Antenna Systems & Technology - Winter 2011 - (Page 13)

FEATURE ARTICLE Design Considerations in the Selection of High Frequency Materials for PCB Base Station Antennas By Allen F. Horn and Art J Aguayo • Rogers Corp. Antennas are a critical component in wireless communication infrastructure, and as such, careful consideration of key properties is needed. Designers of cellular base station PCB antennas have many choices when selecting high frequency laminates. This article will provide the pros and cons of various laminate systems for use in PCB antennas. Comparison of electrical properties for these materials will be discussed. Insertion loss measurements will be presented as well as a detailed review of Passive Intermodulation testing and results for various laminates systems. Passive intermodulation (PIM) is the generation of unwanted frequencies due to non-linearities in the current-voltage relationship of passive elements. The most common source of PIM generation is usually at connections between devices. However, when the lowest PIM values are required in a planar circuit antenna system, the circuit laminate properties can play a significant role as well. In a high frequency circuit laminate, the copper foil roughness is a key variable. With the use of low profile copper foil, laminate PIM performance can be improved by 10 to15 dB, and consistent values of better than –153 dBc can be achieved. Due to the very low values of interest, development of a measurement system and understanding to the repeatability of the measurements is of major importance. This article will help base station PCB antenna designers, as a primer in the selection of materials. Figure 1. Four basic constructions for PTFE based Dk 3.0 materials (0.030 inches) Laminate Options Antennas used in base transceiver stations are, in concept, simple RF circuits when designed for a single frequency. These circuits can be built out of coaxial cable and thin metal plates or out of printed circuit materials, PCB’s (or a combination of both). However, today many antennas designed need to operate at multiple frequencies (due to air interphases, GSM, CMDA, WCDMA, LTE) and the complexity of these designs has increased in order to accommodate the various frequencies that can range from 700 MHz to close to 2,700 MHz depending on the region of the world. Selecting an approach, metal/cable or PCB has many variables within performance, reliability, cost and time to market and which approach is best also depends on the organization and its skill sets. When following the PCB approach for antennas, one needs to select laminate materials that have the right electrical and mechanical properties and in the case of base stations, meet the cost targets in this very competitive market. Key electrical properties are: Dielectric constant, Dk (< 3.5) Dissipation factor, Df (< 0.003) Passive Intermodulation, PIM (< -153 dBc) When selecting a high frequency PCB material, designers have two basic types of material options. The first is based on PTFE resin (thermoplastic) while the second option is based on thermoset hydrocarbon resin. PTFE based materials have the option of being reinforced with woven glass, or with ceramic filler and woven glass. For the thermoset based materials, these all have both woven glass and ceramic filler (lower Dk versions have hollow ceramic filler particles). Figure 1 shows cross sections of four basic PTFE based Dk 2.97 – 3.0 materials. Starting in the upper left corner and going in clockwise direction, the first sample is composed of layers of PTFE/ceramic with PTFE/woven glass. The second is entirely made of woven glass layers that have been coated with PTFE. The lower right hand corner shows a sample made of PTFE/ceramic resin coated on woven glass and the last example is made by combining layers of woven glass that has been coated with PTFE/ceramic and pure layers of PTFE. All samples have copper foil cladding on the outside. The range of Dk and Df for these materials is summarized in Table 1 based on published data sheet values from the top four major suppliers of high frequency materials. When comparing Article continues on page 14 Dk various frequencies PTFE/woven glass 2.2 2.50 – 2.65 3.0 3.2 2.5 - 2.65 2.97 – 3.0 3.2 2.55 3.0 3.30 3.40 Df (various frequencies) 0.0009 0.0016 – 0.0021 0.0023 0.0024 0.0012 – 0.0017 0.0012 – 0.0020 0.0032 0.0022 0.0023 0.0020 0.0022 PTFE/woven glass, ceramic Thermoset/ woven glass, ceramic Table 1. Data sheet property values for antenna grade materials WWW.ANTENNASONLINE.COM WINTER 2011 ANTENNA SYSTEMS & TECHNOLOGY 13 http://WWW.ANTENNASONLINE.COM

Table of Contents for the Digital Edition of Antenna Systems & Technology - Winter 2011

Antenna Systems & Technology - Winter 2011
Editor’s Choice
Reducing Measurement Times and Improving Economic Competitiveness for Antenna and RCS Applications
Troubleshooting in Mediumwave Multiplexing Antennas
Design Considerations in the Selection of High Frequency Materials for PCB Base Station Antennas
Antennas
Distributed Antenna Systems
Wireless Communication
Industry News
Wireless Network Planning from the Desktop Using High Resolution 3D NEXTMap Digital Surface Elevation Data

Antenna Systems & Technology - Winter 2011