Balanced Amplifying Microstrip Patch Antennaat 2.4 GHz

*S.K. Behera, **R. K. Mishra and ***D. R. Poddar*National Institute of Technology/ Dept. ofECE, Rourkela, Orissa,

** Berhampur University/Department of Electronic Science, Berhampur, India

***Jadavpur University/Department of Electronics and Telecommunication Engineering, Kolkata, India

(5)

(6)The following gives sample design of a balanced amplifier.

The disadvantages include matched load to dissipate power inthe decoupled port; no virtual ground and hence less compactstructural realization.

(2)

(1)

(3)

In the above equations,

r. = S + SI2S2I~In 11 I-S22~

(4)For unconditional stability of the transistor, the necessary andsufficient condition is expressed by the following two

inequalities, in terms ofL\ (= SllS2rS21S12).

K = l-lsll-IS2J +ILlI2

> 1

21S 12S 2I1

From Fig 2 (a) for the VSWR, the matching at the input andoutput ports over the desired frequency band is observed to beexcellent. The performance of the amplifier is observed in Fig 2(b) for the noise figure and the gain. Both these observationsindicate a satisfactory design. It is also seen that the antenna is

III BALANCED AMPLIFYING ANTENNA

Using the optimization feature of the EDA software AWRMicrowave Office™, the amplifier was designed for optitnizedgain and noise figure. Normally, active integrated antennaamplifier design still follows the procedure of microwavetransistor amplifiers. The only difference being that theradiating patch acts the load in transmitting case and as sourceimpedance for the receiving antenna. If Zs is the complex sourceimpedance and ZI is the input itnpedance of the patch antenna,then the transducer power gain GT is defined as the ratio ofpower delivered to the load ZI to the power available froln thesource [7]. The expression for GT in terms ofrs and r l is

I " "_ 1- rsl- 1 12 l-Ir,l-GT -Il-r. r 1

2S21 11- S rl 2

In s 22 l

_ l-lrsl

2

1 12 1-lrl l

2

G - " S" "T 11- S r 1- _1 11- r r I-

II sol

II BALANCED CONFIGURATION

The balanced amplifier employs two quadrature hybrids.Reflections of the input signals due to poor matching arechanneled back to a matched load where they get absorbed.Same phenomenon occurs at the output port also. Therefore,theoretically, both at input and output ports one will seematched loads. A schematic of this configuration is depictedin Fig 1.

The real advantages of an ideal balanced configurationinclude good isolation with improved stability; good inputand output external matching; cancellation in the load ofproducts and harmonics like 2jj+h, 2h+jj, 3jj, 3h, ... andattenuation by 3dB of products like jj :th, 2jj, 2h... Furtherthe balanced amplifier has clear edge over the push-pullcounterpart in terms of the output impedance matching. It isalso more stable due to good isolation between its input andoutput sides. These characteristics have inspired us toconsider a balanced configuration for the active antennainstead of the conventional push pull configuration. Thisamplifier is simulated using AWR Microwave Office™ [6].

I INTRODUCTION

Most high power amplifying antennas contain twoindependent devices without any internal transversalconnection between them. Invariably these devices areconnected in the push-pull [1] configuration. Usually thepush-pull configuration is used for relatively narrow bandcommercial applications from UHF to S-band. Due to itspopularity with the circuit designers, it has beenimplemented in active integrated antenna.

However, there are a variety of alternativeconfigurations to combined external components such as180-degree splitters/ combiner (baluns [2]), 3 dB quadraturecouplers (like branch line or Lange couplers), in-phasecouplers (like Wilkinson couplers), etc. In microwavecircuits the balanced [3] configuration also finds wideapplication. This paper proposes the balanced configurationas an alternative to Push-Pull configuration.

Abstract- In this paper a balanced amplifier embedded poweramplifying antenna is proposed. The advantages of an idealbalanced configuration include good isolation with improvedstability; good input and output external matching;cancellation in the load of products and harmonics areconsidered in designing a balanced configuration for the activeantenna. The noise factor is slightly more than 2.21dB, whereas the gain is more than 10dB without any distortion ofradiation pattern at the design frequency of 2.4 GHz.

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properly matched (S22 around -22dB) at the desiredfrequency of 2.4 GHz. The noise factor is slightly more than2.21dB, where as the gain is more than 10dB. The radiationpattern at 2.4 GHz is undistorted, which suggests that theantenna is radiating in its dominant mode and the radiationis not contaminated by harmonic interferences.

IV DUAL FEED ACTIVE ANTENNA

The inherent disadvantage with the balanced amplifieris the use of the matching load on the return path. Thissection examines the possibility of elimination of this loadfrom the output port, using dual feed concept. It will alsodiscuss the results obtained using this concept. As is wellknown, the output from the two amplifiers differs by 90degree in phase. We propose to use the rectangular patchantenna with orthogonal inset feeds at two different points,as shown in Fig 3. The feeding microstrip lines at theseplaces differ by quarter-wave length. Thus the feed willinherently introduce a phase difference of 90 degree. Thisthrows a challenge in antenna design, because of the mutualimpedance between the feed lines. Due to the existingmutual impedance, electrical isolation of the feed linesbecomes a daunting task. The following procedure isadopted for this design.

