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REDUCING THE SIZE OF HELICAL ANTENNAS BY MEANS OF DIELECTRIC LOADING

Mike B. Young, Kevin A. OConnor, and Randy D. Curry, Senior Member IEEE

Department of Electrical & Computer Engineering Center for Physical and Power Electronics

349 EBW, University of Missouri, Columbia, MO 65211 USA

Abstract Helical antennas have long been known as an excellent

choice for point to point communications. The lower the desired operational frequency of the antenna, the larger the physical size of the antenna. In order to overcome this issue, the University of Missouri-Columbia's Center for Physical and Power Electronics has been actively researching the properties of dielectric loaded antennas. Based on simulation models, it has been found that the addition of a dielectric material within the core of a helix will translate into a reduction in the operational frequency of the antenna. Using this knowledge, a 18.6mm diameter x 81mm tall helical antenna was designed around a core with a dielectric constant of r=45. This yielded a 65.4% decrease in the antenna's operational frequency from 5.13GHz to 1.775GHz as well as a 95.86% decrease in the physical size of a comparable 1.75GHz air core helical antenna. The program CST Microwave Studio was used to simulate these antenna designs derived from Krauss's formulas. The purpose of this paper is to describe the simulations and design steps that have shown to significantly reduce the physical size and center frequency of an axial mode helical antenna by loading it with a high dielectric core.

Index TermsDielectric Loading, Dielectric Core, Helical, Helical Antenna, Helix

I. INTRODUCTION

Helical antennas are extensively used in industry and were first introduced by Kraus in 1947 [1]. Applications for helical antennas include line of sight communications, particularly ground to satellite communication systems for which size and weight are limiting factors. Advantages of a helical antenna include circular polarization, high gain, wide bandwidth, and high directivity[2].

Most helicals consist of a flat or parabolic ground plane[3] attached to a single wound conductor. Varying the pitch angle [4] as well as the circumference [5] is not uncommon in order to achieve unique design goals.

In this article, the authors have designed and fabricated an axial-mode helical antenna loaded with a high dielectric core (r' = 45.0) with the goal of reducing its operational frequency without having to increase its

physical size. The final antennas radiation efficiency was evaluated from 100MHz to upwards of 4GHz.

II. THEORY

The formulas governing the operation and behavior of helical antennas have been long known, but are cumbersome to derive and analyze. Over the years, good approximations of the properties of helices have been made. Elical antennas are best described as an approximation between two basic radiating elements; a linear antenna, and a loop antenna. If one winding of the helix was unwound, we would have a wire of length L. The pitch angle of the antenna is best described as the angle of inclination of the spiral and is designated as . The pitch angle is directly related to S, or the spacing between each turn. As the pitch angle increases, so does the gap spacing S. C is the circumference of the spiral and is equal to *D, where D is the diameter of the spiral if viewed from above. If both ends of the helix were to be slowly stretched apart, the pitch angle would increase to 90 as the coil would be stretched out into a rod, also known as a linear antenna [6]. On the other hand, if both ends were to be compressed, the pitch angle would decrease all the way to 0, as would S.

At this point the coil becomes more and more compressed until we simply have a loop antenna. The operation of a helical antenna is best demonstrated as an approximation of everything between the minimum and maximum pitch angle , or a mix of linear and loop antenna theory [8].

Helical antennas operate in two distinct modes; the normal and the axial modes [1]. The normal mode occurs when the length of one turn of the helix is significantly smaller than the design wavelength ( L> ) [7]. When operating in this mode the gain is concentrated in the direction that the antenna is pointing, as seen in Figure 3.

Loading the antenna with a high dielectic material affects the phase velocity of the propagating electromagnetic wave [2]. Many numerical methods have

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been developed that model the behavior of the propagating wave through a lossy dielectric [9]

III. SIMULATION IN CST MICROWAVE STUDIO

Simulations were performed using CST Microwave

studio 2010. The simulation was based on actual dielectric material fabricated by the University of Missouri-Columbia (UMC) that has an average dielectric constant of 40 over a wide frequency range. The properties of the dielectric material were measured on an Agilent network analyzer attached with a dielectric measurement probe. The network analyzer sweeps from 0-4.5GHz and records the probes values of ' and '' as a function of frequency. The probe was first calibrated by sampling dielectrics with known constants; air and water. Measurements of UMC's dielectric material were then taken multiple times and averaged to ensure a high level of precision. The resulting table of dielectric parameters ' and '' vs. frequency of our material is shown in Figure 1. This table of parameters was then imported into CST's dielectric dispersion list for our material, which is modeled as the green CAD cylinder shown in Figure 2. The inner white cylinder is modeled after a nylon rod used to hold the final material together. The nylon rod was simulated as lossless and was set to have a dielectric parameter of '=3. The complete CAD model was created using all calculated dimensions and is comprised of a conductor wound around our hollow 18.63mm diameter dielectric cylinder threaded with a nylon rod. All of which are positioned on a large ground plate and fed through a 50 coax connection. The radiating coil is fed by port1, which simulates signal injection from a coaxial feed. Simulation accuracy was set for -60dB. A manual decoupling plane was set up at the location of the ground plane to aid in CST's automatic decoupling plane recognition algorithm. This helps to reduce erroneous false sidelobes in order to better view the true magnitude of real sidelobes [10].

