Near-Field Coupling Method for a Complex Navy Ship Environment Patrick Deschênes #1, Martin Coulombe #2, Robert Paknys *3, Amy R. Pinchuk #4 # InField Scientific Inc. 171 avenue Labrosse, Pointe-Claire, Quebec, H9R 1A3, Canada 1 [email protected] 2 [email protected] 4 [email protected] * Concordia University, Department of Electrical Engineering Montreal, QC, H3G 1M8, Canada 3 [email protected] Abstract— This article describes a method to solve near-field In addition, an Ideal Receiver is inherently based on all of coupling based on Hu’s formulation [1] which is applied to a the obstructions and transmitters being in the far-field of the complex navy ship electromagnetic environment where the receive antenna, which is not generally the case in the transmit antenna, receive antenna and obstacles may all be shipboard application. Therefore, a coupling analysis method located in the near-field. The method was developed for use with is required which takes into account the near-field responses boundary value based computational electromagnetic software packages to overcome their inability of calculating received of the antennas. power when using an aperture illumination antenna model. InField Scientific Inc. along with Professor Robert Paknys of Concordia University developed a Near-Field Coupling I. INTRODUCTION Method (NFCM) to solve this problem. Based on [1], the In a navy ship environment, the topside systems, structures near-field coupling of antennas can be calculated using the and obstacles are often located in the near-field zone of the transmitted fields of both the transmit and receive antennas in antennas. Without knowing the antenna internal geometries a shared environment which may include obstacles. and being only provided with limited information, coupling The next section provides information on the ship calculations between antennas may be a complex task, electromagnetic environment for which the near-field especially when far-field approximations are invalid. coupling method was developed. It is directly followed by the Furthermore, exact models of the directional antennas are not explanation of the technique along with verification examples widely available either because of their complexity or due to and an actual ship application. proprietary information. An accepted practice for simulating the main beam of a II. GENERAL OVERVIEW OF NAVY SHIP PLATFORM transmitting antenna when the internal geometries (radiator components, feed, etc) are not known is to model the antenna The analyses described in this article and in our companion by its aperture illumination [2], [3]. It is straightforward using paper [5] are required to support the mid life refit of the Computational Electromagnetic Software (CEM) such as Canadian Forces Halifax Class Frigate, Fig. 1. The length of FEKO [4] to model a transmit antenna using aperture the ship is 134.1 meters and the beam is 16.4 meters. The illumination. Unfortunately, the aperture illumination cannot Weatherdeck antenna farm comprises approximately 70 be used by most boundary value based software packages to antennas, covering frequency ranges from High Frequency derive the received signal which is generally required for (HF) to 40 GHz. There is unavoidable spectrum overlap source-victim coupling calculations. between systems and the source victim coupling analysis is A possible solution is to use an Ideal Receiver which is required to mitigate Electromagnetic Interference (EMI) conceptually similar to a point source. The Ideal Receiver problems. characteristics are derived from the simulated radiation pattern of the antenna under study. However, verification tests performed by the authors using this approach demonstrated that the accuracy was strongly dependent upon the exact location of the Ideal Receiver within the antenna geometry. Certain Ideal Receiver locations received no power while other positions received the peak coupling power. Since this location is arbitrary, it is impossible to predict or bound the accuracy of any given result. Fig. 1 Computational model of the Canadian Forcers Halifax Class frigate. 