A New Type of Turnstile Antenna

Ivana Radnovic1, Aleksandar Nesic1 , and Bratislav Milovanovic2

llMTEL Institute Blvd. M. Pupina 165b, 11070 Belgrade, Serbia

20epartment of Telecommunications, Faculty of Electronic Engineering University of Nis, Nis, Serbia E-mail: ivana@insimtel.com

Abstract

This paper presents a new type of turnstile antenna, realized with two crossed dipoles connected in parallel. Feeding of the dipoles in phase quadrature to obtain an omnidirectional radiation pattern is achieved by optimizing their impedances to be complex conjugates. The dipoles are realized with aluminum strips. A balun - the transition from the antenna to a coaxial cable - is accomplished with lumped parameters: two capacitors and two inductors. The concept of the turnstile antenna presented can be used in the VHF and UHF frequency ranges. The measured results show very good agreement with those obtained by simulation. The antenna is also characterized by simple manufacturing and low cost.

Keywords: Turnstile antenna; omnidirectional radiation pattern; horizontal polarization

1. Introduction

Turnstile antennas are used in cases when there is a need for an omnidirectional radiation pattern with horizontal polarization:

mostly in the VHF and UHF ranges, for broadcast transmitters. They are usually realized with conventional folded or crossed dipoles in the horizontal plane. These are fed with currents of the same intensity and in phase quadrature, or with so-called batwing radiating elements. An omnidirectional radiation pattern is obtained in such a manner [I, 2J.

This paper presents a new method of feeding two orthogonal dipoles in phase quadrature that is similar to the feeding of anten­nas with circular polarization [3]. Higher gain can be obtained by using arrays of these orthogonal dipoles (with two or more of them).

2. Concept and Design

In this paper, we will show the solution for obtaining feeding in quadrature without the use of a phase shifter. If we feed crossed

dipoles in parallel - one having an impedance Zdl = (50 - j50) n, the other having an impedance Zd2 = (50 + j50) n - then the total

impedance of the antenna will be Z = (50 + jO) n. We will con­

sider first the case of a capac itive dipole, with an impedance

Zdl = (50 - j50) n that is obtained by suitable choice of the

strip's length (/1) and width (wI) (Figure 1 a). The strip ' s thickness

is chosen to be 2 mm. The corresponding plot presenting its impedance as a function of frequency is given in Figure I b. The

second, inductive, dipole, with an impedance Zd2 = ( 50 + j50) n,

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is designed in the manner shown in Figure 2a: a feeding strip enters between two slots incised into the dipole. Tne lengths and

widths of these slots ($, $z), the width of the outer strips of the

slotted dipole (wz), and the length of the dipole D2 (Iz) are then

optimized to obtain required impedance. The Zd2 impedance variation with frequency is shown in Figure 2b. Since the length of the aluminum strips when operating in the VHF range is about 60 cm, it is important to provide mechanical firmness for the dipole arms. For this purpose, the middle parts of both dipoles are

1 c=========���c==========�I· t � I

Figure 1a. The layout of the dipole with capacitive reactance.

o z1,1

m:8 L���r:.....:::.:�_..:.-..-:.., -:-:-' -� 5(lO t 40.0

30.0 20.0 10.0

0.0 ·10.0 ·20.0 ·30.0 -40.0 ·50.0 -60.0

-,' " ,--I -t

+­---I-:�:8 - r +- -t-·�.O " r -+-�� ... ·100.0 r-:::-' ' - 1m

·110.0 ---J MHz .120.0+---...::.....,-�-�-�-��-�-�-�� 88.0 �.O 92.0 94.0 96.0 98.0 100.0 102.0 1().4.0 lffi.O 108.0

. Figure ] b. A plot of the impedance Zdl as a function of fre­

quency for the dipole in Figure la.

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

Figure 2a. The layout of the dipole with inductive reactance.

'ZlOIZl,l 120.0 110.0 100.0

90 .0 SIl.O 70.0 60.0

5O.0 L_-------�7 40.0 / ' 31.0 20.0 / 10.0 ,,, ,,'"

0.0 i" " -10.0 , ;;-, ' ·20.0 -31.0 " .- t R;-

Im -40.0.j....<C'---����-�-�-�-�-�-��. """"'"

88.0 90.0 92.0 94.0 96.0 98.0 100.0 102.0 104.0 100.0 108.0

Figure 2b. A plot of the impedance Zd2 as a function of fre­quency for the dipole in Figure 2a.

