Proceedings of Asia-Pacific Microwave Conference 2007

Design ofA Novel Variable Phase Shifter

Jing-ke Zhou, Qing-Xin Chu, Ji-Song YangCollege of Electronic and Information Engineering

South China University of TechnologyGuangzhou, Chinaqxchugscut.edu.cn

Abstract A new type of variable phase shifter is designed byusing the Ansoft HFSS simulation software, which is based on thesuspended T-junction and its equivalent circuit. It can easilyrealize the characteristic of equal power and phase shift.

Keywords- electrically adjustable antennas; variable phaseshifter; AnsoftHFSS; equivalent-circuit

1. INTRODUCTIONElectrically adjustable antennas with phase shifters have

been used widely in the modem mobile communicationsystems. When the antenna positions are stationary, the beamsteering should be achieved by phase controlling, and thephase shifter is generally applied to control the beam.

The phase shifter is often a power divider with thecharacteristic of adjustable phase. Fig. 1 shows a highfrequency phase shifter unit having pivotable tapping elements[ 1 ]. The variable phase angle means that the lines havedifferent electrically effective lengths.

The arms of the phase shifter shown in fig. 1 are installed inthe same side of the pivot, so it is difficult to make equal powerdistribution in these four outputs as this structure is asymmetry.We proposed a novel variable phase shifter in this paper,whose lines are installed at different sides. This symmetricalline branching system can obtain the same power at both endsof this line, as shown in fig.2.

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Fig.2 The novel variable phase shifter

It can be seen from fig. 1 and fig.2 that the suspended T-junction is a key part in the pointer phase shifter. And theequivalent circuit of T-junction can help to design the phaseshifter more accurately. In [2] [3] and [4] we have proposedthe equivalent circuits of suspended microstrip T-junction andsuspended stripline T-junction. In this paper, a new type ofvariable phase shifter is proposed based on the suspended T-junction and its equivalent circuit.

II. STRUCTURE DESCRIPTION

To fabricate the phase shifter in symmetric structure, wehave done some special designs with the feed line and therotating arm, as shown in fig.3.The power can be divided moreequally and stably while phase shifter rotates different angledue to these special designs.

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We usually use the double coupling T-junction to makethe energy coupling more toughly. In order to make the CADof the phase shifter more precise, we should propose theequivalent circuit of the phase shifter based on the equivalentcircuit of the double coupling T-junction.

Fig.4 shows the proposed equivalent-circuit of the phaseshifter, where /, and z.. is the electric length and thecharacteristic impedance of the ith part (i 1, 2).

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Fig.4 the equivalent circuit of the phase shifter

The scattering parameters of the phase shifter can beeasily calculated from the equivalent circuit shown in Fig.4.

After the optimization, we can get the dimension of phaseshifter, where the characteristic impedance were calculated byCITS25.

Zo0 50Q 101 = 80mm Z02 = 40.2Q 102 = 20mm

Z03 34.8Q 103 23mm Z04 29Q 104 = 12mm

Z06 38.8Q 101 lOmm Z07 20Q 101 = 8mm

Z08 6Q 108 5.7mm Z05 = 50Q 105 = 90mm

Z05 50Q I 82mm

The frequency range is from 1.7GHz to 2.2GHz, in whichmost base station antenna systems operate. The U-shapesubstrate used to separate the conductor is made up of Teflon,whose permittivity is = 2.1.

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Fig.5 The calculated results of equivalent circuit

The magnitudes and phases of S-parameters are shownin Fig.9 by the equivalent-circuit. We can see that the returnloss SI, is lower than -12dB, S21 and S41 is nearly -6.5dB,which proves that the powers at different outputs are equal. Sothe proposed model is acceptable in the frequency range of ourinterests.

III. DESIGN PROCEDURE AND SIMULATED RESULTS

To prove the validity of the proposed equivalent-circuitmodel of the phase shifter and get the actual phase shifter, wefabricate the phase shifter based on the above dimension data,and compare the calculated results of the equivalent circuitwith its simulated results by Ansoft HFSS software andmeasured results.

Fig.6 The photo of the phase shifter

A comparison of the magnitudes and phases of S-parameters is shown in Fig.9 by the equivalent-circuit, HFSSand measurement, a good agreement is demonstrated, whichproves that the proposed model is reasonable in the 1. 71GHz-2. 17GHz. But we haven't consider the influence ofdiscontinuity point such as holes and steps in the equivalentcircuit, the return loss and the insert loss in the simulated andmeasured results are bigger than those of equivalent circuit.

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We compare the simulated and measured results togetherwhen the rotating angle is 30 degree and -30 degree, as shownin fig.8 and fig.9

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Fig.9 Comparison between simulated and measured results (-30 degree)

We can see from the fig.8 and fig.9 that the powerdistribution nearly keeps the same as the rotating anglechanging. And the simulated S-parameters of phase shifterhave a good agreement with the measured results, which hasdemonstrated the validity of the simulated results.

IV. CONCLUSIONS

Based on the equivalent circuit of suspended T-junction,a novel variable phase shifter is devised in this paper. Thisvariable phase shifter has achieved a good performance fromthe test and the analysis. The calculated results of theequivalent circuit have a good agreement with the simulatedand measured results, which has demonstrated the validity ofthis equivalent circuit.

ACKNOWLEDGMENT

This work is supported by the Science Fund of China(U0635004) and (No. 60571056).

REFERENCES

[1] M. Gottl, R. Gabridl, M. Markof, High-frequency Phase Shifter UnitHaving Pivotable Tapping Element, Pub. NO.:US 6,850,130 BI, Pub.Date: Feb. 1, 2005.

[2] J. Q. Gong, Q. X. Chu, "Equivalent-circuit Model of SuspendedMicrostrip T-junction," 2005 Asia-Pacific Microwave Conference(APMC'05), Su Zhou, Dec. 4-7, 2005, pp. 2826-2828..

[3] Q. X Chu and J.K. Zhou, "Modeling of Suspended Stripline T-junction,"IEEE Antennas and Propagation Society International Symposium,Albuquerque, 9-14 July, 2006, pp.3409-3411..

[4] J. K. Zhou and Q. X. Chu, "Equivalent Circuit of Suspended Stripline T-junction," Cross Strait Tri-Regional Radio Science and WirelessTechnology Conference, Macau, Oct.24-27, 2006, pp.230-232.

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