Electronic beam tilting using a single reactively loaded circular wire loop antenna

H. Scott and V.F. Fusco

Abstract: The near-field current distribution along a circular loop wire antenna is modified by the inclusion of an electronically controllable reactive load, a varactor diode, into the antenna to provide beam pattern shaping. Modifications to a 4 1 balun are shown which permit varactor biasing while facilitating the impedance match of the circular loop, VSWR 1.3 to 1.9; t h s VSWR is superior to that previously reported for a reactively loaded wire antenna. In addition, it is shown that a reactively loaded dipole structure can provide only 10" beam tilt while the circular wire loop structure provides up to 27" beam tilt.

1 Introduction

Consideration of published work makes it clear that the addition of a reactive load(s) into a wire dipole antenna affects not only the current distribution and thereby the resultant far-field pattern, as desired, but also the impedance match to the antenna; normally the latter is affected in an adverse way. This approach has been studied theoretically for single and double reactively loaded dipole element [l] situations where it is necessary to preserve gain while enabling the pattern maximum to follow the direction of the incoming signal to provide better signal reception. In addition, beam tilting using a single reactively loaded dipole wire antenna provides the potential for a reduction of array size (i.e. array thinning) and hence cost over the classic methods of phased antenna array beam tilting [l, 21.

The integration of loads into dipole antenna structures is relatively well researched with two main results; the first is to position the load in order to maintain the optimum current distribution for matchng purposes [3] and the second is to choose a reactive load to induce an asymmetric current distribution and hence tilt the beam pattern [4]. However, the first methodology requires the current distribution to be a symmetrical resonant distribution while the second requires it to be asymmetrical. Thus it is difficult to use a single reactive load to carry out both these functions simultaneously. Unfortunately, to date ths has meant that impedance matching is a major issue when beam tilting using reactive loading is required. The best VSWR performance reported to date for reactively loaded dipole antennas has been achieved for the special case of fixed beam tilt [5]. Here a fixed beam tilt of 30" was obtained for a 0.82 asymmetrically fed reactively loaded dipole; reported VSWR was 2.2 and the antenna parameters were chosen to force resonance within the structure, hence improving the match of the structure to 50R. For the best reported example of electronically controlled beam tilt 32" has been

0 IEE, 2002 IEE Proceedings online no. 20020580 doi: 10.1049/ip-map:20020580 Paper first received 14th November 2001 and in revised form 25th April 2002 The authors are with the High Frequency Electronics Laboratories, School of Electrical and Electronic Engineering, Queen's University, Belfast, Ashby Building, Stranmillis Road, Belfast BT9 5AH, Northern Ireland, UK

IEE Proc. -Microw. Antennus Propag., Val. 149, No. 5/15, OctoherlDecember 2002

predicted accompanied by VSWRs in excess of 25 in a 50 !2 system [l]; no measured results were cited.

Previously it has been shown that, by introducing two reactive elements into a square loop placed over a ground plane, 30" beam tilt can be obtained [6]. In this paper, the, advantages of varying the near field current distribution of a reactively loaded circular wire loop antenna in terms of its far field characteristics and impedance matching perfor- mance are demonstrated and contrasted with those obtained for a reactively loaded dipole antenna. In addition it is illustrated how to electronically tilt the beam pattern while preserving a good impedance match to 50R using only one reactive element.

2

The inclusion of a reactive load in a wire antenna will cause, if placed off-centre, an asymmetrical modification of the near-field current distribution phase and hence cause far- field beam tilt. In this paper a dipole of length approxi- mately 1 h at 1 GHz (0.305m) was chosen to demonstrate thls theory. For this antenna length the resultant electric field pattern gives good directivity and maximum beam tilt capability for a single capacitive reactive load. For simulation purposes the dipole is centre fed, divided into 100 equal length segments, and has a capacitive reactive load placed at segment 76, Fig. la. The structure is simulated at 1 GHz using NE@ [A. For ease of practical implementation the load was made capacitive as ths could be implemented using a varactor diode. It should be noted that a variable inductor, if available, would allow beam tilting in directions opposite from those cited in this paper, ths aspect will not be pursued here. Initially capacitance values of 1 pF and lOpF were chosen for the simulations because these are close to the published limits for the varactor diode used here [8].