I

Dipole 2 Is 112 � I

1 J I ..1..-'

/ -'

q p � I Dipole 1

Figure 3. The layout of the two crossed dipoles forming the turnstile antenna.

mechanical ly supported along their axes by aluminum bars with a square cross section (see Figure 4). These supports were also included in the simulation. The dipoles were optimized at a fre­quency fc = 100 MHz using the WIPL-D program package [4]_

After separately simulating both dipoles, we made a model of the complete antenna structure, consisting of the dipoles connected in paral lel. The layout is shown in Figure 3. The total input imped­ance, Za (between the points p and q) was around 50 Q. There

was a reactive part that slightl y deviated from 0 Q at the operating frequency (100 MHz), as seen in Figure 5_ This was due to the

IEEE Antennas and Propagation Magazine, Vol. 52, No. 5, October 2010

mutual coupling of the feeding strips in the center of the antenna. The variations of Za as well as of the reflection coefficient with frequency are given in Figure 5 and Figure 6, respectively.

The calculated gain of the turnstile-antenna model at fc = 100 MHz was -2_15 dBd (O dBi). The deviation from the ide­

ally circular radiation pattern in the horizontal plane was 2.4 dB.

The structure presented is symmetrical. However, the feeding is realized through a coaxial cable, which is an unsymmetrical structure. Due to this fact, there is a need for a balun as a transition from the symmetrical feed points (p - q) to a coaxial cable_ A balun with lumped L-C elements was chosen [5, 6J (Figure 7). In our

Figure 4. The WIPL-D model of the turnstile antenna, with supporting aluminum bars.

Z QJ Zl.l 60.0 -

50.0

40.0

31.0

20.0

10.0

0.0

-10.0 - ,/ -20.0 -//" I

;" j. ... '�

,'­-31 .0 +--����-�-�-�-�-�-�-'-�

88.0 90.0 92.0 94.0 96.0 98.0 100.0 102.0 104.0 100.0 108.0

Figure 5. The impedance of the turnstile antenna as a function of frequency.

-6.0 -7.0 .a.o -9.0

-10.0 -11.0

-12.0 -13.0 -14.0 -15.0 -16.0

-17.0

� l �"\.

� - I--

-

T I I

� 'K .zj t-� �y+-j �tMm3

88.0 90.0 92.0 94.0 96.0 98.0 100.0 102.0 104.0 100.0 108.0

. Figure 6. The simulated reflection coefficient of the turnstile antenna as a function of frequency.

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Figure 7. A detail of the WIPL-D model, showing the balun with lumped parameters.

p

C Za 2

C q

Za 2

L

Figure 8. A circuit diagram of the balun with lumped parame­ters, used as a transition from the antenna to the coaxial cable.

Z (0 Z1,1 B(tO

70.0

60.0

50.0

40.0

-- - - .. , ,

30.0 - � " 20.0

10.0

0.0

, , ,

'--------- -- ---_ 1m .1O.0+--�-��-�-�-�-�-�-�-----�' · MH

88.0 90.0 92.0 94.0 96.0 98.0 100.0 102.0 104.0 100.0 108.0

Figure 9. The simulated impedance of the turnstile antenna.

case, at frequencies of about 100 MHz, the values for Land C were calculated to be L:::: 79.6 nH and C:::: 31.8 pF. Figure 8 represents the circuit diagram of the balun. After simulation of the complete antenna structure with the balun, an impedance of

Za = (51.66 - jl.ll) n was obtained at f = 100 MHz. The varia-

tions of the antenna impedance and S,' with frequency are shown

in Figure 9 and Figure J 0, respectively. The simulated radiation pattern of the turnstile antenna in the fP plane is given in Fig­

ure II. All simulations were carried out using the WIPL-D pro­gram package [4].

