#
Moment Method Analysis of Antennas on Cylindrical Structures by Using G2DMULT and Design of Hat Fed Reflector Antennas

Doktorsavhandling, 2001

This thesis deals with two aspects of research interest: moment method analysis of antennas on cylindrical structures by using G2DMULT, and design of hat fed reflector antennas. Both of them are two-dimensional (2D) structure problems, one being cylindrical and the other rotationally symmetric.
Many antennas consist of three-dimensional (3D) elements, such as dipoles, slots or patches, in the vicinity of 2D structures, such as cylinders of arbitrary cross section or rotationally symmetric bodies. When dealing with these electromagnetic problems, it is very efficient to find the radiation characteristics of the 3D elements and the mutual couplings between them by using a spectrum of 2D solutions (S2DS). We refer to the 3D elements as excitation elements, as they determine the spectral variation of the solutions. G2DMULT is a general algorithm based on the S2DS technique. The associated FORTRAN program G2DMULT calculates the spectral Green's functions of 2D MULTiregion structures by using the method of moment (MoM). In this thesis the implementation and applications of G2DMULT for cylindrical multiregion structures are presented. The spectral electric and magnetic field integral equations (EFIE and MFIE) and their moment method solution formulations are derived. In order to speed up the convergence and increase the accuracy, three pulse-approximated triangular basis and test functions are implemented in G2DMULT. The boundary conditions at different material interfaces are treated carefully. The self-impedance and the radiation efficiency of a dipole near a lossy dielectric cylinder with arbitrary cross section are calculated and verified against measurements. As applications of G2DMULT, base station antennas such as Allgon's Metro-III antenna have been analyzed. Several other application cases, including the scattering from dielectric coated cylinders, are also simulated and compared with simulations and measurements done by others.
The other type of 2D structures - the rotationally symmetric structures, also referred to as bodies of revolution (BOR) - has not yet been implemented in G2DMULT. Therefore, a commercial code called V2D based on the FDTD method was chosen as an analysis tool. The hat-fed reflector antenna is a BOR structure consisting of a rotationally symmetric and almost-parabolic reflector and a waveguide-fed rear-radiating feed, referred to as the hat feed. This thesis presents the procedure of designing hat-fed reflector antennas. This was applied in two industrial projects, to design millimeter wave radio link antennas for Ericsson Microwave System AB (EMW) and a level gauging antenna for Saab Marine Electronics AB (SME). In such designs it is important to locate the feed correctly relative to the reflector. The location is determined by the phase center of the feed, which for the hat feed is ring shaped. Therefore, we derived a formula to calculate the location of the ring-shaped phase center of feeds which radiate around a support tube such as the hat feed. The aperture efficiency can be improved up to 0.5 dB by using this formula. Primary-fed reflectors of these types are often provided with a vertex plate to reduce the return loss of the feed. In order to reduce the effect of the vertex plate on the far-out sidelobes, a new type of vertex plate with Gaussian thickness profile is introduced and the formula to determine the dimensions of this vertex plate is derived. A new version of the hat feed, the Chinese hat feed, is also presented in this thesis. The Chinese hat feed provides an opportunity to control the beamwidth of the radiation pattern that makes it suitable for use in a shallow reflector. A hybrid method, which combines the MoM program G1DMULT with the V2D code, is used to analyze the increased sidelobes due to scattering from screw heads in reflector antennas. Several ways to reduce this scattering are presented.

G2DMULT

reflector antenna

paraboloid

dipole

base station antenna

screw heads

vertex plate

self impedance

G1DMULT

ring-shaped phase center

radiation efficiency

FDTD

cylindrical structures

V2D

ring-focus

moment method