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Scan Impedance for An Infinite Dipole Array: Accurate Theory Model Versus Numerical Software Sergey N.Makarov 1, Angelo Puzella 2, and Vishwanth Iyer 1 1 Worcester Polytechnic Institute, Worcester, MA Raytheon Company, Sudbury, MA ElectroScience Laboratory, OSU, Columbus, Ohio November 13 th 2008

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Outline Dipole array model with the sinusoidal current distribution and its limitations Analytical solution of VanKoughnett and Yen Extension of the solution to feeds/gaps of finite widths Comparison of the model with numerical simulations (Ansoft HFSS, CST MWS) Possible applications and extensions of the model

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Array model with the sinusoidal current distribution – strip dipoles

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Solution for the co-polar electric field Floquet theorem

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Scan impedance (comparison) Reaction integral Scan impedance

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Scan impedance (Refs.) R. C. Hansen, Phased Array Antennas, Wiley, New York, A. A. Oliner and R. G. Malech, “Mutual coupling in infinite scanning arrays,” in Microwave Scanning Antennas, Vol. II, R.C. Hansen, ed., Academic Press, 1966, Chapter 3, pp L. Stark, Radiation Impedance of a Dipole in Infinite Arrays, Hughes Aircraft Company Technical Report No. FL60-230, L. Stark, “Radiation impedance of a dipole in an infinite planar phased array,” Radio Science, vol. 1, March 1966, pp

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Comparison with numerical simulations (Ansoft HFSS, PML)

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Motivation Establish a more accurate (full wave) analytical solution for the infinite dipole array Compare this solution with numerical software using the different solvers and the different terminations (PML/Floquet port) Provide quantitative benchmark results for analytical and numerical models of the infinite dipole array

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Base analytical solution A. L. VanKoughnett and J. L. Yen, “Properties of a cylindrical antenna in an infinite planar or collinear array,” IEEE Trans. Antennas Prop., vol. AP-15, no. 6, Nov. 1967, pp The idea is to treat an infinite collinear 1D array as one infinitely long dipole with multiple feeds. A gap between two array elements is to be considered as another feed since it possesses some nonzero gap voltage. This voltage is indeed not necessarily equal to the feed voltage. The solution for a 2D array is then obtained as a combination of the (coupled) 1D array solutions.

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Base analytical solution - geometry

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Why is the infinite dipole (a true 3D TM to x field)? Because the analytical full-wave solution is known: the Pocklington integral equation is solved via Fourier transform in the space domain

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Pocklington integral equation and feed models

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Analytical solution to Pocklington integral equation R. H. Duncan and F. A. Hinchey, "Cylindrical antenna theory," J. Research NBS (D-Radio Prop.), vol. 64D, 1960, pp T. T. Wu, "Theory of the dipole antenna and the two-wire transmission line," J. Math. Phys., vol. 2, 1961, pp R. H. Duncan, "Theory of the infinite cylindrical antenna including the feedpoint singularity in antenna current," J. Research NBS (D-Radio Prop.), vol. 66D, no. 2, 1962, pp Y. M. Chen and J. B. Keller, "Current on and input impedance of a cylindrical antenna," J. Research NBS (D-Radio Prop.), vol. 66D, no. 1, Jan.-Feb. 1962, pp R. L. Fante, "On the admittance of the infinite cylindrical antenna," Radio Science, vol. 1, no. 9, Sep. 1966, pp R. W. P. King and T. T. Wu, "The imperfectly conducting cylindrical transmitting antenna," IEEE Trans. Antennas Prop., vol. AP-14, no. 5, Sep. 1966, pp E. K. Miller, "Admittance dependence of the infinite cylindrical antenna upon exciting gap thickness," Radio Science, vol. 2, no. 12, Dec. 1967, pp R. W. P. King, "The linear antenna - eighty years of progress," Proceedings of the IEEE, vol. 55, no. 1, Jan. 1967, pp R. A. Hurd and J. Jacobsen, "Admittance of an infinite cylindrical antenna with realistic gap feed," Electronics Letters, vol. 4, no. 19, Sep. 1968, pp L.-C. Shen, T. T. Wu, and R. W. P. King, "A simple formula of current in dipole antennas," IEEE Trans. Antennas Prop., vol. AP-16, no. 5, Sep. 1968, pp This integral is to be found numerically

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Integration contour (Miller)

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Test of the MATLAB’s solution Methodka=0.01ka=0.04ka=0.08 Chen and Keller [20]; orig. Eq. (22): constant gap field; total current; script circ1.m j j j4.38 Chen and Keller [20]; orig. Eq. (22): square-root gap field; total current; script circ1.m j j j4.63 Miller [23], orig. Eq. (1): constant gap field; outer current; script circ2.m j j j3.25 Miller [23], orig. Eq. (1): square-root gap field [16],[25]; outer current; script circ2.m j j j j j j3.41 Table 1. Antenna input admittance in mS for g/a=0.05 obtained using different models. The corresponding values obtained in Ref. [25] are marked bold.

