Yagi-Uda Antenna Design

Yagi-Uda Antenna Design Muhammad Luqman Azmer, Abdul Mukhlishiina Lahuddin, Raja Ammar Akif Raja Ismail, Ahmad Zaim Ali Mokhtar, Muhd Aizat, Muhammad Nawawi Mat Yusof Abstract-This paper is related with the YagiUda Antenna (also known as a Yagi) is another popular type of end-fire antenna widely used in VHF and UHF bands (30 MHz to 3 GHz). We were given a task to design an antenna that operate in range of 600-800MHz and it can reach at least 7dB gain. We chose to design a Yagi-Uda Antenna.

I.

INTRODUCTION

The Yagi-Uda Antenna (also known as a Yagi) is another popular type of end-fire antenna widely used in VHF and UHF bands (30 MHz to 3 GHz) because of its simplicity, low cost and relatively high gain. The most noticeable application is for home TV reception and these can be found on the rooftop of the house. A typical one is shown in Figure 1.

Figure 2 The main feature of this type of antenna is that it consists of three different element: the driven element, reflector and director, as shown in Figure 2. Some people consider the Yagi-Uda antenna an array, since it has more than one element. However, it has just one active element and feed port; all the other elements (the reflector and director) are parasitic. Thus, some people consider it an elemental antenna array. The main characteristics and design recommendations of these element into three important parts which is the driven element (feeder), the reflector and the directors. The driven element (feeder) is the heart of antenna. It determines the polarization and central frequency of the antenna. For a dipole, the recommended length is about 0.50 λ to ensure good input impedance to a 50 Ω feed line.

Figure 1 Yagi and Uda were two Japanese professors who invented and studied this antenna in the 1920’s. S. Uda made the first Yagi-Uda antenna and published the result in Japanese in 1926 and 1927, and the designed was further developed and published in English by his colleague Professor Yagi a year later. Since then a significant of mount of work has been done theoretically and experimentally.

The reflector normally slightly longer than the driven resonant element to force the radiated energy towards the front. It exhibits an inductive reactance. It has been found that there is not much improvement by adding more reflectors to the antenna, thus there is only one reflector. The optimum spacing between the reflector and the driven element is between 0.15 and 0.25 wavelength. The length of the reflector has a large effect on the front to back ratio and antenna input impedance. The directors usually 10 to 20% shorter than the resonant driven element and appear to direct the

radiation towards the front. They are of capacitive reactance. The director to director spacing is typically 0.25 to 0.35 wavelengths, with larger spacing for longer arrays and smaller spacing for shorter arrays. The number of directors determines the maximum achievable directivity and gain. The special configuration (long reflector and short directors) has made the Yagi-Uda Antenna radiate as an end-fire antenna. The simplest three element Yagi-Uda antenna (just one director) already shows an acceptable end fire antenna pattern. The radiation towards the back seems to be blocked/reflected by the longer element, but not just by the reflector; the reflector and the director produce push and pull effects on the radiation. Induced current are generated on the parasitic element and form traveling wave structure at the desired frequency. The performance is determined by the current distribution in each element and the phase velocity of the traveling wave.

Figure 3

Figure 4

Table 1 (in mm)

III.

RESULT AND ANALYSIS

Return loss (S11) Figure 2.1, Reflector, Driven Element and Directors repestively.

II.

DESIGN

The models (refer to Figure 3 and Figure 4) of the Yagi-Uda antenna was designed by using CST simulation which implemented with the parameters value as shows in Table 1. All the parameters have been tweak to get the best result.

S11 is a measure of how much power is reflected back at the antenna port due to mismatch from the transmission line. When connected to a network analyzer, S11 measures the amount of energy returning to the analyzer – not what’s delivered to the antenna. The amount of energy that returns to the analyzer is directly affected by how well the antenna is matched to the transmission line. A small S11 indicates a significant amount of energy has been delivered to the antenna. S11 values are

measured in dB and are negative. S11 is also sometimes referred to as return loss, which is simply S11 but made positive instead (Return Loss = - S11). So if the antenna Return Loss is 8 dB, S11 is -8 dB. The third and final method to measure an antenna’s ability to accept power is VSWR (voltage standing wave ratio). VSWR evaluates the ratio of the peak amplitude of the voltage of the wave on the transmission line versus the minimum amplitude of the voltage of the wave. A VSWR of 1 is ideal; this indicates that there is no reflected power at the antenna port. When the antenna and transmission line are not perfectly matched, reflections at the antenna port travel back towards the source and cause a standing wave to form. The worse the mismatch, the larger the amplitude of these reflections. Based on Figure 5 below, shows the result of the simulation with the applied parameters value.

Figure 5 Bandwidth (sometimes just referred to as impedance bandwidth) refers to the range of frequencies a given Return Loss can be maintained. Since Return Loss is a measurement of how much power the antenna accepts from the transmission line, the impedance of the antenna must match the impedance of the transmission line for maximum power transfer. However the impedance of the antenna changes with frequency, resulting in a limited range that the antenna can be matched to the transmission line. The Bandwidth is a measure of this range. Based on the Figure 4, shows the measurement of the bandwidth range, BW on the return loss curve.