First the amplifier is designed for a 50-ohlTI termination.Then the antenna is simulated. The starting point assumesno mutual coupling between the feeding lines. So to guessthe feed points we calculate the position where the self­impedance shall be 50 ohms. Then a simulation is done andS-Parameters are found out. Then by trial and error method,the feed positions were continuously changed. The next stepis the use of the antenna as an integral part of the balancedamplifier. For this the antenna is assumed to be a three-portdevice, with two input ports (to be connected to theamplifier output) and an imaginary output port matched tofree space. Then S-parameters for a three-port circuit aregenerated, using the S-parameter information of the inputports and the imaginary output port. This S-parameter file isthen used for simulation in the AWR Microwave Office™.This is used as a three-port sub-circuit. The two input portsare connected to the amplifier and the output port isterminated on a 376.6-ohm port. Fig. 4 (a) and (b) show theS-parameters at these three ports and between themrespectively.

The simulation schematic is shown in the Fig. 5 below.The results of the simulation for the gain and noise figurebetween the port 1 and port 2 are shown in Fig. 6. Theradiation pattern of the dual fed antenna is shown in Fig. 7.It is observed that the patterns of the mono-fed and dual fedantennas are identical. It suggests the same mode is beingexcited in both the cases and harmonics are not interfering.

The reason for this can be that the antenna due to its symmetryhas two distinct locations for excitation of the desired mode. Bydesign optimization, these locations are being used for feeding.The 90-degree phase shift between the outputs from the twoends of the balanced amplifier is being cancelled out due to useof additional quarter-wave length on one of the feeding lines ofthe antenna.

V CONCLUSION

This paper suggested balanced amplifying antenna andfound out its characteristics. For implementing the balancedamplifying antenna, first a balanced amplifier was designed. Itscharacteristics were observed for noise factor and gain. Then aninset fed rectangular antenna was used to replace the 50 Ohmmatched termination in the output port of the hybrid coupler.Again the noise and gain characteristics were observed. Finallya dual fed patch antenna replaced the hybrid. In all the cases itwas observed that it is not possible to obtain minimum noisefigure and maximum gain simultaneously.

REFERENCE

[1] K.Inoue, K.Ebihara, H.Haematsu, T.Igarashi, H.Takahashiand lFukaya, "A 240 W Push-Pull GaAs Power FET forW-CDMA Base Stations", 2000 IEEE MTT-S Digest, pp.1719-1722.

[2] R. Basset, "Three Balun Designs For Push-Pull Amplifier",Microwave July 1980, pp. 47-53.

[3] S.Song and R.Basset, "S-Band Amplifier Modeled forWireless Data", Microwave & RF, November 2000, pp. 53­160.

[4] S. Cripps, RF Power Amplifiers for WirelessCommunications, Artech House Boston London, pp. 294­302.

[5] L.Max, "Balanced Transistors: A New Option for RFDesign", Microwaves, June 1977, pp. 40-46.

[6] "AWR Design Environment", Applied Wave Research Inc.[7] G. Gonzales, Microwave Transistor Amplifiers: Analysis

and Design, 2nd ed., Prentice Hall, Upper Saddle River, NJ,1997.

[8] "Low Current, High Performance NPN Silicon BipolarTransistor", Agilent (HP) Technical Data 5965 - 8919E.

ONLINE RESOURCES

[1]www.educatorscorner.com/experiments/pdfs/exp98/exp98tech.pdf

[2] http://literature.agilent.com/litweb/pdf/5967-5486E.pdf

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I :, .1

U1IIIl..,,.....

1,1

Figure 1 Schematic of Balanced Amplifier

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11.1

~.lS J" ~,4

'fRPN:Y(Qia_ f,....-q fOUl'

Figure 2 (a) VSWR at the input (#1) and output (#2) ports. (b) Response of Balanced Amplifier

Figure 3 Orthogonal Dual Fed Patch Antennas

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.f'S

~1Gl't:t

Figure 4 (a) S-Parameters of the Equivalent Three Port for Antenna at each port (b) Equivalent Three S-Parameters of Port forAntenna between ports

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Figure 5 Simulation Schematics of the Dual Fed Balanced Anlnl"lhT1lncr Antenna

F....'f'JIClHl)

Figure 6 Gain and Noise Figure of the Dual Fed Balanced Amplifying Antenna

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..........................DtIpIt!J..Figure 7 Radiation pattern of dual-fed Balanced Amplifying Antenna (Red line Ee and green line Eel»

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