Design of a “Birdcage-like” Antenna for Collinear Arrays

Anda R. Guraliuc, Andrea A. Serra, Paolo Nepa, Member,IEEE and Giuliano Manara, Member,IEEE

Dep. of Information Engineering University of Pisa

Pisa, Italy [anda.guraliuc, andrea.serra, p.nepa, g.manara]@iet.unipi.it

Abstract— In this paper, an antenna solution for collinear arrays working in the UHF band is presented. It consists in a cylindrical dipole with vertical slots on its surface, assuming a “birdcage” shape. The antenna belongs to the category of folded dipoles and it can be suitable for air traffic control (ATC) applications. This antenna relates to a collinear antenna structure and, more precisely, to a vertical stacked array antenna. It provides omnidirectional radiation patterns and a good isolation between array elements. Two prototypes were manufactured and analyzed. Reflection coefficient and mutual coupling between two adjacent elements are shown.

Keywords- Collinear dipole array, folded dipole, mutual coupling.

I. INTRODUCTION Linear arrays of vertical dipoles are commonly used in

several UHF/VHF band applications [1-2]. An important one, is the Air Traffic Control (ATC) application in the 225MHz - 400MHz frequency band, where dipoles between two and four are vertically staked at a certain distance. Single elements are realized with empty metallic tubes, operate independently in different channels and are fed separately through coaxial cables that lye side by side inside of a metallic tube. The latter is internal and concentric with dipoles, it supports the entire structure and acts as grounding for lightning protection. This type of antennas are required to provide omni-directional radiation patterns, vertical polarization and at least 25dB port isolation between adjacent elements [3-5].

Designing a collinear array to satisfy the previous requirements is not an easy task. In particular, isolation is a major issue due to mechanical and electrical aspects. Each element is feed independently through coaxial cables, which lye together inside of the grounding metallic tube. This cause reduction in antenna performance like radiation pattern distortion, reduction in antenna gains and impedance matching, but especially reduction in isolation between elements.

Fig. 1 show a traditional dipole antenna configuration used in ATC applications. It is mounted on a supporting metallic tube and connected at its end through screw or plates. The connection dipole – metallic tube cannot be realized in the middle point, which is usually used for feeding, otherwise it

would results in a short circuit. This configuration assumes a folded dipole connotation [6]. Currents at dipole ends find a low impedance path along the grounded metallic tube and two major consequences outcome: a) the dipole is electrically longer and its operational frequency changes, b) currents flow along this path from a dipole to another and give rise to undesired high coupling.

The purpose of this paper is to present an UHF antenna solution, operating in the band of ATC applications, with reduced mutual coupling even when mounted on a single metallic supporting tube (no multiple sections). The proposed radiating element is a folded slotted dipole in a birdcage shape. Its geometry is obtained by realizing some slots on the surface of a traditional cylindrical dipole. The antenna design was performed using the commercial software CST MWS®. Simulated and measured performance of a collinear array in the UHF/VHF band at 225-400MHz is shown. Results were compared with the traditional dipole collinear array, when the elements are connected through non-conductive Teflon tube. Results showed improved isolation between the birdcage antennas in the collinear array.

Figure 1. Traditional dipole antenna configuration.

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II. “BIRDCAGE” ANTENNA DESIGN In ATC applications an efficient solution to mitigate the

inter-element coupling in a collinear dipole array is realized by separating the single radiating elements with non-conducting, e.g. Teflon tubes (in this case, an additional grounding structure or technique is necessary for safety and protection requirements), as shown in Fig. 1. It is worth mentioning that the supporting tube inside each dipole must be metallic in any case in order to shield the radiating elements from feeding cables that lye inside them. In the following, authors named the aforementioned structure (dipoles with an inner metallic tube, separated by Teflon tubes in a collinear array configuration) as “traditional dipole” and it was considered as a reference for performance comparisons. To the best of the authors’ knowledge, for ATC applications, the traditional dipole array structure represents the best solution in terms of isolation.

The proposed radiating element is a folded slotted dipole in a birdcage shape. The birdcage is a half-wave cylindrical dipole with some slots on its external surface. A generic layout is shown in Fig. 2 where the antenna is composed of some alternated metallic strips and slots, giving rise to the birdcage shape. The metal strips are connected together at the upper and lower ends of the dipole. One of the metallic strip is cut at its center to get the feeding point. Section AA’B’B (Fig. 1b) takes a cylindrical shape once the AA’B’B surface is wrapped around z-axis.

Two birdcage-like dipoles are studied, namely Birdcage-2S and Birdcage-4S, with two slots/two metal strips and four slots/four metal strips, respectively. The geometrical parameters are summarized in Table I. The antennas were designed to operate in the ATC UHF band by using the CST MWS® simulation package. The Birdcage-4S is made of four

vertical slots and four vertical metal strips, all of the same width (m=s=31.4mm). From a top point of view, the antenna is made of eight equal 45° sectors (β=ϕ=45°, Rout=40mm). The Birdcage-2S is made of two vertical 55mm wide slots and two vertical 165mm wide metal strips (m=165mm, s=55mm). From a top point of view, the antenna is made of four unequal sectors (β=135°, ϕ=45°, R=70mm). The radius of the supporting metallic tube is Rtube=20mm.

III. ANTENNA MEASUREMENTS Some prototypes of Birdcage-2S and Birdcage-4S were

fabricated, and measurements were carried out. The antennas were made by wrapping a copper foil tape, suitably shaped to reproduce the typical slots of the analyzed antenna (Fig. 2b), on a plastic cylindrical support. Fig. 3 shows the fabricated prototypes. In Fig. 3b the inner metallic tube is visible.

a). b). c).

