This article was downloaded by: [University Of South Australia Library] On: 11 August 2014, At: 09:18 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Electromagnetic Waves and Applications Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tewa20 High-power broadband-loaded monopole antenna with sleeve ground plane for portable applications Waqas Mazhar a , Farooq Ahmad Tahir b & Farooq Ahmad Bhatti a a Military College of Signals (MCS), Rawalpindi, Pakistan b School of Electrical Engineering and Computer Science, National University of Sciences and Technology (NUST), Islamabad, Pakistan Published online: 28 Feb 2014. To cite this article: Waqas Mazhar, Farooq Ahmad Tahir & Farooq Ahmad Bhatti (2014) High-power broadband-loaded monopole antenna with sleeve ground plane for portable applications, Journal of Electromagnetic Waves and Applications, 28:7, 802-814, DOI: 10.1080/09205071.2014.891952 To link to this article: http://dx.doi.org/10.1080/09205071.2014.891952 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
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This article was downloaded by: [University Of South Australia Library]On: 11 August 2014, At: 09:18Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Electromagnetic Waves andApplicationsPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tewa20
High-power broadband-loadedmonopole antenna with sleeve groundplane for portable applicationsWaqas Mazhara, Farooq Ahmad Tahirb & Farooq Ahmad Bhattiaa Military College of Signals (MCS), Rawalpindi, Pakistanb School of Electrical Engineering and Computer Science, NationalUniversity of Sciences and Technology (NUST), Islamabad, PakistanPublished online: 28 Feb 2014.
To cite this article: Waqas Mazhar, Farooq Ahmad Tahir & Farooq Ahmad Bhatti (2014) High-powerbroadband-loaded monopole antenna with sleeve ground plane for portable applications, Journal ofElectromagnetic Waves and Applications, 28:7, 802-814, DOI: 10.1080/09205071.2014.891952
To link to this article: http://dx.doi.org/10.1080/09205071.2014.891952
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.
This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
High-power broadband-loaded monopole antenna with sleeve groundplane for portable applications
Waqas Mazhara*, Farooq Ahmad Tahirb and Farooq Ahmad Bhattia
aMilitary College of Signals (MCS), Rawalpindi, Pakistan; bSchool of Electrical Engineering andComputer Science, National University of Sciences and Technology (NUST), Islamabad, Pakistan
(Received 2 October 2013; accepted 4 February 2014)
This paper presents a novel design (loaded antenna with sleeve as a ground plane)for broadband-loaded monopole antenna. Monopole antenna is usually known for itsnarrow bandwidth characteristics. To achieve broad impedance bandwidth, the nar-row bandwidth characteristics of wire monopole antenna is overcome by propercapacitive or inductive loading. The high capacitive behavior of antennas at low fre-quencies is controlled using RF transformer in conjunction with proper matchingnetwork. The proposed antenna is capable of handling high power up to 100WattsCW. The bandwidth characteristics of the proposed antenna are investigated throughoptimization routine namely Genetic Algorithm in CST Microwave Studio. A band-width of 10:1 within the frequency range of 84–890MHz is achieved with antennalength of 1.9 m. By switching the antenna to different configurations with length of1.4 m, bandwidth of 750MHz (from 250MHz to 1 GHz) is also achieved. Theantenna radiation pattern is measured using anechoic test chamber. The antennameasured and simulated results are in good agreement.
Recent advances in mobile technology like spread spectrum band, frequency hoppingtechniques usually require antennas capable of supporting broad impedance bandwidthwith omni-directional radiation pattern. High-frequency broadband antennas are widelyused in military applications.[1–3] The monopole antennas are the most suitable choicefor this type of applications due to their omni-directional radiation pattern. However,monopole antenna has one fundamental limitation of low-resonance bandwidth, whichone can overcome by proper loading of antenna.
