International Journal of Applied Engineering Research

ISSN 0973-4562 Volume 9, Number 22 (2014) pp. 12011-12022

© Research India Publications
http://www.ripublication.com

Size Reduction of Yagi-Uda Antenna by Altering the

Diameter and Spacing between the Elements

S. Daya Murali1, K. Ch. Sri Kavya2, Sarat K Kotamraju3, R. Sai Divya1,

Y. Satish1, K. Pradeep Chandra1, N. Sri Keerthi1, G. Jaya Sree1
1
Department of ECE, K L University
(Koneru Lakshmaiah Education Foundation), AP, India
2
Women Scientist, Department of ECE, K L University
(Koneru Lakshmaiah Education Foundation), AP, India
2
kavya@kluniversity.in
3
Associate Dean (P&D) & Professor, Department of ECE,
K L University (Koneru Lakshmaiah Education Foundation), AP, India

Abstract:

Miniaturization of antennas helps in civilian, military and satellite applications

for present trends in many means such as their installation, mobility etc. Yagi-
Uda antenna with reduced lengths has been proposed used for such
applications. In this work investigation of Radiation Characteristics, Return
loss, Directivity are simulated and practically examined. A five-element Yagi
antenna fed with co-axial feed and made of aluminum is developed in this
work. About 40% of size reduction is obtained by rearranging the directors
and by decreasing the diameter of the elements.

Keywords: Yagi-Uda, Directors, Spacing, Diameter, Co-axial Feed,

Directivity.

Introduction:
An Antenna is a transducer which is used to transmit and receive the electromagnetic
waves, converting them into electric currents and vice versa. A typical antenna finds
its usage in every domain location of life whether it is broadcasting or space
exploration. Point to point communication, wireless LAN are other important fields
which cannot be imagined without an antenna [18]. Antenna can be developed in any
medium whether it is air, space, soil or water [11]. Several critical parameters
affecting on antenna performance are directivity, gain, electric field intensity,
resonant frequency, impedance, gain and band width [8], [17].

Paper Code: 26786 - IJAER

12012 S. Daya Murali et al

Yagi-Uda antenna is famous as a Yagi antenna. It is a directional antenna

consisting of an array of dipole and additionally closely coupled parasitic elements.
Its design is exclusively based on dipoles. The design of a Yagi-Uda antenna requires
proper understanding of how the components are structured and by varying the
diameter and position of these parasitic elements (reflector and directors) changes the
radiation characteristics of the antenna. This Yagi antenna is used as T.V receivers in
olden days because of low cost and easy to construction [9], [16], [20].
A Yagi antenna, as shown in Fig.1, typically has one driven element (dipole),
one reflector which is slightly 5% longer than the driven element and several directors
which are slightly 5% shorter than the driven element. Here driven element is active
element; reflector and directors are passive elements. The length of the reflector
should be greater than the driven element such that its impedance will be more and
hence it can be acts as a inductive element. Similarly the length of the director is less
than the driven element such that its impedance will be less and hence they acts as a
capacitive elements [1-3]. Now-a-days, instead of using a normal rod as reflector,
these are replaced by rectangular plate as reflectors or parallel rods in series
arrangement simulating a reflecting plate. The reflector just acts as mirror and reflects
the EM waves. So position of placing reflector changes the radiation pattern [21].
Normally a reflector will possess a gain of 4-5 dB in the forward direction but more
than one reflector has little benefit [6].

Fig.1: Geometry of proposed 5-element Yagi-Uda antenna

Size Reduction of Yagi-Uda Antenna 12013

Generally every director will provide a 1dB of gain in the forward direction. The
function of these directors is to enhance the radiation in particular direction. The
number of elements will depend on the gain and the limits of the antenna structure [8],
[12]. The length, spacing and diameters of these elements have a large effect on
radiation parameters like forward gain, backward gain ratio and input impedance [4].
Half wave dipole is used as a driven element in our proposed Yagi antenna. The
resonance of driven element will be occurred when its electrical length is half of the
wavelength of the frequency applied to its feeding point [5], [7]. The dipole is fed
with a co-axial cable. Therefore there must be proper impedance match with the cable
input and the antenna feed [13-14].
In order to have appropriate gain it is better to place these elements with equal
spacing and to achieve good antenna beam in desired direction. To improve input
impedance it is better to use folded dipole instead of ordinary dipole [24]. For real life
applications of these types of antennas the SWR should be in the range of 0 to 1. For
Omni-directional antenna noise was added from all directions but where as in case of
Yagi-Uda antenna since it is a Uni-directional antenna noise was added only from one
direction which is better suits for the communications [10], [19], [22], [25].

Design Specifications:
Typical Yagi-Uda antenna designed for 600 MHz.

