Analyzing Advances in Antenna Materials
Enhanced laminate materials provide the physical and electrical requirements needed by designers of printed-circuit antennas for modern wireless communications systems.

By Art Aguayo, Sr. Market Development Manager • Advanced Circuit Materials Division, Rogers Corp.

High-performance antennas are critical to the operation of both fixed and mobile wireless communications systems. Because of evolving system requirements, these printed-circuit antennas must be low in loss and provide high gain at microwave frequencies. The choice of circuit-board materials for effective antenna designs is critical, but can be simplified by knowing more about the latest available laminate materials engineered for antennas as well having a fuller understanding of the key material parameters that impact antenna performance.
Practical antennas can be fabricated in a number of different ways, with planar printed-circuit antennas typically formed with microstrip or stripline circuits and circuit structures (Figure 1). Antennas can be single-sided, double-sided or multilayer configurations. Single-sided printed-circuit antennas consist of a dielectric insulator with circuitry etched from a copper conducting layer. A double-sided printed-circuit antenna has two copper conducting layers on either side of the insulator, allowing two sets of transmission lines to be fabricated. The two conductive layers are typically connected electrically by means of plated through holes (PTHs) in the insulator layer. A multilayer antenna extends the two-sided concept to three or more conductive layers and multiple dielectric insulator layers, with the multiple conductive layers connected electrically by means of PTHs. Antenna arrays involve multiple elements, where each element is driven by its own feed. Altering the phase shift between elements allows the antenna to be electronically steered without physically moving the antenna. Signal feed points for the various designs are usually selected at a point along the etched circuit transmission lines that provides a close match to the characteristic impedance of the system, typically 50 Ω in high-frequency systems.

The laminated substrate materials used to fabricate printed-circuit materials are characterized by a number of different performance parameters including dielectric constant, dissipation factor, coefficient of thermal expansion (CTE) and thermal conductivity. The dielectric constant of a material describes its capability as an insulator, or to polarize and hold a charge. For example, the dielectric constant of a vacuum is unity or one. But since circuit traces cannot be held in place in a vacuum, practical dielectric materials, formed of such materials as thermoset resins and polytetrafluoroethylene (PTFE) always have dielectric constants greater than one. Since the dielectric constant of a material is always referenced to the value for a vacuum, the parameter for practical circuit board materials is referred to as the relative dielectric constant (εr). Values of relative dielectric constants for materials engineered for antennas are typically in the range from 2.2 to 10.3 as measured in the z-axis (through the thickness of the material). Materials with lower values of permittivity are good insulators for lower-frequency signals requiring high isolation in densely packed circuits, such as mobile communications handsets or in multilayer circuit designs. Materials with higher relative dielectric constants offer greater capability to store charge and generate larger electromagnetic (EM) fields in some antenna designs, but with limited isolation between conductors.

However, it is not just the value of the relative dielectric constant that is important; the consistency of that value across a laminate panel is just as critical for an effective antenna design, especially for physically large arrays requiring more surface area of a laminate panel. High-frequency antennas based on stripline or microstrip circuitry typically employ planar structures, such as resonators, to achieve resonance at a desired operating frequency, such as 2.50 GHz for WiMAX. Since these structures are based on the physical wavelengths of the required frequency based on a material with a given relative dielectric constant, such as 3.0, variations in the dielectric constant across the length and width of a laminate material can result in variations in the desired operating frequency of the antenna. Put simply, a variation of only 5 percent in the dielectric constant can result in frequency variations of as much as 60 MHz for a WiMAX antenna designed for a center frequency of 2.50 GHz (30 percent of the allocated bandwidth of 194 MHz). Obviously, laminate materials with the tightest possible tolerances for relative dielectric constant provide the most predictable and repeatable frequency performance for printed-circuit antennas.

As an example of a practical laminate material engineered for antenna applications, the RO3200 series materials are ceramic-filled laminates that are reinforced with woven fiberglass for structural stability (Figure 2). Although designed for low-cost applications, such as Global Positioning System (GPS) antennas and microstrip patch antennas, the laminate materials provide excellent electrical performance with durable mechanical stability. In order to cover a wide range of applications, they are available with three different dielectric constants — 3.02, 6.15 and 10.2 — in support of antenna designs through 40 GHz. The tolerances of the relative dielectric constant for these materials varies with the value of the dielectric constant, with tolerances of ±0.04, ±0.15, and ±0.5, respectively, for the 3.02, 6.15 and 10.2 materials when measured at 10 GHz.

The dielectric constant of a laminate material can vary over the length and width of a circuit board but also as a function of temperature, a parameter known as the thermal coefficient of dielectric constant. Such variations are difficult to compensate for, but can be minimized by engineering substrates with carefully chosen filler materials, such as glass fibers, to stabilize the dielectric constant with changes in ambient operating temperature. As an example, both RO4730 materials and RO3730 materials employ ceramic fillers (of different types) to stabilize the dielectric constant with short term changes in temperature compared to traditional PTFE/woven glass materials, Figure 3.
By modifying the shapes and types of fillers used in their dielectric materials, laminate suppliers have also responded to the needs of antenna designers for more physically stable substrates while even reducing the weight of the materials. For example, the recently introduced RO4730 LoPro antenna grade laminates are low-density thermoset resin materials with dielectric constant of 3.0 that incorporate hollow glass microspheres as the filler material. The glass microspheres help achieve low density resulting in about 30 percent the weight of similar-sized glass-reinforced PTFE laminate materials, while also delivering the outstanding passive intermodulation (PIM) performance valued by antenna designers working on digitally modulated communications systems. The laminates have demonstrated PIM performance of better than -154 dBc in two tone 43 dBm 1,900 MHz testing.