Figure 4 Based on S11 graph, the resonance frequency is 0.66 GHz as it have the highest gain which is 48dB. The operating bandwidth is between 0.60.72 GHz as it have at least 10dB gain. Radiation Pattern The radiation pattern of an antenna is a plot of the far-field radiation properties of an antenna as a function of the spatial co-ordinates which are specified by the elevation angle (θ) and the azimuth angle (φ). More specifically it is a plot of the power radiated from an antenna per unit solid angle which is nothing but the radiation intensity. It can be plotted as a 3D graph or as a 2D polar or Cartesian slice of this 3D graph. It is an extremely parameter as it shows the antenna’s directivity as well as gain at various points in space. It serves as the signature of an antenna and one look at it is often enough to realize the antenna that produced it. Because this parameter was so important to our software simulations we needed to understand it completely. 3-dB Beamwidth The 3-dB beamwidth (or half-power beamwidth) of an antenna is typically defined for each of the principal planes. The 3-dB beamwidth in each plane is defined as the angle between the points in the main lobe that are down from the maximum gain by 3 dB. This is illustrated in Figure above. The 3-dB beamwidth in the plot in this figure is shown as the angle between the two blue lines in the polar plot. In this example, the 3-dB beamwidth in this plane is about 75.6 degrees. Antennas with wide beamwidth typically have low gain and antennas with narrow beamwidth tend to have

higher gain. Remember that gain is a measure of how much of the power is radiated in a given direction. So an antenna that directs most of its energy into a narrow beam (at least in one plane) will have a higher gain.

IV.

FUTHER ANALYSIS AND DISCUSSION

Further Analysis is done for Yagi-Uda antenna to determine the behaviour of the antenna when certain parameter is being changed. First parameter being manipulated is the radius of the element. The radius for our actual designed antenna is 5mm. But when we change the radius into 4mm the result is as shown in Figure 7.

Figure 5, 3D radiation pattern from CST result which shows a directional pattern. Antenna plots are the road map for the antenna user. Plots tell you where power is being radiated or received (since they are reciprocal). They also tell you how much degradation you can expect if the antenna is not aimed properly. Sometimes it is desirable to communicate with more than one station. Antenna plots will assist in the proper aiming of the antenna for optimum performance on all the desired signals. The narrower the beamwidth, the greater the difficulty in properly aiming the antenna. Remember that weather phenomenon such as wind may also affect antenna performance or dictate the type of antenna mounting.

Figure 6, polar radiation pattern from CST result.

Figure 7 Figure 7 shows that the return loss of the antenna is at -40 dB and the resonance frequency is at 0.66 GHz with the bandwidth from 0.6 GHz to 0.72 GHz. Compare to the actual result the return loss for the when uses the 5mm radius is at -48 dB which the different is 8 dB whereas the resonance frequency and the bandwidth remain the same. Second parameter that has being manipulated is the distance between each element. The parameter is changed into 0.2 time the wavelength from the actual parameter and the result is shown in Figure 8.

Figure 8 This Figure 2 shows that the return loss of the antenna is at -24 dB which gives significant loss in dB compare when changing the radius of the

elements. The resonance frequency and the bandwidth still remain the same. The third parameter that has being manipulated is length of driven element and all other elements. The length is change to 30% smaller from final design and the result is shown in Figure 9.

between 600-800 MHz have been achieved. We have designed a Yagi-Uda antenna that operate between 600-720 MHz (bandwidth) and have 48dB gain at its resonance frequency (0.66 GHz). By using CST Studio the parameters have been tweak for antenna to achieve widest bandwidth and highest gain. The optimum radius for all elements is 0.5 mm. The optimum driven element length is 0.468 x wavelength (428.58 mm) which is 200.58 mm. The optimum distance between elements is 0.26 x wavelength.

VI.

ACKNOWLEDGEMENTS

We would like to thank you everybody whose involve in this project especially Dr. Mohd Haizal Jamaluddin for guiding us throughout this project. Figure 9 REFERENCES Figure 9 shows that the change in resonance and the bandwidth which is 0.93 GHz and 0.85 to 1 GHz respectively whereas the return loss is the same as the actual result which is -48 dB.

Book 1. 2.

In this further analysis, we can conclude that the change in radius has less changed in return loss compare to when changing the space between the elements. But changing the length of driven element has no effect to the return loss instead it changes the resonance frequency and the bandwidth of the radiation.

V.

CONCLUSION

From the result, it can be conclude that the task given which is to design an antenna that operate

3.

"Antenna and Wave Propagation”, U. A. Bakshi, 1st Edition (2011). "Antenna Theory: Analysis and Design”, C. A. Balanis, 3rd Edition (2005). “Antenna Theory and Design”, W. L. Stutzman, G. A. Thiele, 3rd Edition (2012).

Website 1. 2.

3.

"Yagi-Uda Antenna", http://www.antennatheory.com/antennas/travelling/yagi.php "Yagi-Uda antenna." Wikipedia. Wikimedia Foundation, n.d. Web.29 May 2015. "Antenna (radio)." Wikipedia. Wikimedia Foundation, n.d. 17 May 2015.