P.B.L.Kishore
1 , Dr.A.Jhansi Rani 2
1 S𝒕𝒖𝒅𝒆𝒏t(𝑴. 𝑻𝒆𝒄𝒉) , 𝑽. 𝑹. 𝑺𝒊𝒅𝒅𝒉𝒂𝒓𝒕𝒉𝒂 𝑬𝒏𝒈𝒊𝒏𝒆𝒆𝒓𝒊𝒏𝒈 𝑪𝒐𝒍𝒍𝒆𝒈𝒆, p.b.l.kishore@gmail.com
2 𝑷𝒓𝒐𝒇𝒆𝒔𝒔𝒐𝒓, 𝑽. 𝑹. 𝑺𝒊𝒅𝒅𝒉𝒂𝒓𝒕𝒉𝒂 𝑬𝒏𝒈𝒊𝒏𝒆𝒆𝒓𝒊𝒏𝒈 𝑪𝒐𝒍𝒍𝒆𝒈𝒆, jhansi9rani@gmail.com
Abstract: Small size antennas with more bandwidth are essential for present day technology. These large bandwidth antennas are mainly used for MIMO applications. The log periodic (LP) antennas are most widely used for enhancing the Bandwidth. The Bandwidth increase by accomplishing a miniaturization factor greater than 2 is achieved by coupling the currents from the booms of the LP to an annular ring, thereby providing an efficient well-matched radiator at frequencies below the cut-off frequency of the LP.
The sinuous antenna will also give higher bandwidth but it requires the separate beam forming network which makes the antenna as complex structure. This problem is simplified by log periodic antenna. The printed 4-arm LP aperture has the ability to radiate both senses of circular polarization (CP) in the broadside direction and can also be integrated with 180 0 balun lines in order to simplify the beam former. Printed 4arm LP configurations are for dual-polarization use (both linear and circular) in wide range of applications like high-gain reflector feeds, MIMO, and even detection of cosmic microwave background radiation. The frequency of operation for these applications ranges from the L-band to K/Ka band. The concept of bandwidth extension is not limited and it can be enhanced by changing the size of the antenna. keywords : Log-Periodic, circular polarization, impedance matching.
Log-periodic antennas are frequency independent antennas; also it obeys self- complimentary principle Log-periodic antennas are mostly used for frequency large bandwidths.
That is these are not depends on frequency of operation. The log-periodic history starts from isbell antenna innovated by isbell. Log-periodic
(LP) antennas are introduced in the late 1950s.
Log periodic antennas are designed for the specific purpose of having a very wide bandwidth.
“The achieve bandwidth is typically infinite”. The actual bandwidth is dependent on how large the structure is and how exact the smaller features are on the antenna, it was noted that wideband antennas are often defined by angles instead of lengths, so that they are more frequency independent (because angles are independent of wavelength for any frequency). Their electrical properties repeat periodically with the logarithm of frequency [1], with the lowest and highest operating frequencies determined by the lengths of the shortest and longest resonant teeth. LP antennas are widely used in varieties of wideband communication systems due to their frequency independent characteristic. It has been shown that by using the recently developed surface microfabrication process, bandwidths in the excess of a decade are achievable with planar two-arm LP antennas.
The concept of MIMO, incorporating antenna arrays at bothsides of the radio link may be used to increase the channel capacity in wireless communications. Parallel subchannels areformed by uncorrelated fading processesbetween the individual transmission paths of the multi dimensional transmission link
[2]. The dimension of the MIMO communication

channel depends on the diversity concepts applied at transmit and the receive antenna. The diversity concept is determined by spatial-, polarization- and pattern diversity techniques. Depending on the properties of the underlying multipath channels each of those concepts may double the communication capacity over the same physical link, so that by combining those orthogonal diversity techniques large transmission capacities can be retained. Applying dual-polarized antenna modules at the receiver and at the transmitter, polarization diversity concepts may effectively be used in order to meetthe requirements of compact transmission terminals with high transmission capacity [2].
II. FOUR ARM LOG-PERIODIC ANTENNA
Fig. 1 . Four arm log-periodic antenna
Log-periodic antennas are often defined by angles instead of lengths, so that they are more
“Quasi frequency independent” (because angles are independent of wavelength for any frequency).
“Quasi” implies that at the lower end, the true FI principles are not maintained. In other words, the
LP is FI only after its nominal turn-on frequency when its teeth are resonant. A novel antenna composite with greatly extended bandwidth of a baseline printed 4-arm log-periodic topology is introduced. Bandwidth increase by accomplishing a miniaturization[ is achieved by coupling the currents from the booms of the LP to an annular ring, thereby providing an efficient well-matched radiator at frequencies below the cut-off frequency of the LP. The annulus, the turnstile
(booms of the LP), and the LP form the radiators of this novel quasi frequency-independent ringturnstile-LP antenna. A 160 mm diameter antenna is designed and fabricated to demonstrate the proposed concept. Measured VSWR and extensive far-field characterization verify multioctave dual circular polarization operation. Also, the ring-turnstile-LP configuration is integrated with two planar impedance transformers/feeds to demonstrate small size, simple beam former and dual-CP operation over bandwidths as wide as those of more complex sinuous and singly polarized spiral antennas. Detailed computational studies are conducted for better understanding the impact of various structural parameters on antenna’s unidirectional performance and relevant conclusions are provided.
The printed 4-arm LP aperture (Fig.1.) has the ability to radiate both senses of CP in the
Frequency band of operation 2-16GHz.(UWB)
VSWR 1 to 2
Return loss (S11)
Input Impedance
Below -10db
50ohms
Antenna impedance input power
100ohms
40W at 5GHz
Impedance transformer chebychev
Polarization Linear and circular
Dielectric material FR4 (
 r
=4.4)
α β
R
0
τ
No. of Port teethes gap
35
0
10
0
80mm 72mm 0.7-0.9 22 6mm
Table 2. Physical Specifications of Antenna broadside direction and can also be integrated with 180
0
balun lines in order to simplify the beam former. A beam forming network (BFN) produces the desired mode (e.g., beam) at its output port by establishing the correct relative phase between arms, effectively filtering out all other modes. A beam former can have multiple output ports, each producing a different characteristic mode of the spiral antenna.
However, errors in the beam forming network, mismatched terminations, large spirals capable of

