Design of Antenna Elements for EISCAT 3D s Phased Arrays

Transcription

1 TECHNICAL REPORT Design of Antenna Elements for EISCAT 3D s Phased Arrays Gunnar Isaksson

2 Design of Antenna Elements for EISCAT 3D s Phased Arrays Gunnar Isaksson Luleå University of Technology Department of Computer Science, Electrical and Space Engineering EISLAB

3 ISSN: ISBN Luleå

4 Design of antenna elements for EISCAT 3D s phased arrays ISBN Gunnar Isaksson September 5, 2012 Abstract Design of the basic crossed polarization antenna element in the VHF band for EISCAT 3D s phased arrays has resulted in two competing designs that has some similarities but also differences. The virtues of the designs and the sometimes arbitrary design decisions leading to them are further elaborated in this article. 1

5 1 Requirements In this section some of the unique properties for the antenna elements are discussed in order to find the most important requirements. Also electrical and mechanical characteristics such as VSWR bandwidth and life cycle are discussed. These requirements are of course conflicting and will need to be balanced towards each other in a design. 1.1 Cross polarisation The antenna shall have two linear and orthogonal polarisations. 1.2 Antenna foot print Phasedarrayantennascansufferfromgratinglobesifthemainlobeissteered far away from bore sight and the individual elements are spaced too wide apart. However if theelement spacing isat most λ/2at thehighest frequency of operation, then grating lobes will not appear. Now, λ/2 is about the standard length of a straight half wave dipole which is a common construction element in antenna designs. In order to arrive at a usable design that gives some head room to λ/2 in all directions, the radiating elements will have to be turned and mangled to arrive at a small foot print. 1.3 Directivity In a phased array, it is desirable to steer the main lobe as far away as possible from bore sight and that can only be done if the individual antennas have a low directivity. Since the radiation is only into one hemisphere a directivity of 5 9 db seems a reasonable goal. 1.4 Front to back ratio The antennas will be locatedfar northin regionswhere deep snow is common during winter season. This will change ground elevation and conductivity during the year and creates a unique challenge for the arrays. There are a few ways to deal with that challenge but one way is to elevate the array a few meters above ground and let the snow simply fall through to the ground. Doing so creates yet another problem with ground reflections that creates 2

6 nulls in the radiating pattern of the array. This will require the use of antennas with a high front to back ratio and perhaps also a metallic mesh below the antennas. 1.5 Antenna efficiency One often used method in antenna designs have been to introduce more or less obvious losses somewhere in the antenna to increase it s bandwidth. Ground loss is often used and not so obvious. This is not acceptable since the antenna noise temperature will become an important part of the total system noise temperature. Efficiency shall not be sacrificed in order to arrive at a good bandwidth. 1.6 Power level For transmission, the antennas should be able to work with pulses of up to 500 Watts. It s not obvious that there will be any economical savings to design receiving antennas for a lower power level since all antennas have to be mechanically sturdy to withstand the climate. 1.7 Center frequency Both receiving and transmitting antennas is assumed to have a center frequency of 235 MHz. 1.8 Bandwidth The receiving antenna should have a bandwidth of 30 MHz where VSWR< 2 and the transmitting antenna a bandwidth of 5 MHz where VSWR< 1.2. These two requirements can possibly be joined in a single design. 1.9 Beam width The beam width in E and H plane should be as wide as possible which is consistent with the requirement of a low directivity. If the antenna optimization pushes for a low directivity, then the beam width will also become as wide as possible. 3

7 1.10 Environment The antennas are to be located in an Arctic environment where temperatures can varyfrom+30 C down toto 40 C andwind cansometimes reach storm conditions. Fortunately these two extreme conditions doesn t occur at the same time. Severe storms occurs mostly when the temperature is above the freezing point Life cycle The antennas and associated mechanical structures should have a total life length of 40 years. It is assumed that such a long life is accomplished via periodic maintenance. Manufacturers are supposed to state necessary maintenance intervals. 2 Design decisions The requirement for a high front to back ratio means that the antenna must have a reflector below the driven element and the low directivity requirement that no directors should be used. The only possibility consistent with these two requirements is a two element design with a driven element and a reflector. Ideally, the best reflector would be an infinite ground plane or a fine mesh of conducting wires. Due to the snow this would also require a radome above the antennas or they would else become buried in deep snow during long periods in winter time. In the chosen design, it s instead decided to use a small reflector for each antenna made out of conducting tubes. The driven element and the reflector can be designed in several ways and would create a lot of alternatives but only two of these are discussed here. A Quad element (a closed square loop) is used as a reflector due to its small foot print. It works equally well for both polarisations and have a smaller foot print than normal crossed reflectors made out of straight wires/tubes. To make the Quad into a reflector it s resonant frequency is just made somewhat lower than the operating frequency. For a driven element two different designs is used. First is a shortened dipole. It is cut a bit short and capacitively loaded at the ends to keep it s resonant frequency. Another alternative to fit a dipole into a small foot print is to bend the antenna tips down into an inverted V configuration. These two approaches are parametrized and optimized to reach as good performance as possible. 4

8 2.1 Antenna nr 1 The first antenna has as driven elements crossed shortened dipoles mounted above a reflector made out of a Quad element. Figure 1: Antenna nr 1 5