978-1-4673-2060-3/12/$31.00 ©2012 IEEE 256 III. NEAR-FIELD COUPLING METHOD EXPLANATION simplification technique of placing the Hu plane directly on The Near-Field Coupling Method uses Hu’s formula [1] to one of the antenna apertures can be used for most of the calculate the coupling: coupling calculations in order to reduce the required number 2 of simulations. E2 H1 E1 H2 nˆdS P S It should be noted that the above simplification introduces r 0 (1) P 16P P the inherent aperture field assumptions (η ≈ 377 Ω and plane t t1 t2 wave) into a portion of the Hu formulation which otherwise would have been exact. This would only introduce errors if Pr is the power received by either Antenna 1 or 2. the aperture field is significantly distorted, in which case to Pt is the power transmitted by either Antenna 2 or 1. begin with, the aperture illumination model would most E1, H1 is the field produced by Antenna 1 if it is probably not be applicable. Notwithstanding, this inaccuracy assumed to be transmitting. can be avoided by positioning the Hu plane at a location other E2, H2 is the field produced by Antenna 2 if it is than coincident with one of the antenna apertures and running assumed to be transmitting. the two simulations to calculate the field distributions from S0 can be any surface that separates the two antennas. each of the transmitting antennas. nˆ is the normal (the sign of nˆ does not matter). IV. VERIFICATION EXAMPLES Pt1 and Pt2 are the power transmitted by Antennas 1 and 2 respectively so that Often, we only have limited information about an antenna, e.g. its beamwidth, directivity, and sidelobe level. Under 1 * these conditions an aperture model is the best approach. (2) Pt1 ReE1 H1 nˆ1dS 2 S1 Accepting this limitation, the NFCM allows us to compute the and antenna coupling at any antenna separation with either or both 1 of the antennas modeled using aperture illumination. It * ˆ (3) Pt 2 ReE2 H2 n2dS overcomes the near-field inaccuracy problem of the Friis 2 S2 formula and those of the ideal point source receiver. The following examples illustrate the use of the NFCM and for S1 is the aperture of Antenna 1 and nˆ1 is the verification purposes compare the results with other solution aperture’s outward normal. methods. Antenna 2 has aperture S2 and outward normal nˆ2 . A. Identical aperture antennas The Hu formula is useful because it derives the power The first example is a calculation of the coupling between received by either antenna, P , from the transmit properties of r two identical aperture antennas operating at a frequency of both antennas without explicitly requiring their receive 3 GHz (wavelength λ = 0.1 m). One aperture is located at z = properties. This is key for the coupling analysis since CEM 0, and oriented such that −a/2 ≤ x ≤ a/2, −b/2 ≤ y ≤ b/2. The software packages are able to use aperture illumination to aperture size is a × b = 1 m × 0.5 m. Without loss of model antennas as transmitters; however, they are not able to generality, it is assumed that the aperture field is y-polarized model them as receivers. The Hu method is based on the and has a uniform amplitude and phase. This gives a reciprocity theorem and it is formally exact. directivity of about 28 dBi and a side lobe level of -13 dB. The shared Electromagnetic (EM) field plane S , henceforth 0 This is a large antenna (10 λ × 5 λ) with a narrow beam in the referred to as the Hu plane, could be set anywhere between the +z direction and high gain. A second identical aperture is two antennas provided that the coupling energy goes through placed with its center at (x, y, z) = (0, 0, d). Its main beam the Hu plane. points in the −z direction towards the first antenna. The goal In general, the NFCM would require two simulations to is to calculate the coupling between the two antennas. calculate the field distributions on the Hu plane: one The simplest solution for coupling calculations is to use the simulation for each antenna assuming that each is transmitting. Friis formula, (This is in contrast to most coupling calculations which require one antenna transmitting and the other receiving.) A 2 simplification is possible if the Hu coupling plane is P Coupling r G G (4) positioned at one of the aperture fields, then one of these t r Pt 4d simulations would not be required since the field distribution can be approximated by the known aperture illumination. For where the transmit gain and receive gain are example, if the Hu plane coincides with the Antenna 1 G G 1028/10 631. The far-field distance is 20 m; when aperture, then the field distribution on the shared plane due to t r Antenna 1 would not have to be simulated because it would be d < 20 m the Friis formula begins to fail. equal to the Antenna 1 known aperture illumination. The only To implement the Hu formula and the NFCM, it is simulation required would be to calculate the Antenna 2 convenient to put both the Antenna 1 aperture S1 and the Hu transmit field distribution on the Hu plane. This plane S0 at the same place, z = 0. Then, = nˆ = zˆ . 257 Following the methods of [3] to model the apertures, we The far-field zone of the IFF Interrogator is approximately assume a y-polarized aperture field having E1 = yˆ E0 and 9.89 meters and for the Communication Link antenna it is 5.21 meters. The shipboard distance between both antenna H = − xˆ (E /η) where E = 1 V/m and η ≈ 377 Ω.