When antenna PIM performance is a concern, it is important to note that the choice of copper conductor is as important as the choice of dielectric material. For the RO4500 series laminates, for example, Rogers offers reverse-treated electrodeposited copper foils to minimize insertion loss and PIM. Rogers uses a proprietary surface modifier to bond these foils to the dielectric materials with strong adhesion and evaluates the performance of the laminate materials using two-tone PIM test methods. The low-loss RO4730 LoPro antenna grade laminates are compatible with RoHS, lead-free processes and manufacturing practices typically used with conventional FR-4 substrate materials.

Handling Power
An important consideration for antenna designers concerns the power-handling capabilities of a laminate material during transmit mode. While receive mode involves only low-level signals, high-enough transmit power levels can result in variations in electrical performance. [1] Although the power-handling capability of a printed-circuit antenna is generally limited by the width of the conductive transmission lines and the ground-plane spacing, another key laminate material parameter, the dissipation factor, also provides some insight into how signal losses in the dielectric material can be expected to translate into temperature rises at elevated power levels. The dissipation factor, which is also known as a laminate’s loss tangent, is the ratio of the material’s loss to its capacity. By coupling low loss tangent and smooth foils, one can lower the overall insertion loss of the RF feed as can be seen in Figure 2 comparing RO3730 antenna grade material with conventional PTFE/woven glass Dk 3.0 products.

Although all laminate materials suffer some dissipative losses, materials intended for antennas with high transmit power levels should exhibit the lowest values possible to ensure adequate power-handling capability and less RF energy converted to heat. Of course, the power-handling capability of a printed-circuit antenna is often not limited by the laminate material itself but by connections to the antenna, such as a stripline feed. The weakest connection point on a printed-circuit antenna, which is generally also the lowest-temperature point on the design, such as a feed’s solder joint, is often the main point of concern for reliability.

Two important laminate parameters that relate to a printed-circuit antenna’s performance at high power levels are thermal conductivity and coefficient of thermal expansion. The thermal conductivity is a measure of the amount of heat that passes through a unit area of a laminate of unit thickness, with higher values indicating a better capability of conducting heat away from the copper transmission lines and through the dielectric material. The coefficient of thermal expansion (CTE) describes the physical changes that take place to a laminate material as a function of changes in temperature. Ideally, the dielectric portion of a laminate material intended for higher-power antenna applications should match the CTE of the copper conductors, which is typically around 17 ppm/ºC. While the CTE is defined in all three axes of a laminate material, an important consideration for multilayer antennas is a low CTE in the z-axis, to ensure the reliability of PTH connections.

For example, the RO4500 laminate materials, available in panel sizes as large as 50 inches by 110 inches and dielectric constants of 3.3 through 3.5 and tolerances of ±0.08 at 10 GHz, achieve low CTE in all three axes. For a laminate with dielectric constant of 3.3, the CTE values in the x, y and z axes are 13, 11 and 37 ppm/ºC, respectively, when tested over an ambient temperature range of -55°C to 280ºC. The thermal conductivity of this same material is 0.6 W/m/K at 100ºC.

Another temperature-related parameter of concern to antenna material specifiers is the temperature coefficient of dielectric constant, which describes the influence of short-term temperature changes on a laminate’s dielectric constant. The RO4730 LoPro laminate materials, for example, exhibit a temperature coefficient of dielectric constant of about 23 PPM/ºC, which allows a certain amount of design flexibility over a wide temperature range. The laminates also exhibit dimensional stability of better than 0.05 percent, indicating minimal physical changes in the material over wide temperature ranges when used in printed-circuit antennas. When comparing laminate materials, it is important to note the test conditions for both thermal parameters, including the range of temperatures over which the CTEs for all three axes were tested.

Modeling Antennas
Given the availability of modern computer-aided-engineering (CAE) software design tools, antenna designers can now enter many of the material parameters mentioned above into a software program to evaluate the influence of different parameters on different printed-circuit antenna designs at different frequencies. Because of the radiated EM field nature of printed-circuit antennas, most designers opt for an electromagnetic (EM) simulation tool for antenna modeling purposes.

For a printed-circuit antenna designer, an EM simulator can predict the way that current is distributed in the conductive metal traces and how much coupling will occur between traces. Maxwell’s field equations are the basis for these software tools, which are available as two study planar structures in two dimensions (2D), as full three-dimensional (3D) tools or as combinations (2.5D) of the two types of structures. EM simulation tools use a variety of technologies to solve Maxwell’s equations, including the method of moments (MoM) for planar 2D tools and finite-difference, time-domain (FDTD) techniques for full 3D simulators. EM software tools are available from a large number of suppliers, as both stand-alone programs and as integrated suites of software tools. Generally, the tradeoff for EM simulators is accuracy for computer processing time, since the solution of matrices of the field equations that represent a printed-circuit antenna can be time consuming, even for modern multiple-processor computers.

In the end, a printed-circuit antenna designer must reconcile a set of requirements when choosing a laminate material. The requirements, which may include weight, thickness, loss, stability of the dielectric constant, cost and other factors, will be dictated by the particular application. By comparing the key material parameters of different laminates as they relate to printed-circuit-antenna requirements, antenna designers can simplify the selection process and find the best tradeoffs in cost, mechanical requirements, and electrical performance for a given application.

References
[1.] “Handling Continuous Power in Bonded Stripline on RT/duroid Laminates” Design Note 3.3.1, Advanced Circuit Materials Division, Rogers Corp., www.rogerscorp.com

Art Aguayo has been with Rogers Corp. for the past 20 years and is currently a senior market development manager responsible for wireless telecom and aerospace and defense. He holds a Master of Science degree in Engineering from Arizona State University. He can be reached at art.aguayo@rogerscorp.com.

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