from the supporting boom. Everything else in the antenna depends on the energy that the driven element radiates.
The Reflector
The reflector is placed behind the driven element, on the side opposite to the direction in which you want to transmit. It is slightly longer than the driven element. Usually only one reflector is used, because the gain improvement from additional reflectors is small, although more can be added for other reasons, such as a wider bandwidth.
The Directors
The directors sit in front of the driven element, in the direction of the beam. They are slightly shorter than the driven element. You can use any number of them, and the gain increases as you add more. Yagis with 40 directors and more have been built. The parasitic elements are typically solid rods, meaning that they are short-circuited dipoles with no break at the centre, and because a half-wave dipole has a point of zero RF voltage at its centre, they can be attached to a metal boom at that point without insulation.
Typical spacing between elements ranges from about one tenth to one quarter of a wavelength, depending on the design. The exact lengths and spacings vary from one design to the next, which is why you should not mix and match dimensions from different antennas.
Why Parasitic Elements Add Gain
A parasitic element receives energy from the driven element and re-radiates it, but with a different phase that depends on its exact length. The waves from all of the elements superpose, and the result is that they add up in the forward direction and cancel in the reverse. That is the whole trick. The reference article gives a simplified explanation in terms of reactance. An element that is longer than a half wavelength has inductive reactance, so the phase of its current lags and the delay exceeds 90 degrees. This is how the reflector behaves. An element that is shorter than a half wavelength has capacitive reactance, and the phase delay is smaller than 90 degrees. This is how a director behaves.
Now add the distance between the elements. The signal takes time to travel from the driven element to the passive one and back, so spacing also affects the phase. In an idealised case with a spacing of a quarter wavelength, the wave re-radiated by the director cancels the wave from the driven element in the backward direction, while the wave re-radiated by the reflector adds constructively there, and the opposite occurs in the forward direction. In reality, the phase delays do not reach those extremes, so the elements are given the correct lengths and spacings so that the waves from the driven element and from the parasitic elements all arrive at the front of the antenna in phase. The reference describes this as the crest of the forward wave from the reflector reaching the driven element just as the crest of the wave is emitted from that element, and then reaching the first director just as the crest is emitted from that one, and so on. Waves in the reverse direction interfere destructively, so little signal is radiated to the rear.
The result is a unidirectional beam. The main lobe lies along the axis perpendicular to the elements, off the end with the directors. To understand the terms that describe it, see our guide to the antenna radiation pattern, which explains lobes, beamwidth and front-to-back ratio.
Gain, Front-to-Back and Bandwidth
The more elements, the more gain, but there is a price. In its basic form, the Yagi has a narrow bandwidth of about 2 to 3 percent of the centre frequency. The reference notes a trade-off between gain and bandwidth, with the bandwidth narrowing as more elements are used. Bandwidth is defined here as the range of frequencies over which the gain is within 3 dB of the maximum. That is acceptable for a fixed frequency or for a single narrow band, but it limits how well a high-gain Yagi covers a wide band such as 10 metres. A designer who wants a wider bandwidth can use thicker elements or additional reflectors, and television antennas often use larger-diameter conductors for this reason.
| Characteristic | What the reference says | Practical meaning |
|---|---|---|
| Gain | Moderate to high, up to about 20 dBi, growing with more elements | More directors give more forward gain |
| Front-to-back ratio | Up to about 20 dB | Weaker signals from behind the beam |
| Bandwidth | About 2 to 3 percent of the centre frequency in basic form | Narrower with more elements; tune for the part of the band you use most |
| Polarisation | Linear, horizontal or vertical depending on mounting | Match the polarisation to the stations you want to work |
| Weight and cost | Light, inexpensive and simple to build | Popular for portable and fixed stations |
For an introduction to how gain figures are quoted and compared, our article on dBi and dBd explains why the same antenna can have two different gain numbers.
Impedance and the Feed System
When the parasitic elements couple to the driven element, they change its feed-point impedance. Mutual coupling tends to lower the impedance of the driven element, which is why folded dipoles, which have a large radiation resistance, are often used as the driven element. The coupling reduces their impedance to the typical range of 50 to 75 ohms. A plain half-wave dipole has an impedance of about 73 ohms in free space, and with parasitic elements nearby it drops lower, so many amateur Yagis include a matching system, such as a gamma match, a hairpin match or a balun, to bring the impedance up to 50 ohms. Because the matching details differ from design to design, follow the manufacturer’s instructions for your antenna.