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A passing remark: dipole antenna as a half wave transmission line resonator According to Shen, Wu, and King [26]: "The foregoing discussion seems to support the following picture of a dipole antenna. An outgoing traveling wave of current is generated along the dipole antenna when it is driven by a time- harmonic source. It travels along the two arms of the dipole with a speed almost equal to the speed of light, and decays slowly…, as a result of radiation. It is reflected at the ends of the dipole…. After it is reflected, the current wave travels in the opposite direction with the same speed and decays in the same manner as before. The current distribution on the antenna is just the result of the superposition of the outgoing current wave and all the reflected waves. This description of the current along a dipole antenna is analogous to that for a lossless transmission line…" The transmission line is the infinite wire with the second conductor at infinity

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Array solution Here we assume equal gaps and feeds (for simplicity only)

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Why is the array solution simpler than the solution for the infinite dipole? First guess: a double sum of the integrals in the complex plane? Integrals go away! Impulse train or Dirac comb The rest of the solution remains the same

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Centerpiece of the VanKoughnett and Yen’s model for wide feeds Periodic component; gaps are shorted out Periodic component; feeds are shorted out Express one in terms of another Total periodic component by superposition No current in the gap: only if I S is constant there Fortunately, the square root feed model is close to this assumption!

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Proof (periodic current component) Discuss the feed – the center

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Solution for a planar array Solution for the linear array: Solution for the planar array:

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Comparison with numerical simulations: E-plane scan (a /2 array) HFSS – Floquet port; SCT MWS – Floquet port

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Comparison with numerical simulations: D-plane scan (a /2 array) HFSS – Floquet port; SCT MWS – Floquet port

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Comparison with numerical simulations: H-plane scan (a /2 array) HFSS – Floquet port; SCT MWS – Floquet port

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Scan impedance error for all scan angles/planes (the half-wave array)

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MATLAB script array3.m for ph = 1:length(phi0) for th = 1:length(theta0) th kx0 = k*sin(theta0(th))*cos(phi0(ph)); % phase progression factor ky0 = k*sin(theta0(th))*sin(phi0(ph)); % phase progression factor Km = a*(kx0 + 2*M*pi/dx); % vector (for outer summation) temp = ka^2 - Km.^2; % vector (for outer summation) root = sign(temp).*sqrt(temp); % vector (for outer summation) BESSELJ = besselj(0, root); % vector (for outer summation) %F = sinc(Km*gtd/pi); % uniform gap field F = besselj(0, Km*gtd); % square root gap field root1 = sqrt(1 + (N*dy/a).^2); % vector (for inner summation) root2 = sqrt((2*h/a)^2 + (N*dy/a).^2); % vector (for inner summation) BESSELH = j*zeros(1, length(M)); for n = 1:length(N) BESSELH = BESSELH + exp(-j*(N(n))*ky0*dy)*... (besselh(0, 2, root*root1(n)) - besselh(0, 2, root*root2(n))); end Cm = F./(temp.*BESSELJ.*BESSELH); for p = 1:length(X) EXP = exp(-j*2*pi*M*a/dx*X(p)); Is(p) = 1e3*(4*ka/(const.eta*dx/a))*sum(EXP.*Cm); % Is in mA/V end q = 1; % 0 for tip zero current or 1 for center zero current V0_prime = -Is(end-1+q)/Is(P/2+q); Per = 1:P/2; I(Per) = Is(Per) + V0_prime*Is(Per+P/2); % I (per) in mA/V Per = P/2+1:P+1; I(Per) = Is(Per) + V0_prime*Is(Per-P/2); % I (per) in mA/V Im = I.*exp(-j*kx0*X*a); % Current solution Zs(ph, th) = 1e3/Im(P/2 + q); % Scan impedance (Ohm) end

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Limitations of the analytical model: no angular current symmetry

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Possible extensions and applications of the analytical model Array of strip dipoles A loaded infinite dipole array (capacitive, or inductive, or resistive loading between the dipole ends) An infinite dipole array with an (infinite) array taper (Gaussian amplitude taper, Gaussian phase correction, a load taper, etc.) A lossy ground plane, dielectric layers, etc. Straightforward Rather challenging, but perhaps important Challenging Straightforward

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Array of strip dipoles Green’s function remains the same if w/2 is replaced by a C. M. Butler, “ A formulation of the finite-length narrow slot or strip equation, ” IEEE Trans. Antennas Prop., vol. AP-30, no. 6, Nov. 1982, pp A predefined transcendental profile

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Test (array of /2 dipoles) a)b) c)d)

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Comparison with numerical simulations: E-plane scan (a /2 array)

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Comparison with numerical simulations: D-plane scan (a /2 array)

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Comparison with numerical simulations: H-plane scan (a /2 array)