supporting higher-order modes, manufacturing imperfections, and data-acquisition errors can lead to contamination, which is manifested as a power loss in the desired mode. It has been established that if a pattern is contaminated, or if an arbitrary set of complex voltage weighting coefficients are applied to the antenna terminals such that the modal phasing between arms is not satisfied, then the radiation pattern of the spiral is a vector superposition of its characteristic modal patterns.The log-periodic antenna having two specifications one is physical specifications and another is electrical specifications. The physical specifications are shown in the following Table 1.
And the electrical specifications are shown in table.2.
III. MATCHING NETWORK
For a center-fed configuration, where each pair of arms of the LP is excited with a voltage source in order to simplify the external beamformer and make the antenna bidirectional, a micro-strip line matching network is integrated to the aperture. The matching network not only transforms the nominal impedance of the LP to 50 but also functions as a balun, there by simplifying the external beam former to a single 90 0 hybrid for dual-CP use. Each pair of arms is fed with impedance transforming micro-strip line where the boom of one arm acts as the ground plane and the signal line is attached to the opposite arm near the center with a metal via. Thus, the 4-arm LP aperture now has two inputs which are excited with appropriate phases for dual-CP. This method of exciting a 4-arm planar LP with a pair of micro-strip lines has been reported before.
However, in all these cases, 2 arms of the 4-arm
LP and one of the two micro-strip lines are shifted to the opposite side of the substrate to facilitate the feed mechanism. Herein, the feed lines are routed underneath each other near the center. This way the log-periodic aperture preserves its selfcomplementary shape. Note that the conventional plating allows a reliable via fabrication to achieve both low reactance and vias’ ability to sustain high currents when antenna operates in the highpower CW mode. The nominal impedance of a single arm (to ground) of the ring-turnstile-slot LP is found to be 90. As previously mentioned, this differs from the expected value of 133 due to the finite size of the aperture and more importantly substrate and superstrate loading. A voltage source that is placed across apairofarmsshouldhaveanominal impedance of
100 ohms. Thus the impedance transformer that excites a pair of arms needs to transform 100 ohms to 50 ohms. However, on the chosen substrate 180 corresponds to a line width of only
0.08 mm. This is below the tolerance of most fabrication techniques but more importantly it will severely limit the antenna power handling capability. HFSS model for impedance matching is shown in Fig. 2.
IV. SIMULATION AND MEASUREMENTS
This four arm log-periodic antenna was designed and simulated by using High Frequency
Structured Simulator (HFSS). HFSS is an interactive software package for calculating the electromagnetic behavior of a structure using finite element method. This HFSS can give the all measurements of antenna. And it also supports to parametric analysis. For designing log-periodic antenna the substrate used is Taconic having dielectric constant of 3 in the shape of circular.
Fig. 2. chebychev impedance transformer
Fig. 3. Antenna Design in HFSS
Above this created a four arm log-periodic

antenna then it forms a planar log periodic antenna. The radiation box is drawn with 2d
2
/
 distance. This creates the near and far fields for the calculation of the antenna behavior. The designed log-periodic structure was shown in Fig.
3. The feeding technique used here is micro-strip impedance matching transformer. The parametric study was very useful tool for getting proper parameters for getting good results. After parametric study the gap between LP and ring turnstile is achieved as 0.5mm. the S
11
and VSWR
Fig.4. Reflection Coefficient Results
Fig.5. VSWR Result results are shown by following fig. 4 and fig. 5.
VI. CONCLUSION
A four arm Ultra Wide Band (2-16 GHz) logperiodic antenna is demonstrated. The band width enhancement is achieved by coupling a ring turnstile slot to log-periodic antenna. HFSS is used to simulate the results. The feed was done by chebychev impedance matching technique on simulation but in fabrication the feeding is by balun. This large band is useful for MIMO applications to provide large channel capacity.
The unidirectional principle was achieved by providing cavity to one side of the antenna. This dissipates all fields in the backward direction and power is radiated in one direction.
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