Table 1: Design Specifications

Elements General specifications Proposed specifications

Length of reflector 0.55 λ=16.5cm 16.5cm
Length of driven 0.5λ=15cm 15cm
Length of director 1 0.45 λ=13.5cm 13.5cm
Length of director 2 0.40 λ=12cm 12cm
Length of director 3 0.35 λ=10.5cm 10.5cm
Spacing between the Directors 0.2 λ=6cm 5cm
Diameter of the elements 10mm 6mm

Working principle:
The working of yagi uda antenna is mainly depends on the working of the normal
ordinary dipole which is working as a driven element. The input is given to the driven
element through the co-axial cable or transmission line. The driven elements radiates
in all directions i.e. forward and backward. As Yagi-Uda antenna is a forward
radiating antenna we have to place a reflector at the back end. The length of the
reflector is larger than the driven element and therefore the current leads the voltage
and hence no radiation will be there at the back end. While in the forward desired
direction we have to place a parasitic element whose length is smaller than the driven
element thus current lags the voltage and hence it acts as a director. The phase and
amplitude of the currents depends on length of the elements and spacing between the
12014 S. Daya Murali et al

elements. Hence proper spacing and length of parasitic elements causes constructive
addition of E-field in forward direction and destructive addition in backward direction
[23].

Fig.2: Geometry of 5-element Yagi in HFSS

Fig.3: Hardware setup of Proposed Yagi-Uda Antenna

Radiation pattern
The radiation fashion of the antenna enhances how the relative strength of the field
that is radiated in multiple directions from the antenna. The radiation pattern is
nothing but a "reception pattern", since it also narrates the receiving and acceptance
properties of the antenna. The proposed Yagi antenna has radiation view displayed in
fig 4 has maximum radiation of 20.33 at an angle of 90 degrees. It is clear from the
Size Reduction of Yagi-Uda Antenna 12015

Fig.4 & Fig.5 that the radiation pattern of proposed Yagi shows an end-fire radiation.

Fig.4: Radiation Pattern of Proposed Antenna

Fig.5: Radiation Pattern of Proposed Antenna

Directivity:
As number of director’s increases the directivity also increases in the forward
direction. But this leads to affect the input impedance of the antenna and also
12016 S. Daya Murali et al

increases the physical complexity of the antenna. Here we have decreased the spacing
between the elements this leads the antenna to achieve high directivity. As shown in
the Fig.6 and Fig.7, the proposed Yagi is showing maximum directivity of 7.16 at an
angle of 90 degrees.

Fig.6: Directivity Pattern of designed Yagi Antenna

Fig.7: Directivity Pattern of designed Yagi Antenna

Size Reduction of Yagi-Uda Antenna 12017

Practical Results:

Fig.8: Experimentation of proposed Yagi-Uda Antenna using Network Analyzer

(Facility available in RF&MW Research Centre, KL University)

Return loss:
Return loss defines the wastage of signal power resulting from the reflection caused at
discontinuities in a transmission line or optical fiber. These discontinuities may be
either a mismatch with the load that is terminated or a device inserted in the
Transmission line. It is usually expressed in decibels (dB). Two lines or devices are
said to be well matched if the return loss is more. An antenna is said to be perfectly
matched if the return loss is of about-10dB.The return loss obtained for our design is-
17.653db which can be shown in the Fig.8.

VSWR:
The Voltage Standing Wave Ratio describes how better the impedance matching is.
VSWR is frequently called as SWR. A high VSWR indicates that how much signal is
reflected rather than being radiated by the antenna. VSWR and reflected power
express the same in different ways of measuring [15]. Generally the vswr is calculated
at the desired operating frequency. The Proposed antenna has obtained SWR of 1.4 as
shown in the Fig.9.
12018 S. Daya Murali et al

Fig.9: Return loss of Yagi using Network Analyzer

Field characteristics:

Fig.10: SWR Measurement of Yagi using Network Analyzer

Size Reduction of Yagi-Uda Antenna 12019

Fig.11: E-Field distributions of proposed yagi-uda Antenna

Fig.12: H-Field characteristics of proposed Yagi-Uda Antenna

12020 S. Daya Murali et al

Discussions
For designing of Yagi-Uda antenna to operate at 6 MHz we required spacing between
the elements is 10 cms. But in this work we have designed for 1 MHz where the
spacing should be 6 cms. But by reducing the spacing between the elements to 5 cms
and diameter of the elements to 6mm we made the antenna to operate at 6 MHz.
Hence optimization of geometrical length has been achieved.
The proposed Yagi–Uda antenna has been modeled using a software HFSS.
HFSS is a commercial finite element method solver for EM structures. The software
includes a linear circuit simulator with integrated optometric for input and matching
network design. The radiation characteristics are investigated using network analyzer.
Network analyzer is an instrument that measures the electrical parameters of
networks.

Conclusions
In this work, simple, light weight and optimized antenna based on a modification of
the classic Yagi–Uda antenna has been developed and implemented practically.
Further impedance and radiation characteristics can be improved by using folded-
dipole as driven element and gain can be improved by using more number of
directors.

Acknowledgments
The authors especially thank the support given from Department of Science and
technology DST, Government of India through the WOS–A scheme with file number:
SR/WOS–A/ET-33/2011 and also with SR/FST/ETI-316/2012 FIST program. The
authors are also highly thankful to the management of K L University for supporting
and encouraging this work.

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