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MATLAB script array4.m for ph = 1:length(phi0) for th = 1:length(theta0) th kx0 = k*sin(theta0(th))*cos(phi0(ph)); % phase progression factor ky0 = k*sin(theta0(th))*sin(phi0(ph)); % phase progression factor Km = a*(kx0 + 2*M*pi/dx); % vector (for outer summation) temp = ka^2 - Km.^2; % vector (for outer summation) root = sign(temp).*sqrt(temp); % vector (for outer summation) BESSELJ = besselj(0, root); % vector (for outer summation) %F = sinc(Km*gtd/pi); % uniform gap field F = besselj(0, Km*gtd); % square root gap field root1 = sqrt(1 + (N*dy/a).^2); % vector (for inner summation) root2 = sqrt((2*h/a)^2 + (N*dy/a).^2); % vector (for inner summation) BESSELH = j*zeros(1, length(M)); for n = 1:length(N) BESSELH = BESSELH + exp(-j*(N(n))*ky0*dy)*... (besselh(0, 2, root*root1(n)) - besselh(0, 2, root*root2(n))); end Cm = F./(temp.*BESSELJ.*BESSELH); for p = 1:length(X) EXP = exp(-j*2*pi*M*a/dx*X(p)); Is(p) = 1e3*(4*ka/(const.eta*dx/a))*sum(EXP.*Cm); % Is in mA/V end q = 1; % 0 for tip zero current or 1 for center zero current V0_prime = -Is(end-1+q)/Is(P/2+q); Per = 1:P/2; I(Per) = Is(Per) + V0_prime*Is(Per+P/2); % I (per) in mA/V Per = P/2+1:P+1; I(Per) = Is(Per) + V0_prime*Is(Per-P/2); % I (per) in mA/V Im = I.*exp(-j*kx0*X*a); % Current solution Zs(ph, th) = 1e3/Im(P/2 + q); % Scan impedance (Ohm) end

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A loaded infinite dipole array (capacitive or inductive loading) One line in a MATLAB code! Total periodic component by superposition Unloaded array Loaded array

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Test (a /2 array) Lumped LCR boundary Dipole feed

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Scan impedance at zenith CScan resistance, ΩScan reactance, Ω pF pF pF pF Scan impedances at 7 at 10 GHz and for different capacitive end-to-end loads. Two sets of numbers correspond to Ansoft HFSS data and to the analytical solution (marked bold), respectively. The analytical solution is obtained with the script array3.m.

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Scan impedance at different elevation angles

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Optimization of a wideband loaded strip dipole array (only at zenith – array5.m) Ben A. Munk, "A wide band, low profile array of end-loaded dipoles with dielectric slab compensation," 2006 Antenna Applications Sym., Allerton Park, Monticello, IL, 2006, pp for nh = 1:length(h) % first optimization loop for nr = 1:length(R); % second optimization loop r = R(nr); nr for nl = 1:length(C) % third optimization loop c = C(nl); for m = 1:length(f) [MAIN BODY - single calculation] end temp2 = max(max(max(RL))); temp3 = max(max(max(RL(:,:,1)))); if (temp2 < temp1) & (temp3 < -10) % at least -10dB RL everywhere NH = nh; NR = nr; NL = nl; temp1 = temp2; RL_out = RL; end Four nested loops; total about1,100 data points

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A 4:1 non-scanning array (array5.m)

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A 2:1 scanning array (array6.m) About 20,000 data points

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Array taper: preliminary remarks The quadratic amplitude taper leads to a single sum of the 1D integrals for the 2D array The Gaussian amplitude taper needs this sum to be expressed analytically, as a function of k and

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Conclusions For the infinite planar array of dipole antennas, we have extended the analytical model of VanKoughnett and Yen to the case of a finite feeding gap and a non-uniform field distribution in the gap. As a result, we were able to compare the accurate theory model with the numerical simulations for infinite dipole antennas arrays using two major antenna software packages - Ansoft HFSS v. 11 and CST Microwave Studio To our knowledge, such a quantitative comparison for the dipole array has been performed for the first time. The analytical array model seems to be a useful tool for array optimization MATLAB scripts are available online:

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Acknowledgements Authors are thankful to Dr. H. Steyskal for his continuous interest and support of this work, and for critical comments, and to Dr. R. C. Hansen for his interest and important comments. We would like to thank especially Mr. Robert Helsby of Ansoft Corporation/ANSYS for his encouragement and patience throughout different stages of this research. We are thankful to Mr. Mark Jones, now with Ansoft Corporation/ANSYS, for valuable insight, time and resources invested into this project. We are grateful to Mr. Frederick Beihold, CST of America, for numerous and extensive test and validation results related to the present study, and useful discussions, and to Dr. David Johns, CST of America, for the support. This paper has been completed when one of the authors (SNM) was on sabbatical leave with Lawrence Livermore National Laboratory, Livermore, CA.

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