Figure 3. 3D Farfield Gain plot at 1.629GHz.

IV. CONSTRUCTION

Construction of the antenna proved to be a challenge. We first had to manufacture the dielectric material to be used in the antenna. Using proprietary materials and processes, 3/4" diameter x .5" tall cylinders of compressed material were produced. An example of a similar cylinder produced is shown below in Figure 4. The material appeared to have much more tensile strength than shear strength. This made machining a bit of a cumbersome task. Due to the presence of nano-particles within the compound, all machining was done in a fume hood on a small combination mill/lathe. To begin, a small amount of surface material was shaved off of each cylinder in order to remove residue deposited during the manufacturing process. This left us with a cylinder that was 18.6mm in diameter. Each cylinder was hollowed in order to create the desired wall thickness. To accomplish this, each block was individually mounted on the end mill, and bored out to the proper diameter with boring bits. This method was simpler, more accurate, and risked damaging the material less than using a drill press.

Figure 1. Dielectric dispersion curve.

Figure 2. CST cad model of the helical antenna.

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Figure 4. Machined rod composed of dielectric cylinders.

The resulting hollow cylinders were held together using a threaded nylon rod and nylon nuts. Small amounts of a cyanoacrylate based adhesive was applied to between each block and allowed to dry overnight to form the final rod as shown above in Figure 4.

The constructed and simulated antennas have a pitch angle of 12 and a total of six turns. The spacing between the ground plane and the beginning of the first turn of the helix was kept to a minimum. This spacing adds capacitance to the line and its value can be tweaked in order to finely tune the antenna where small changes have a large effect. A nylon rod and nylon screws were used to attach the cylinder to a large brass ground plane. Next the antenna was wound with 3/16" copper wire and attached to a SMA connector on the other side of the brass ground plane. More cyanoacrylate was placed under the wire during the winding process to adhere it to the dielectric cylinder. The final constructed antenna is shown in Figure 5.

Figure 5. Final antenna in our anechoic chamber.

V. RESULTS

The antenna was placed on an Agilent network analyzer and swept to 4.5GHz. The results are shown in Figure 6.

Figure 6. S11 parameters.

Evaluating the S11 parameters, it can be seen that that the antenna is best matched at 2.3GHz, with the S11 parameters reaching -38dB. For comparison, a typical air-core helical antenna with a 18.6mm diameter and a pitch angle of 13 will have a center frequency of approximately 5.13GHz. With the addition of the dielectric, a downshift of approximately can be expected [11]. In this case, from Figure 1, the value of ' at 5.13GHz is approximately 36. From this we can expect a downshift from 3.746GHz to 863MHz.

-50

-40

-30

-20

-10

0

0.00E+00 2.00E+09 4.00E+09

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The antenna was placed in our anechoic chamber in order to plot find its gain as a function of frequency. The results are shown below.

Figure 7. Gain(dB) vs. Frequency for the antenna shown in Figure 5.

Gain was recorded from 1.6GHz to 1.95GHz at a maximum of 4dB. The measured gain levels shown above match up extremely well with the simulated gain of Figure 3.

With gain occurring at 1.775GHz, further improvement in the antennas radiation efficiency can be achieved by better matching the S11 parameters at this frequency. From Figure 6, the S11 parameters for the antenna at 1.775GHz is a poor -8dB. An appropriate matching circuit would help reduce the reflection coefficient and allow for better coupling from the signal source to the antenna. This would help to further increase the gain.

VI. IMPROVEMENTS

In order to further reduce the operational frequency and improve its gain, the University of Missouri of Columbia developed a newer dielectric with a higher ' and a lower ''. The dielectric dispersion chart for the new material is shown on the next page in Figure 8. [12]

Figure 8. Dielectric dispersion for UMC's newest high dielectric material with =100.

At our current antennas center frequency of 1.775GHz, the new material has a loss tangent comparable to that of the old material. Evaluating the two dielectrics at 500MHz the newer material has a loss tangent that is 82% lower than the older one used to construct our current antenna. This will allow for us to ultimately achieve higher gains at low frequencies.