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The calculated gain of the turnstile antenna at 100 MHz was -2.15 dBd. The deviation from the ideally circular radiation pat­tern in the horizontal plane was 2.5 dB, reaching a lowest value of 1.2 dB at f = 1 02 MHz.

3. Realization and Results Obtained

The antenna was realized with aluminum strips of 2 mm thickness, and aluminum supporting bars with a square cross sec­t ion (10 mm x 10 mm). The dipoles and the balun were connected in parallel with the coaxial cable through the metallic plate, P, placed on a plastic cylindrical stand (Figure 12). This plastic cyl­inder also served for fastening the antenna onto a vertical mast (Figure 13). The measured gain of the realized antenna was around -3 dBd at 100 MHz, with a peak-to-peak variation of about 2 dB. The measured VSWR is given in Figure 14. There was good agreement between the measured results and those obtained by simulation.

Mag]l!m S 1.1 -5.0

-10.0 -15.0

·20.0 --t +

·30.0

.25 .0 -1= -35.0 - -·40.0

·45.0+--��-�-�-�-�-�-�-�-' == 88.0 90.0 92.0 94.0 96_0 98.0 100.0 102.0 104.0 100.0

Figure 10. The simulated return loss of the turnstile antenna.

100MHz

. Figure 11. The simulated radiation pattern of the turnstile antenna in the fP plane (gain in dBi).

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

Figure 12. A photograph of the balun employed in the turnstile

antenna.

Figure 13. A photograph of the realized turnstile antenna.

fie _ l;hamoI s ..... � I .... £_ ....... s ...... l!{- !!.-. s-h _188.000IXXI .... B _ Slop c- � .. .J ......... poro� �="' ... ro

3Oro --2Oro

.�.� �ro oro 10.00 h

� '/ I----2Oro . '\...,., "'" / Iooro

\11 !'Oro lJ fsoro

�1 $&_ .. .0000 lr!4H2 $lop 108'OOO .... Hz � s..... Oil �ll Cl-l'at Ln

Figure 14. The measured return loss of the realized turnstile antenna.

IEEE Antennas and Propagation Magazine, Vol. 52, No.5, October 2010

4. Conclusion

A new type of turnstile antenna has been investigated and realized. The antenna is significantly simpler than those of the same type used so far. Due to a simple manufacturing process, the cost is also very low. Such an antenna can potentially be used in higher frequency ranges, including microwaves. For additional gain, an array of two or more turnstile antennas, one above the other, spaced a half-wavelength apart, could be formed and attached to a common mast.

5. Acknowledgments

The Authors would like to thank Mr. Lj. RadoviC for his help in realization of the experimental model. This work was supported by the Serbian Ministry of Science and Technological Develop­ment.

6. References

1. J. D. Kraus, Antennas, New York, McGraw-Hili Book Com­pany, 1988.

2. H. Kawakami, G. Sato, and R. W. Masters, "Characteristics of TV Transmitting Batwing Antennas," IEEE Transactions on Antennas and Propagation, AP-32, 12, December 1984.

3. A. Nesic, I. Radnovic, and M. Mikavica, "New Printed Antenna Array With Circular Polarization," Electronics Letters, 32, 9, April 1996, pp. 785-786.

4. Branko Kolundzija, Jovan Ognjanovic, and Tapan K. Sarkar, WIPL-D Pro v5.1.

5. Henry Jasic, Antenna Engineering Handbook, McGraw Hill Book Company, 1961.

6. Winfried Bakalski, Werner Simburger, Herbert Knapp, Hans­Dieter Wohlmuth, and Arpad L. Scholtz, "Lumped and Distributed Lattice-Type LC-Baluns," IEEE International Microwave Sympo­sium, Seattle, WA, June 3-7, 2002, pp. 209-212.

7. Aleksandar Nesic, Ivana Radnovic, and Dusan Nesic, Antena sa horizonta/nom po/arizacijom i kruinim dijagramom zracenja, pat­ent pending 2009/0294.

Ideas for Antenna DeSigner's Notebook Ideas are needed for future issues of the Antenna Designer's Notebook. Please send your suggestions to Tom Milligan and they will be considered for publication as quickly as possible. Topics can include antenna design tips, equations, nomographs, or shortcuts, as well as ideas to improve or facilitate measurements. EiV

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