After optimisation by repeated simulation, using as goal criteria the trade-off between minimum variation in input impedance and best achievable beam tilt, the configuration for the reactively loaded dipole antenna was found, Fig. la. Examination of the magnitude and phase of the current distribution obtained for Fig. la is approximately symme- trical for the lOpF capacitor but notably asymmetrical when the capacitance is reduced to 1 pF, Fig. 2a. These results are as expected and are consistent with previously

Simulation of reactively loaded wire antennas

27 1

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Fig. 1 a Di pole b Circular loop

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Fig. 2 Current distributions for reactively loaded diople and circular loop antennas a Di pole b Loop

published theoretical work [9], which discusses in detail both the operating principle of near field current modification for beam tilt and the method of moments adapted to include reactive elements as used here [7] for simulation purposes.

272 IEE Proc.-Microw. Antennas Propag., Vol. 149, No. 516. OctoberlDecember 2002

when load capacitance is reduced to 1pF the current distribution becomes asymmetrical, Fig. 2h. This asymme- try again provides the beam tilt mechanism for the antenna.

The VSWRs of the optimised loop configuration, Fig. lb, before impedance matching with a 4:l balun, namely, 3.1 (at lOpF), and 4.2 (at 1 pF) at 1 GHz when simulated in a 50 R impedance system show a marked improvement when compared to the optimised dipole configuration, i.e. 22 (at IO pF) and 38 (at 1 pF). The addition of a 4: 1 balun into the circular wire loop antenna structure resulted in a further improvement with a predicted VSWR range of 1.7 to 1.9 in a 5 0 0 system. For the exact range of capacitive variation cited in the varactor datasheet, a maximum beam tilting potential of 30" in the xy plane was predicted for the optimised loop antenna; this is 12" more than predicted for the optimum reactively controlled dipole antenna case.

3 4: 1 balun structure

A simple balun structure providing the necessary 4:l impedance transformation was designed [IO]. The balun was constructed using coaxial cable with a dielectric constant of 2.2 making it less than one-third of the size of the loop. The cross-polar effects caused by the balun were limited by placing it orthogonal to the principal radiation plane of the loop, Fig. 4. In order to facilitate inclusion of varactor diode DC bias two additional components were required to be included in the balun, a DC bloclung capacitor and a choke, Fig. 4.

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i i capacitor

A

Fig. 4 Loop ccntennu structure: dicryruin und photogruplz

JEE Proc.-Microw. Antennus Propng., Vol. 149. No. 516, OctoberlDecernber 2002

The DC signal travels along the RF path to the anode of the diode through a bias T. To prevent the same DC voltage reaching the cathode of the diode, by travelling through the half-wavelength coaxial cable loop, a 1 nF DC blocking capacitor has been inserted at the end of the coaxial cable. This capacitor allows the required potential difference to be set up across the diode provided a DC short-circuit, RF open-circuit, to ground, can be realised. Consequently a choke measured to present -60dB loss at 1 GHz was included, Fig. 3. Inferior choke attenuation would lead to RF leakage to ground, hence gain reduction. The physical length of the varactor diode was 1.7mm, therefore it occupies less than one segment, 3"; in the simulation model it is assumed to occupy one segment.

4 Characterisation

Measurements of input impedance against bias voltage and the respective radiation characteristics for the reactively loaded wire antennas are now presented. When measured, up to 10" (18" predicted) beam tilt was demonstrated by the dipole antenna, and the measured VSWR ranged from 7 to 10. Measurements were carried out inside a 3.7m x 2.4m x 2.4m anechoic chamber with reflectivity -40dB at 1 GHz at the back walls and a general reflectivity better than -30 dB at 1 GHz. The actual beam steer of the loaded dipole antenna is less than predicted since the simple bias arrangement used for the dipole configuration uses a thin ground return wire parallel to one of the dipole arms. Due to its internal resistance one cannot provide the correct bias voltage across the varactor diode such that the lower capacitance values can be realised. It should be possible to improve this value by using a different bias scheme and/or by placing two or more varactor diodes in series. However, as the VSWR values for the dipole structure are large it was decided not to pursue further developments to ths structure.

Fig. 5 shows the simulated and measured VSWR for the reactively loaded loop antenna. The difference between the predicted and measured results occurs since the balun structure is modelled in the simulation by changing the reference impedance level for the structure from 5051 to 200 R, i.e. no attempt at creating a physically representative model for the balun in the simulator was made. The measured VSWR results for the loaded loop antenna after inclusion of the 4: 1 balun range between 1.3 and 1.9 which shows the utility of the novel balun arrangement proposed here. It should be noted that the simulated results of this section used values of capacitance, 9.6 pF at 0 V and 0.78 pF at 29V as stated on the varactor datasheet.

