Ring loading of annular sector radiating line antenna

P.H. Rao and V.F. Fusco

Abstract: A modified annular sector radiating line antenna has been designed to improve the efficiency of the basic Anserlin antenna by introducing smooth or serrated annular rings around the antenna. Design curves generated for the modified Anserlin antenna are utilised to produce a four-element series-fed linear array with 18 dB sidelobe taper at 2.1 GHz without the need for an amplitude tapering network.

1 Introduction

In modem communication applications, due to operational requirements, circularly polarised and slant 45" polarised antenna arrays are a more popular choice than linearly polarised antennas. In [I-31, extensive theoretical analyses were presented for circular sector, annular ring and annular sector microstrip antennas. These antenna are capable of providing circular polarisation and slant 45" polarisation over a wide bandwidth [4]. To obtain wideband circular polarisation, an annular sector two-port radiating line (Anserlin) antenna had previously been developed, which could achieve a wideband impedance match 'and 3 dB axial ratio bandwidth [5] .

It has heen shown that Anserlin antenna radiating efficiency can be improved by changing the dimensions of the antenna in terms of ratio of outer radius (RI) to inner radius (R,) [6] (Fig. I). Hence in an array environment, different element geometries would be required for sidelobe shaping using the inherent S,, distribution across each of the radiating elements in a series-fed array [6].

In the present approach, improvements have been made to the basic Anserlin antenna element concept in order to improve its efficiency without modifying the basic Anserlin radiating element, which forms the kemel of the new structure. Here R I , RI are optimised and then fixed (Fig. 1). Then we load the antenna element extemally with smooth or serrated rings of various dimensions without modifying the geometry of the basic antenna in order to increase the magnetic current path length around the antenna and thereby improve its radiating efficiency. 'this parametric study yields design curves, which are then exploited in the design of an array. In order to take advantage of the optimised S,, performance, a four-element series array was designed to provide a weighting on the antenna in terms of S,, variation, without the addition of any further amplitude weighted feeding network.

0 IEE. 2003 IEE Procecdiqc online no. 2W30550 doi: 10. IC49jipmap:20030550 Paper h i M u d 22nd August 2W2 and in revisal form 3rd March 2W3. Online publishing date: 15 August 2003 The authors are with High Frequency Elecrrolucs Labonlories, School OC El%trical and Electronia En@neeting. The Queen's University of Belkst, Ashby Building, S t r m n i l l i s Road, BeWast-879 SAH, UK

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1 YX 2

semi-ngid cables with SMA connecton

Fig. 1 Anserlin antmna loaded with annular ring

2 Antenna element design

In the present approach, the design of the antenna mainly depends on maintaining constant impedance along the antenna structure from its input to output port. A travelling wave exists on the structure, which in turn results in wideband performance of the antenna. Details of the design methodology, and details of the starting dimensions used for the optimisation study, are given in [q. Normafiy, this type of antenna is fed at port one and terminated with a non-reflecting load at the other port in order to generate circular polarisation. For our study, the centre frequency of operation of the antenna has been chosen to be 2.1 GHz, operating bandwidth I S 2 . 5 GHz. The basic Anserlin antenna element structure, Fig. 1, was refined using H.FSS [7l to obtain the design dimensions in Table 1. The design optimisation process was subjected to as small as possible ground plane (11, x I&) that gave reasonable far field pattems and VSWR over the frequency band of interest. At

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Table 1: Anserlin element dimensions

Outer radius R, Inner radius R2 Width to height ratio of antenna

H Size of ground plane

1,

h W, W7

35 mm

9.5mm 3.6 7mm

115mmx1

20 mm

8 mm

33mm 3 mm

15"

2.1 GHz the measured parameters of the basic element are, SI,, -24dB boresight axial ratio, 2.1dB, gain, I.6dBi.

Unfortunately, the basic Anserlin antenna, in spite of its many advantages such as small sue, widehand circular polarisation and multi-octave impedance bandwidth, ex- hibits low radiation efficiency. To incorporate an efficiency variation into a basic unmodified Anserlin antenna element, the problem was quantified in the form of improving S2, of the antenna by loading it with ring elements without unduly affecting the basic antenna element circuit port impedance values or its far field radiation characteristics.

The approach adopted was to produce an open cavity in the form of an annular ring placed around the antenna element in order to reduce S,,, by radiation, for a fixed antenna SI,, hence increasing the radiation from the antenna. Ring size optimisation was camed out in terms of the dimensions of the smooth annular ring and the spacing between the annular ring and the antenna aperture d, d, in Fig. 1. In another configuration an annular ring plate with a serrated edge was introduced to increase the magnetic current path around the antenna and hence its power loss by radiation. The number of serrations and the spacing between the serrated plate and antenna were optimised for S2, improvement over a range of frequencies.