Figure 2. Birdcage antenna configuration: a). 3D configuration of a four slots birdcage; b). Section AA’B’B takes a cylindrical shape once the edges AA’ and BB’ are wrapped around z-axis; c). Cross section of four slots birdcage.

TABLE I. GEOMETRICAL PARAMETERS OF BIRDCAGE ANTENNAS

Birdcage-4S Birdcage-2S

R[mm] 40 70 L 410 410

Circumference [mm] 251.2 440 s(n) [mm] 31.4(4) 55(2)

[°] 45(4) 45(2) m(n) [mm] 31.4(4) 165(2)

β [°] 45(4) 135(2) Rtube=20mm

h=10mm

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Fig. 4 and Fig. 5 show the simulated and measured

reflection coefficient for each prototype, i.e. prototype A and B for the Birdcage-2S, and prototype C and D for the Birdcage-4S. Due to inaccuracies in the realization of the prototypes, a frequency shift between simulated and measured reflection coefficient was noticed. However it is not significant in this analysis.

Fig. 6 shows the simulated gain for the traditional dipole,

the Birdcage-2S and the Birdcage-4S. It was noticed that the proposed birdcage antennas have similar gain over the bandwidth.

It is well known that the mutual coupling between collinear dipoles is due to radiation or parasitic currents induced on the outer conductor of the feeding coaxial cables and, based on what previously asserted to the supporting grounded tube. Radiation coupling can be reduced by properly positioning the antennas in a collinear array and at a certain distance. The coupling due to the parasitic currents is more difficult to control and, because of the mechanical issues, is often dominant. On the contrary, when coupling by parasitic currents on the coaxial cables is minimized, radiation coupling is the dominant factor. Thus, when cables and tube effects are neglected, a 30dB [7] isolation requires at least 1.5 inter element spacing, corresponding to a 1.5m spacing between the centers of two dipoles at 300MHz.

When a non-conductive tube is used to separate adjacent array elements, cables and connecting tube effects can be ignored.

Our aim is to show that birdcages forming a collinear array separated by a metallic tube provide lower coupling than two traditional dipoles separated by a Teflon tube. In both cases, feeding cables lye inside the supporting structure and both antennas and cables coupling are accounted. Measurements have been carried out, using a Vector Network Analyzer (VNA), in an open space with no surrounding buildings. The lower antenna was at 3.3m from the ground.

Fig. 7 and Fig. 8 show the measured mutual coupling comparison. For traditional dipoles a -12dB average mutual coupling was obtained when the antennas are at 0.9 and -21dB for 1.6 inter-dipoles distance. Two Birdcage-2S provide a 30dB average isolation for d=0.9 and 39dB for d=1.6 . Two Birdcage-4S provide a 38dB average isolation for d=0.9 and 39dB for d=1.6 . An improvement of more than 18dB is gained with the proposed configuration.

Figure 6. Simulated gain for the traditional dipole (TD), Birdcage-2S and

Birdcage-4S.

Figure 4. Simulated and measured reflection coefficient: Prototype A and B

for the Birdcage-2S antenna.

Figure 5. Simulated and measured reflection coefficient: Prototype C and D

for the Birdcage-4S antenna.

(a) (b)

Figure 3. Prototype of Birdcage antenna: (a) Birdcage-2S; (b) Birdcage-4S.

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IV. CONCLUSIONS Mutual coupling between dipoles working in the ATC UHF

band was investigated. An antenna solution was proposed to overcome isolation issues due to metallic supporting structures. The designed antenna was a slotted folded dipole and two different solutions have been implemented through numerical simulations and prototyped. An array system consisting in a pair of cylindrical birdcage-like dipoles was built and analyzed. An improvement of 18dB was noticed between two adjacent birdcage antennas placed on a metal tube with respect to two traditional dipoles separated by a non-conductive tube. According to this results, the proposed antenna can be suitable for low coupling implementation.

ACKNOWLEDGMENT The authors would like to thank Telsa S.p.A.

(www.telsasrl.it), Bergamo-Italy, for funding the research activity and for giving technical support during the experimental activities.

REFERENCES

[1] P. Patrick, D. Haitu, “Multiple band collinear dipole antenna”, USA Patent 2010/0302116A1, December 2010.

[2] E. B. Joffe, “A comparison of the coupling between collocated VHF antenna on a common mast in various configurations”, IEEE Int. Symp. On Electromagn. Compat., pp. 488-493, August 1997.

[3] D. Campbell, “Highly decoupled collinear antennas”, Int. Symposium on Antennas and Propagation, vol. 16, pp. 439-445, 1978.

[4] D. P. Kaegebein, “Parallel fed collinear antenna array”, USA Patent 6,057,804, May 2000.

[5] E. G. Price, R. F. Gordon, “Parallel fed collinear dipole array antenna”, USA Patent 6,720,934, Aprile 2004.

[6] C. Balanis, “Antenna theory-analysis and design”, John Wiley & Sons, 2nd Edition, 1997.

[7] W. Wasylkiwskyj, W. K. Kahn, “Mutual coupling and element efficiency for infinite linear arrays”, Proceedings of the IEEE, vol. 56, no. 11, pp. 1901-1907, November 1968.

Figure 8. Measured mutual coupling between: two adjacent Birdcage-4S on metallic tube, two adjacent Traditional Dipoles (TD) on PTFE tube, and two

adjacent Traditional Dipoles (TD) in open space.

Figure 7. Measured mutual coupling between: two adjacent Birdcage-2S on metallic tube, two adjacent Traditional Dipoles (TD) on PTFE tube, and two

adjacent Traditional Dipoles (TD) in open space.

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