Antenna Systems & Technology - Fall 2013 - (Page 17)

FEATURE ARTICLE between the uplink and downlink frequencies is only about 2 GHz. On the other hand, for commercial Ka band operation (30 GHz uplink and 20 GHz downlink) the difference between uplink and downlink frequencies is about 10 GHz. The problem is seen to be significantly more difficult for the MMW operation. The design of SATCOM antenna radomes also requires appropriate selection of suitable dielectric materials and wall construction. Some parameters of interest are the material strength, specific gravity and performance under environmental conditions. From the electrical viewpoint, a low loss material is also required wherein the use of dielectric materials having a (preferably) small dielectric constant and a low loss tangent value must be employed. To consider just how a low of a “low loss tangent” that is required, take as an example the loss versus frequency for a monolithic Figure 2. Effect of Loss Tangent on Material Transradome wall having a thickness of approximately 0.3 parency (Thickness = 0.3 inches, Permittivity = 3) inches and a dielectric constant of three is shown in Figure 2. As the loss tangent of the material is parametrically increased, it is seen that the material loss monotonically increases with frequency. For a loss tangent with a value equal to or less than 0.001, the regions of high material transparencies are essentially the same as that of a lossless material. But for material loss tangent of 0.01, or greater, the loss increases rapidly with frequency. For millimeter wave use one should select dielectric materials with a loss tangent smaller than 0.01. In addition, the dual frequency (uplink and downlink) requirement for SATCOM antenna radomes most often drives users toward multilayer radome wall concepts since a monolithic wall generally cannot provide suitable frequency bandwidth except for the case where the dielectric material is very low permittivity and loss tangent very small. Performance of multilayer wall concepts are not unique and many radome manufacturers treat their specific wall designs as proprietary or company confidential. The most common wall types of multi layer radomes use three layers (A-sandwich) or the five layers (C-sandwich) consisting of thin higher dielectric “skin” materials and alternating layers of very low dielectric core materials [4]. The skin materials are most often in the form of “prepreg” materials while the core materials are often in the form of a dielectric honeycomb Figure 3. Radome Wall Optimizaor dielectric foam. The many advantages of multiple layer radomes tion Computer Program Approach above include increased strength, larger frequency bandwidths and improved radome wall transmission with higher angles of incidence (AOI). The electrical design of multiple layer radomes is not a simple “cook book” activity but often must factor in the experience of the radome designer since in many cases the design may need to take into account other factors than just the radome wall transmissivity at 0° AOI versus frequency. For instance, the wall design may have to take into account a maximum reduction of the transmissivity with angle-of-incidence (AOI), a maximum depolarization, or even a maximum axial ratio (AR). One approach toward meeting various wall performance requirements is to use a wall transmission computer in an optimization loop as depicted in Figure 3. In this approach, an error function ERF is defined that is related to the difference between the performance desired and the performance achieved with a given stack-up of skin and core layers permitting a search through thousands of www.AntennasOnline.com Fall 2013 Antenna Systems & Technology 17 http://www.AntennasOnline.com

Table of Contents for the Digital Edition of Antenna Systems & Technology - Fall 2013

Editor’s Choice
Exponential M2M Market Growth Calls for Innovative RF and Antenna Solutions
Managing Distributed Antenna Systems Effectively and Efficiently
Millimeter Wave Antenna Radome Systems for Satellite on the Move Applications
Antennas
Components/Subsystems
Software/System Design
Test & Measurement
Antenna Systems 2013: Conference Preview
Marketplace
Industry News
Addressing Customer Requirements in Public Safety Antennas

Antenna Systems & Technology - Fall 2013