Recently, a number of broadband-loaded antenna designs have been reported in [4].In this paper, we will discuss new approach for broadband impedance matching ofantennas at low frequencies. The designing of low-profile high-performance antenna atlow frequencies is a very much daunting task. In our design, we have deployed bothdistributed and lumped impedance matching techniques. Many researchers have foundthat the load consisting of parallel inductor/resistor configurations are more useful whileexploiting the broadband impedance phenomena at low frequencies.[5,6] The author ofthe papers [5,6] have used Genetic Algorithm (GA) to design broadband monopole
antenna with three inductor/capacitor loads and matching network consisting of RFtransformer and series capacitors along shunt inductor.[7] It is observed in sensitivityanalysis presented in [4] that small change in component values and position has littleeffect on overall system performance.
For broadband antenna, one has to make compromise between impedance band-width and pattern bandwidth. A stub-loaded monopole with enhanced gain is dis-cussed.[8] A distributed loaded monopole antenna is mentioned in [9]. A novel designprocedure for the synthesis of loaded wire antennas is proposed in [10] using stochasticalgorithms, e.g. simulated annealing,[11] (global) evolution strategies,[12] and GA.[13–17]
Because of market demand for miniaturization of communication devices, it isobligatory to explore a method to reduce the dimensions of antennas. In order to takethese requirements in consideration, the loaded monopole antenna having one or moreinternal-lumped elements are introduced.[18] Then changing load values and its posi-tion result in radiation pattern change and in current distribution of antenna which mayresult in wideband behavior of antenna. Using concept of lumped elements, the antennabandwidth can be extended at the cost of lower radiation efficiency. In fact, if antennacould achieve optimal efficiency over wider range, its bandwidth cannot be brokenthrough the Chu limit.[19]
In order to overcome the above-mentioned real-world difficulties, in this paper, anovel design of a capacitive-loaded monopole antenna is proposed and developed. Thedesigned antenna has characteristics of broad impedance bandwidth, compact portablesize, and high-power handling. Instead of using λ/4 wavelength ground plane, sleeveground plane is proposed to make antenna suitable for portable applications.
The optimization of different antenna parameters is done in CST Microwave studiousing GA.[20] GAs are computing algorithms having close resemblance with naturalevolution process.[8] They are very famous algorithms for providing good solution forgeneral spaces as well as poorly defined spaces.
2. Antenna design and analysis
This antenna model is divided into two sections. First consists of RF transformerwith matching network at the antenna input while second section consist of isantenna with distributed load. Distributed matching techniques and lumped componenttechniques are the two fundamental techniques for impedance matching of antennas.Lumped component matching techniques are concerned with matching network at thefeed point or at the input of antenna. On the other hand, in distributed matchingtechnique, the impedance matching is achieved through transforming the geometry ofthe antenna itself. Impedance matching by lumped components is used mostlybecause it is quite difficult to transform the whole shape of the antenna while havingcontrol on antenna performance parameters like S parameters and radiation pattern. Inthis paper, we have exploited both lumped component and distributed matching tech-niques for good impedance and pattern bandwidth of antenna. Antenna geometry isgiven in Figure 1.
2.1. Antenna 1
Dimensions of the antenna without matching network are given in Table 1 below
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2.2. Antenna 2
Dimensions of antenna geometry with a matching network are given below. TeflonSpacer with name C1 and C2 are used for creating the capacitive effect between tworadiating element of antennas whereas abbreviation R1 and R2 is used for powerresistor (Table 2).
Figure 1. Proposed antenna geometry.
Table 1. Table of dimension.
Element name Dimension
Radiator1 (dia × height) 13 mm × 840 mmRadiator2 (dia × height) 12 mm × 635 mmRadiator3 (dia × height) 13 mm × 425 mmC1 (Teflon spacer) (dia × height) 13 mm × 1 mmC2 (Teflon spacer) (dia × height) 13 mm × 1.25 mmSleeve (dia × height) 70 mm × 180 mm
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3. Simulation and analysis
The antenna is designed in CST Microwave Studio Suite using transient analysis. Thedesign is started by taking into consideration the basic monopole antenna with groundplane of λ/4 wavelength. One of the major problems with standard monopole antennasis their size and portability issue at low frequencies where wavelength is quite largeand secondly monopole antennas are usually frequency sensitive with low resonatingbandwidth. Both of the above-mentioned problems can be resolved using the idea ofsleeve monopole antenna, as a result the antenna requires less space and is easily porta-ble while impedance bandwidth is also enhanced. The simulated model consists ofthree radiating elements and two capacitive loads.