In order to demonstrate this, the CST model antenna shown in Figure 2 was re-simulated. The white nylon rod in the center was removed in order for the green dielectric cylinder to be homogeneous. The dielectric dispersion curve shown above in Figure 8 was imported into CST's material properties for the green dielectric rod. It was not necessary to increase the diameter or length of the cylinder as the higher dielectric material will result in a greater frequency downshift in operational frequency. This new expected center frequency of 513GHz will match up well with the points on the dielectric dispersion curve where the loss tangent of the material is extremely low. The simulation was reran and swept from 100-1000MHz. The results of the simulation at 597MHz is shown below in Figure 9.

Figure 9. Gain profile simulation for the same size antenna shown in Figure 2 with UMC's new =100 material.

As can be seen, we are expecting a gain of 12.6dB to occur at 597MHz. The simulations conclude that the antenna will operate in the axial mode from 565MHZ-918MHz with a maximum gain of 17.8dB occurring at 775MHz (assuming that the impedance of the antenna is perfectly matched to its source). The gain of the antenna along the z-axis is plotted in Figure 10.

Figure 10. Simulated Gain vs. Frequency along the z-axis of the antenna.

Due to the limited supply of compounds within our dielectic material, the antenna simulated is still awaiting construction and is expected to be finished within a

1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 -12 -10 -8 -6 -4 -2 0

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Frequency (GHz)

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months' time. Seeing as how well the simulated gain levels of our antenna in Figure 3 matched up with the measured gain levels of the constructed antenna in Figure 7, there is a high level of confidence that the antenna still awaiting construction will match up nicely with the expected gain levels shown in Figure 10.

VII. SUMMARY

A 1.775GHz helical antenna was developed in an extremely small package (18.65mm diameter x 5" height). The downshift in operational frequency from 5.13GHz to 1.775GHz of the antenna is due to the introduction of a high dielectric material within the core of the radiator. This resulted in both a 65.4% reduction in operational frequency and a 95.86% reduction in the physical size of the antenna.

A new dielectric material was developed with a much lower loss tangent at low frequencies in order to improve the gain. This material was used to simulate a solid core helical of the same size (18.65mm diameter x 5" height) that operates in the axial mode at with a maximum gain of 17.8dB occurring at 775MHz.

VIII. ACKNOWLEDGMENT

We would like to acknowledge the help and advice of Everret Farr for selection of the antennas and advice in this regard.

IX. REFERENCES [1] Kraus, J.D. Helical Beam Antenna. April 1947, Electronics, pp. 109-111. [2] Gupta, Ramesh C. and Singh, S.P., "Propagation and Radiation Characteristics of Dielectric -Loaded Axial-Mode Helical Antennas," in Microwave and Optical Technology Letters, Vol. 51, No. 5. May 2009 [3] Djordjevic, A.R.; Zajic, A.G.; Ilic, M.M.; , "Enhancing the Gain of Helical Antennas by Shaping the Ground Conductor," Antennas and Wireless Propagation Letters, IEEE , vol.5, no.1, pp.138-140, Dec. 2006 [4] Zhou, G.; , "A non-uniform pitch dual band helix antenna," Antennas and Propagation Society International Symposium, 2000. IEEE , vol.1, no., pp.274-277 vol.1, 2000 [5] Yu Xinfeng; Gao Min; , "Simulation design of ultra-wideband helix antenna," Radar Symposium, 2008 International , vol., no., pp.1-3, 21-23 May 2008 [6] C. A. Balanis, Antenna Theory, Third Edition. Hoboken, NJ: John Wiley & Sons,2005, pp.566 [7] Volakis, John L., Antenna Engineering Handbook 4th Edition. s.l. : MgGraw-Hill, 2007. [8] E. Weeratumanoon, "Helical Antennas with Truncated Spherical Geometry," M.S. thesis, ECE. Dept., Virginia Tech., Blacksburg, Virginia, 2000.

[9] Tsuboi, H.; Tanaka, H.; Fujita, M.; , "Electromagnetic field analysis of the wire antenna in the presence of a dielectric with three-dimensional shape," Magnetics Conference, 1989. Digests of INTERMAG '89., International , vol., no., pp.EC12, 28-31 Mar 1989 [10] Young, M.B.; Norgard, P.; Curry, R.D.; , "Design, simulation, construction, and characterization of a wideband 900MHz-2.25GHz helical antenna," Antenna Technology and Applied Electromagnetics & the American Electromagnetics Conference (ANTEM-AMEREM), 2010 14th International Symposium on , vol., no., pp.1-4, 5-8 July 2010 [11] Altshuler, E.E.; Best, S.R.; O'Donnell, T.H.; Herscovici, N.; , "An electrically-small multi-frequency genetic antenna immersed in a dielectric powder," Antenna Technology, 2009. iWAT 2009. IEEE International Workshop on , vol., no., pp.1-3, 2-4 March 2009 [12] O'Connor, K.A.; Curry, R.D.; "High Voltage Characterization of High Dielectric Constant Components," 2010 IEEE International Power Modulator and High Voltage Conference, Atlanta, GA., May, 2010. Awaiting Publication