3 Smooth annular plate

The antenna configuration is shown in Fig. Ib, the separation between the smooth annular plate and the antenna (4 is vaned to optimise the basic Anserlin antenna elements' S2, characteristics. The measured results for SI,, S22 and S2, for the case of an annular ring placed at a distance (4 of 3.5 mm from the antenna shown in Fig. 2 are typical, here dl = 20". The measured results for S,, are shown in Fig. 3 for a variety of separation distances (4 of

I"

1 .o 1.5 2.0 2.5 3.0 3.5 4.0

frequency, GHr

Fig. 2 dZ3.5"

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Measured rerum loss and 1rummLwion loss

-2

4-

m -6 - c -8

-10 + 1.8GHr -x- 1.9GHr -2.0GHZ c 2.1GH1 -+ 2.2GHr -2.3GHz

-12 I 0 2 4 6 8 10 12 14 16 18 20

d, mm

Fig. 3 dl=20mm

S,, as a f ic t ion of d

the circular plate from the Anserlin antenna, in all cases a value of d, = 20 mm was maintained in order to minimise overall antenna dimensions. It should be noted that as S2, reduces more energy is radiated. Fig. 3 shows that this happens as the annular ring is moved closer to the Anserlin antenna perimeter. This is a direct result of extra-induced current on the annular ring. The SI, and S2, characteristics are insensitive to the variation in 'd' and broadly remain as in Fig. 2.

From the results it can be seen that a variation of 1.5dB in S2, at 2.0 GHz is possible, and at 2.5 GHz the variation is more than 4 dB. This is a useful improvement over the basic element when not ring loaded.

4 Serrated plate

The serrated plate antenna configuration is shown in Fig. 4; again SI, and S2, are similar to those given in Fig. 2. The separation distance between the circular serrated plate and the antenna (4 is varied to optimise the Anserlin antenna S,, characteristics, the depth of serration (d,) is fixed at

lop view

Side view h

1 2 G W semi-rigid Cables With SMA connectors

Fig. 4 Anserlin antenna with serrated annular plare

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IO", and (d2)= 2Omm is maintained constant in all the parametric studies undertaken. The variation in S2, is larger than in the smooth annular ring case previously presented Electromagnetic simulations [7] show that the magnetic current induced on the serrated circular plate has further to flow and that this is responsible for the increased variation in the S,, of the antenna. As for the smooth annular ring case, SI I and S2, of the antenna are not very sensitive to the presence of the serrated circular plate around the antenna. The variation of S,, with frequency, and as a function of serrated circular plate distance, is shown in Fig. 5. The variation of S,, with frequency, and as a function of serrated plate distance around the Anserliu antenna, is shown in Fig. 6.

-10

0

-10 -5 I

-

-12- " " " " '

m -15 n $ -20

-25

3 0

-35 L 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

frequency, GHz

S2, withfiepiency as a function ofserr~tedplate distance Fig. 5 d,=lOmm,d,-ZOmm,W,=lOmm

0

-2

-l

m D 1 -6 I

-8

-21GHz -22GHz -23GHz

serrated circular plate distance, mm

Fig. 6 S,, with serrated mnirlar plate distance

5

The antenna configuration is the same as shown in Fig. 4, except that the number of serrations on the circular plate around the antenna is varied. The distance between the serrated circular plate and the Anserlin antenna is held at d = 1 mm, the serration depth (d,) is maintained at lOmm and circular plate distance beyond the serrations (d2) is 20" and kept constant. Again, SI, and SZ are only slightly affected, and the antenna maintains its wideband characteristics. The variation of S2, with frequency, and as a function of number of serrations on the circular plate, is shown in Fig. 7.

Variation in number of serrations

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0

-2

4

B B - 4

-8

-1 0

-12 1 0 5 10 15 20 25 30 35

number of serrations

S,, wit/z w r y i n g nimhmher of serrurwnr Fig. 7

Of all the configurations discussed above, the variation of S2, is highest in the serrated circular plate configuration as the number of serrations on the plate increases. In the limit, it approaches the smooth annular ring. Therefore, in the demonstration array we use the annular ring loaded disc, which also has the advantage of ea% of manufacture.

6 Ring loaded four-element array

In order to achieve sidelobe suppression by amplitude weighting of the array elements, a symmetrical series feed network has been employed. A thorough examination of this approach has been given in [7J In addition, a mathematical basis for array sidelobe optimisation as a function of S2, tailoring has also been given in [I, and is not repeated here. In the present configuration, one port of the basic antenna element is excited while the other port is connected to form the feed to its nearest neighbour element, and the output port of the final element is terminated in a matched load. In this way, RHCP is generated (Fig. 8a). As the intrinsic input return loss of each antenna element is better than -15dB, it is assumed that the other port is automatically connected to a 50ohm load, i.e. no further matching network is required. The energy loss due to S,, variation across each element provides the necessary amplitude weighting.

A fourelement array was designed at 2.1 GHz with an interelement spacing of 1 10" (O.77&). The cables used to connect the adjacent elements when measured provide approximately k Y relative phase difference between the elements. The inherent taper provided by S,, variation across each of the elements comprising the array provides the amplitude taper necessdry for sidelobe reduction.