The capacitive loads are simulated in CST Studio by placing the Teflon spacers i.e.C1, C2 between the second and third radiator. The simulated antenna model is shownin Figure 2.
In Figure 2, red ribbon indicates Teflon gap for simulating capacitance and arrowsare showing their optimized position in the antenna geometry. The three radiators aremade up of brass tubes of diameter 12 mm and 13 mm, respectively. The antenna iscapable of power handling up to 100 watts CW.
4. Parametric analysis and optimization
4.1. Effect of capacitve loading on antenna reactance
For efficient power transfer to load i.e. reactive part of antenna’s load should be zeroor close to zero which is quite difficult task for broad range of frequencies. Evenimpedance matching at the input of antenna is useless without making particulararrangement i.e. (proper loading of antenna). In order to achieve this objective, theantennas capacitive reactance in whole frequency range is first minimized with the helpof capacitive loading. For this purpose, load values and positions are optimized usingGA.
Figure 3 shows that antenna high capacitance at lower frequencies is reduced con-veniently by capacitive loading i.e. with no load the maximum imaginary parts is foundto be −678j at 85MHz, which reduces to −541j using one capacitive load and to−447j using two optimized capacitive loads. Once reactance in the most of operativefrequency range is made capacitive, the antenna can be tuned using single inductor atantenna input with RF matching transformer. This technique of capacitive loading
Table 2. Table of dimension.
Element name Dimension
Radiator1 (dia × height) 13 mm × 700 mmRadiator2 (dia × height) 12 mm × 280 mmRadiator3 (dia × height) 12 mm × 320 mmC1 (Teflon spacer) (dia × height) 13 mm × 1mmC2 (Teflon spacer) (dia × height) 13 mm × 1.5 mmRF transformer 2:1Resistor R1 10 ΩResistor R2 250 ΩSleeve (dia × height) 70 mm × 180 mm
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provide great advantage in design of broadband antennas.[21] The graph regarding theoptimized capacitive loads of antenna reactance is shown in Figure 3.
Figure 2. Simulated VHF Antenna.
Figure 3. Antenna reactance after capacitive loading blue curve with symbol (A) representingantenna reactance without capacitive loading, red curve with symbol (B) representing antennareactance with one capacitive load and green curve with symbol (C) representing antenna reac-tance with two capacitive load.
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4.2. Effect of sleeve ground with single radiator element on antenna inputimpedance
Extensive analysis of ground plane geometry is also done for making antenna feasiblefor portable applications. The antenna as a complex load has wide spread in SmithChart as shown in Figure 4. For broadband impedance matching, it is necessary toreduce the spread of antenna impedance on Smith Chart. It is observed that for antennawith single radiator, the magnitude of impedance reduces significantly by increasing theheight of ground sleeve. The height of ground plane sleeve is optimized to achieve theminimum possible spread of antenna load on Smith chart.
4.3. Effect of ground plane sleeve on return loss of antenna having two fixed loads
Parametric analysis of height of ground sleeve with respect to whole antenna geometryi.e. (with fixed loads C1 = 1 mm and C2 = 1.5 mm) is also done showing the significanteffect of height of ground plane sleeve on the antenna characteristic bandwidth asshown in Figure 5.
4.4. Optimiztion
After doing an extensive parameteric anaylsis, all the antenna paramters are optimizedto achieve overall improved performance. The different antenna paramters, design
Figure 4. Smith chart read curve is for antenna 1 having no impedance matching, green curveis for antenna 2 with impedance matching and purple circle is for VSWR = 2.
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goals, and optimization setting are given below in Tables 3–5, respectively. The Sparameter result of the optimized antenna is shown in Figure 9 with blue curve.