Design Is Mostly Computation and Trial
You might hope that there is a simple formula for the dimensions of a Yagi. There is not. The reference article explains that the relationships between element length, spacing, element diameter and performance are too complex for simple formulas. For an antenna with n elements, there are 2n minus 1 parameters to adjust, namely the element lengths and the relative spacings, and a full analysis involves mutual impedances between all the elements. In practice, designs are often empirical, starting from an existing design and modifying it, then checking the result by measurement or by computer simulation. If you want to try simulations yourself, our beginner guide to antenna modeling with EZNEC and 4nec2 shows the first steps.
This is a good reason to build or buy a proven design rather than to scale one by eye. Small changes in the length of an element can shift the resonant frequency and the pattern noticeably, particularly at higher frequencies where the elements are short. If you adapt a design to another band, check it with a model and measure the result.
Multiband Yagis and Traps
Amateur Yagis are sometimes designed to work on several bands. These designs insert a parallel LC circuit, called a trap, at electrical breaks in each element. At the higher frequency, the trap effectively truncates the element so that it is about a half wavelength long. At the lower frequency, the whole element, including the remaining inductance of the trap, is close to half-wave resonance, so a different Yagi comes into being. A second set of traps allows a triband antenna that resonates on three bands. The reference article says that, given the cost of erecting an antenna and a rotator on a tower, combining antennas for three bands in one unit is a practical solution. It also lists the disadvantages: traps reduce the bandwidth on each band, reduce the electrical efficiency, and add mechanical concerns such as wind loading and the ingress of water and insects. For a comparison of directional HF designs, see our guide to Yagi versus hex beam.
Practical Tips for Using a Yagi
- Point it correctly. The beam is narrow, so you need a rotator or a fixed heading toward the area you want to work. See our antenna rotator guide for sizing and control.
- Mind the height. The antenna’s pattern in elevation depends on its height above ground, as well as on the antenna itself.
- Use the right polarisation. A Yagi radiates linearly polarised waves, so mount it for horizontal or vertical polarisation according to the stations you want to work.
- Do not trust the SWR alone. A low SWR shows that the match is good, not that the pattern is right. Check the front-to-back ratio against stations behind the beam.
- Protect the elements and traps. Check for corrosion, loose clamps and water in the traps during your seasonal inspection.
- Add elements with care. More directors bring more gain but also narrower bandwidth and more weight on the tower and rotator.
The Bottom Line
A Yagi works because one driven element feeds power into the antenna and the parasitic elements, a slightly longer reflector behind it and slightly shorter directors in front, re-radiate that power with phase shifts that add up in front and cancel behind. The result is a unidirectional beam with moderate to high gain and a useful front-to-back ratio, at the cost of a narrow bandwidth that narrows further as elements are added. Because the dimensions interact in complex ways, use a proven design, mount it for the right polarisation and keep the beam pointed where the stations are.
Frequently Asked Questions
How does a Yagi antenna work?
Only the driven element is connected to the radio. The reflector and directors are parasitic elements that re-radiate the energy they receive with different phases, so the waves add in the forward direction and cancel behind the antenna.
Why is the reflector longer than the directors?
An element longer than a half wavelength has inductive reactance and a larger phase delay, which makes it behave as a reflector. An element shorter than a half wavelength has capacitive reactance and a smaller delay, which makes it a director.
How much gain can a Yagi have?
The reference article gives moderate to high gain of up to about 20 dBi, depending on the number of elements, with a front-to-back ratio of up to about 20 dB. The exact figures depend on the design.
Why is the bandwidth of a Yagi narrow?
The elements are cut for a specific frequency, and the phase relationships that produce the forward gain change quickly as the frequency moves away. The basic Yagi has a bandwidth of only 2 to 3 percent of the centre frequency, and more elements make it narrower.
Do the parasitic elements need to be insulated from the boom?
Not usually. A half-wave parasitic element has a point of zero RF voltage at its centre, so it can be fixed to a metal boom at that point without insulation. The driven element is different, since its two halves must be insulated from the boom.
What are traps on a multiband Yagi?
Traps are parallel LC circuits placed at breaks in the elements. They make the element act as a half wave on the higher band and allow the whole element to resonate on a lower band, so one antenna can serve several bands, at some cost in bandwidth and efficiency.