Due to size limitations of the measurement anechoic chamber (12' x 8' x 89, the maximum array size which could be accommodated was four-elements and so the achievable taper was limited, but sufficient to prove the ring loading methodology proposed in this paper. The config- uration of the centre-fed antenna array is shown in Fig. Xu, and a photograph of the antenna measured is shown in Fig. 8b. As can be seen, the array is fed symmetrically from its centre. This configuration is equivalent to an array of four right-hand circularly polarised elements with a taper resulting from the S,, variation across each element. Compared to the pdrakl-fed uniform distribution case, the series-fed configuration with no ring loading yields around 4 to 5dB sidelobe level improvement in the RHCP signal generated (Fig. 9). The slight asymmetry in the

341

110

90 _,,, 90 _j,_ 72

1:2 power divider

all dimensions in m m input

a

b

Fig. 8 a Schematic b Photograph

Riny landed 4-element Anserlin away

-180 -135 -90 4 5 0 45 90 135 180

0, degrees

Fig. 9 Comparison of meusured rudiutwn patterns for .serie.s fed unloaded loaded and pardlel fed imifDrm dirtrihution 4-elonent Anserlin array

radiation pattems observed is due to the mechanical asymmetry of the basic Anserlin element feed network ([SI, Fig. I , Fig. 86).

Next we examine the case where S,, further profits by the use of circular loading rings with the dimensions obtained from the design curves of Fig. 3. The centre two elements are excited with a normalised amplitude distribution of unity. The outer two elements (I and IV) are excited with an amplitude distribution of 0.59, which according to [9]

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should give 19.6dB maximum sidelobe level. This equates to an equivalent S2, for the array distribution for elements (I and Iv) of -4.6d8, which can be obtained from the design curves as a circular ring plate loaded at a distance of 1 mm from the antenna.

The resulting coplanar radiation pattem taken in the 4 = 0" plane clearly shows that a sidelobe level suppression of - I8 dB was achieved from the ring loaded configuration, cf -19.6dB predicted, - I I dB uniform distribution and -15dB for an unloaded series fed array. The pattem is symmetrical and crosspolarisation levels, not shown for clarity except for the ring loaded case, were better than - 1 1 dB on boresight for all the array configurations given in Fig. 9. Thus, sidelobe level suppression of -6.5dB better than uniform distribution was achieved. If the number of elements is made greater than four then enhanced sidelobe level suppression should be possible. The intrinsic cross polarisation level of the basic Anserlin element was - 16dB at 2.1 GHz and this crosspolarisation level increased to -12dB for a single element when ring loading was introduced. This is thought to be due to the extra current induced around the slightly asymmetric feed network. The measured gain of a single ring loaded element was 2 dBi.

The measured gain of the four-element array for the parallel fed uniform distribution at 2. I GHz was 5.5 dBi, for the unloaded configuration it was 3.9dBi, and for the ring loaded configuration the gain was 6.5dBi. From the gain figures it is obvious that, in addition to achieving good sidelobe level suppression, the ring loaded configuration is more efficient than the unloaded configuration.

7 Conclusions

A single Anserlin antenna type was designed to provide circular polarisation over a wide frequency range without further modification to the basic antenna element. By loading it with annular rings we provide a method to improve its radiation efficiency, or provide amplitude tapering in a series fed array configuration. Design curves were presented and a four-element array of annular ring loaded elements has been designed and tested. This arrangement provided -18 dB sidelobe level response, while - 1 1 dB was obtained for the uniform power distribution, parallel fed case. The measured gain for the ring loaded array configuration is 6.5 dB, a 2.6dB improvement over the unloaded configuration making the antenna more efficient. The resulting antenna array has low profde and needs a small ground plane, making it a suitable element for configurations where space is limited and where circular polarisation is required.

8 Acknowledgments

This work is supported by a British Commonwealth scholarship scheme.

9 References

Chada, R., and Gupta, K.C.: 'Green's functions for nrcular secto~s, annular rings, and annular Sectors in planar microwave circuits', IEEE T r m . Minow. 7heory Tech., 1981, 29, (l), pp. 68-71 Richards, W.F., Ou, J., and Long, S.A.: 'A theoretical and experimental investigation of annular. annular sector, and circular sector microstrip antennas', IEEE. Tram A n l e n m Propay, 1984, 32. pp. 864-867 HE", W., and Wong, K.L.: 'Single-feed annular-ring-sector microstrip antenna for circular polarisation', Microw Opt. Techno/. Lex, 1999, 22, ( I ) , pp. 7-10 F u w , V.F., and Rao, P.H.: 'Wideband dual slant linearly polarised antenna', submitled lo IEEE T r m . A n l e w Propag., 2002 Drewnisk, J.L., and Mayes, P.E.: 'ANSERLIN a broad-band, low profile, circularly polarised antenna', IEEE. T r m . A n t e m Propag, 1989,37, pp. 281-288

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6 Drewniak, J.L., and Mayes, P.E.: ‘The synthesis of pattems using a . series-fed array of annular sector radiating Ene (ANSERLN elements: IOW profile. nrcularly polarised radialon’, IEEE T r m . Anre- Propag., 1991, 39, pp. 184-189

7 High Frequency Structure Simulator, HP EEsof Design Technology, Sept. 1998

8 Balanis. C.A.: ‘Antenna theory, design and annalyiis‘ (John Wiley and Sons, New York, 1997, 2nd edn.)

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