4.5. Impedance matching
The matching network for antenna is shown in Figure 6. In order to improve theantenna radiation resistance, high-power resistor of 10 Ohm (250Watts) is added in ser-ies of RF input to increase the antenna radiation resistance. The RF transformer (withiron core) of 2:1 with bifilar winding is used to step down antenna high input imped-ance at lower frequencies and iron core to remove the higher capacitive reactance atlower frequencies.
The Return loss with matching and without matching network for antenna 2 isshown in Figure 7.
4.6. Antenna fabrication
The geometry of designed antenna is fabricated and tested. The results of fabricatedantenna are found to be in good agreement with simulated results. The two antenna
Figure 5. Parametric analysis of ground sleeve height with return loss.
Table 3. Genetic algorithm setting.
Population size No of iteration No of solver evaluation Mutation rate Goal
4 × 50 50 5101 0.6 S11 ≤ −10
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prototypes are shown in Figure 8 as a fabricated antenna 1 and fabricated antenna 2.The white color Teflon sockets are used for the support of brass tube antenna radiators.The antenna is fed using custom-designed connector. The assembly of antenna radiatorfor two prototypes and custom-designed connector is depicted in Figure 9.
5. Measured results and discussion
The fabricated antenna results are measured through Agilent Network AnalyzerE8362B. The measured return loss of antenna 1 is shown in Figure 10 with blue curve.The measured return loss of antenna 2 is also shown in Figure 10 which is in reason-able agreement with simulated results. For antenna 1, antenna parameters radiator 1,
Table 4. Parametric values for optimization of Antenna 1.
Parameters Min (mm) Max (mm) Samples Resolution (mm) Optimized value (mm)
Table 5. Parametric values for optimization of Antenna 2.
Parameters Min Max Samples Resolution Optimized Value
C1 0.2 mm 5mm 24 0.2 mm 1mmC2 0.5 mm 4mm 35 0.1 mm 1.5 mmSleeve height 100 mm 200 mm 20 5 mm 160 mmRadiator 1 600 mm 800 mm 20 10 mm 840 mmRadiator 2 550 mm 700 mm 15 10 mm 635 mmRadiator 3 350 mm 500 mm 15 10 mm 425 mmR1 5 Ω 15 Ω 10 1 Ω 10 ΩR2 200 Ω 300 Ω 20 5 Ω 250Ω
Figure 6. Antenna matching network.
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radiator 2, and radiator 3 length are optimized to 700, 280, and 320 mm length, respec-tively, having no matching network with resonating bandwidth from 250MHz to 1GHz. For antenna 2 with matching network, resonating bandwidth changes from 84 to890MHz.
Figure 7. Simulted S parameters plot of loaded antenna for antenna 2 with and without match-ing network.
It has been observed from stimulated results mentioned below that at most of thefrequencies designed, antenna has omni-directional radiation pattern. The current distri-bution and radiation pattern of loaded monopole antenna is controlled with the help ofload values and load positions. The simulated and measured radiation patterns of
Figure 9. Loaded antenna radiator assembly.
Figure 10. Measured and simulated S parameters results, Green curve represents the simulatedresults for antenna 2 having antenna matching network, blue curve represents the measuredresults of antenna 2 and red curve represents the measured results for antenna 1 having no match-ing network.
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Figure 11. Antenna 2D radiation patterns in XZ plane.
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designed antenna at different frequencies are given below in Figures 11 and 12,respectively.
6. Conclusion
In this research paper, novel designs for VHF-UHF and UHF antennas for broadbandapplications are presented. The parametric analysis of different antenna parameters i.e.
Figure 12. Far-field measured E plane radiation pattern in XZ plane.
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loads values, loads position, and ground sleeve is closely analyzed with respect to theantenna performance. In order to achieve wide bandwidth characteristics, impedancematching is done in CST Microwave Suite. For proposed antenna, measured and simu-lated results are in good agreement. An antenna with optimized loads showed signifi-cant improvement in bandwidth and return loss by variation in height of ground sleeve.So it can be considered as key parameter while considering return loss and bandwidthenhancement. The designed antenna is capable of working at high power and in robustenvironment.
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