9 2.2 Antenna nr 2 The second antenna has as driven element a crossed Inverted V mounted above a reflector made of a Quad element. Above the driven element there is a parasitic open sleeve element for increasing the bandwidth. For even wider bandwidth, another set of parasitic elements below the dipole would be beneficial but was deemed unnecessary for this application. Figure 2: Antenna nr Parasitic open sleeve element The element increases antenna bandwidth and can also be used as a matching device without any other side effects since it s very close to the dipole. The parasitic elements can be electrically grounded to the mast tube in order to get better lightning protection. The open sleeve broadband antenna was first patented by Dr. J. T. Bolljahn and a patent application was filed in 1946 and granted 1950 with US patent number 2,505,751. He also used the term open sleeve in his patent application. An often referenced article is King & Wong 1972 [1]. A more modern and very inspiring article is Spence & Werner 2006 [2]. 6

10 3 Simulations and optimizations The software for simulating the designs uses NEC2 [3] for calculating the actual antenna fields and currents. This old and famous software is packaged together with a front end program 4NEC2 [4] that provides a graphical user interface and some very convenient tools for antenna development. The input file to 4NEC2 can be a standard NEC2 formatted input file but it also supports the use of symbolic variables and math expressions. This simplifies the description of an antenna. The program also have a built in optimizer that can use these variables to find a solution that optimizes a quality function with weights to different properties. The optimizer uses either hill climbing or genetic algorithms. If the initial guesses of the different variables is reasonable, a best solution will eventually appear. However it s often not that simple and the program needs some baby sitting and user input to converge into a solution. Starting with the genetic algorithm and finishing of with hill climbing seems to be the best strategy. Since the number of iterations is so large, a fast computer is essential. Nightly runs and sometimes also weekend runs have been used to find the antenna dimensions. Optimization of bandwidth, front to back ratio and VSWR with regard to antenna dimensions will put the solution on top of a multi dimensional hill. When on top of a hill, small perturbations of the dimensions will not be noticeable in the end results and the antennas will not become so sensitive to manufacturing tolerances. 4 Optimization results The optimization was done in free space with weight on the antennas front to back ratio. As a consequence both antennas feed point impedance got close to 75 Ohms. The diagrams that follows is the free space results. 7

11 4.1 Antenna nr 1 Figure 3: Antenna nr 1 VSWR and reflection coefficient 8

12 Figure 4: Antenna nr 1 gain and front to back ratio 9

13 Figure 5: Antenna nr 1 radiation pattern E plane 10

14 Figure 6: Antenna nr 1 radiation pattern H plane 11

15 4.2 Antenna nr 2 Figure 7: Antenna nr 2 VSWR and reflection coefficient 12

16 Figure 8: Antenna nr 2 gain and front to back 13

17 Figure 9: Antenna nr 1 radiation pattern E plane 14

18 Figure 10: Antenna nr 1 radiation pattern H plane 15

19 5 Antenna dimensions In this section, all relevant metric dimensions from the antenna optimization are given in simple pictures. 5.1 Antenna nr 1 Figure 11: Spacing between dipoles and reflector 16

20 Figure 12: Dimension of dipoles 17

21 Figure 13: Dimension of reflector 18

22 5.2 Antenna nr 2 Figure 14: Spacing between parasites, dipoles and reflector 19

23 Figure 15: Parasitic element 20

24 Figure 16: Crossed dipoles 21

25 Figure 17: Reflector 22

26 6 Matching antennas from 75 Ω to 50 Ω In the report a convenient transformer for matching co-axial lines, by Peter Bramham[5] see also[6], theauthor describes avery simple methodtomatch coaxial lines with different impedances. The popular name for this matching technique has become the λ/12 transformer. It uses two short coaxial line sections with an electrical length of nearly λ/12 in series and gives a correct impedance transformation. The lines have the same impedances as is used to a and from the transformer only in an unexpected order. l l Z1 Z 2 Z 1 Z 2 Figure 18: 75 to 50 Ω match The formula for calculating the lengths of the cable segments are: l = λ [ ] 2 2 Z1 +Z 2 4π cos 1 (Z 1 +Z 2 ) 2 Any dielectric within the cables will lower the velocity and the length of the segments has to be multiplied by the velocity factor. Interestingly the total length of the matching section will be about λ/6 which is shorter then the more well known λ/4 transformer. 7 Conclusions The possibility to describe a hypothetical antenna with variables and math expressions and being able to optimize antenna properties with respect to these variables is essential to arrive at a robust solution in a timely manner. Any other approach would have included more guesswork and also been much more time consuming. 23

27 References [1] H. E. King and J. L. Wong 1972, An Experimental Study of a Balun-Fed Open-Sleeve Dipole in Front of a Metallic Reflector, IEEE Transactions of Antennas and Propagation (March 1972), pp [2] Thomas G. Spence, Douglas H. Werner 2006, A Novel Miniature Broadband/Multiband Antenna Based on an End- Loaded Planar Open-Sleeve Dipole, IEEE Transactions of Antennas and Propagation (December 2006) [3] Numerical electromagnetic code, [4] Voors, Arie NEC based antenna modeler and optimizer, arivoors/ [5] Bramham, P. 1959, A convenient transformer for matching co-axial lines, CERN 59-37,(November 1959), [6] McDonald, Kirk T Impedance matching of transmission lines, (July 20, 2005), Princeton University, mcdonald/examples/impedance matching.pdf, page 3 24