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Figs. 6 and 7 shows the radiation characteristics at the reactively loaded wire loop antenna. Figs. 6a and 6b show

273

- measured co-polar result measured cross-polar result

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the measured and predicted xy plane co-polar results (c.f. Fig. 4) for bias voltages of both 0 and -29V, respectively. It can be seen that for both cases the measured and simulated co-polar patterns are in good agreement. Here beam shape is well preserved for both 0 V and -29 V bias with a small increase in directivity as bias voltage increases. Only cross-polar measured results are given, as all simulated results lie below -40dB. In all cases the measured cross- polar results are at least 10 dB less than the maximum in the co-polar plane for both the 0 and -29V bias voltages.

Fig. 7 shows the xz plane (c.f. Fig. 3) measured co-polar and cross-polar patterns for bias voltages of 0 V and -29 V.

The close to omnidirectional pattern expected in this plane is shown for both voltages.

The loaded loop beam tilt results with varactor tuning voltage are compared to NE@ predictions, Fig. 5. Here, by using the integral balun/DC bias network it is demonstrated that 27“ of actual beam tilt, (30” predicted) in the xy plane could be achieved and that the beam tilt response is almost linear with tuning voltage.

5 Conclusions

Beam tilting has been demonstrated using a single varactor diode loaded circular loop antenna while for the first time simultaneously providing a good impedance match to a 50 R system. The varactor is used to electronically modify the near-field current distribution of the antenna and hence tilt its far-field pattern.

Investigation was carried out using the NEC@ simulation tool to find the configuration for a reactively loaded wire dipole and loop antenna which provided the greatest predicted beam tilt associated with the least susceptibility of input impedance to reactive load variation. It was shown that the dipole arrangement presents a considerably more difficult impedance matching problem than does the reactive circular loop antenna. A modified 4:1 balun was created to simultaneously match the loop antenna to 50R while allowing DC bias tuning of the varactor diode.

The radiation and VSWR characteristics of the loop antenna were measured and approximately 27” beam tilting in the xy plane was obtained whde VSWR vaned only from 1.3 to 1.9 over the entire varactor tuning range. In addition co- and cross-polar results exhibiting negligible pattern degradation with bias voltage variation were also demon- strated and the change in gain for the loop over the range of 27” beam tilt was less than 2dB. These results suggest the use of a reactively loaded wire loop antenna as a possible contender for receiver applications where beam modifica- tion is required when the use of a two-element array is prohibited due to space limitations.

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References

FUJIMOTO, K., HENDERSON, A., HIRASAWA, K., and JAMES, J.R.: ‘Small antennas’ (Research Studies Press, 1987) HIRASAWA, K., and FUJIMOTO, K.: ‘On electronically-beam- controllable-dipole antenna’. Proceedings of IEEE AP-S International Symposium, 1980, pp. 692-695 FINZIGER, P.D., LEVIATAN, Y., and ROZENKOVICH, J.: Broad-band reactively loaded dipole antenna’, Microw. Opt. Techn.

Lett., 1989, 2, (6), pp. 208-210 TANAKA, T., NAKAHARA, F., EGASHIRA, S., and SAKITANI, A,: ‘Determination method of the loading impedances of beam- tilting antenna with passive loads’. IEEE Antennas and Propagation Society, AP-S International symposium digest, 1997, vol. 3, pp. 1604- 1607 KIM, K.-C., and KWON, IS.: ‘Beam tilting dipole antenna elements with forced resonance by reactance loading’, IElCE Trans. Comnzun., 2000, E83-B, (l), pp. 77-83 LI, R.L., FUSCO, V.F., and CAHILL, R.: ‘Pattern shaping using a reactively loaded wire loop antenna’, ZEE Proc. Microiv. Antennas Propay., 2001, 148, (3), pp. 203-208 NEC-Win Professional vl . la 0 1997, Nittany Scientific Inc BB833 Silicon Tuning Diode 0 1999-2000, Infineon Technologies A.G HARRINGTON, R.F., and MAUTZ, J.R.: ‘Straight wires with arbitrary excitation and loading’, IEEE Trans. Antennas Propay., 1967, 15, pp. 502-515 JESSOP, G.R.: ‘VHF-UHF manual’ (Radio Society of Great Britain, 1972), Chap. 8

274 IEE Proc-Microw. Antennas Propug., Vol. 149, No. 516. OctoberlDecember 2002