feature article
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 circuitboard 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 printedcircuit 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 Figure 1. This microstrip patch antenna is an example of a
polarize and hold a charge. For high-frequency printed-circuit antenna for wireless communications applications.
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).
14
Dk @ 10 GHz Df @ 10 GHz Therm Cond W/m/°K
Obviously, laminate materials with the tightest possible RO3203 3.02 ± 0.04
0.0016
0.47
tolerances for relative dielecRO3206
6.15 ± 0.15
0.0027
0.63
tric constant provide the most
RO3210
10.2 ± 0.5
0.0027
0.81
predictable and repeatable
frequency performance for Figure 2.
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.
Figure 3. Change in dielectric constant for RO4730 LoPro materials, RO3730 materials and PTFE/woven
glass Dk 3.2.
Antenna Systems & Technology december/january 2010
www.AntennasOnline.com
feature article
Handling Power
thermal parameters, including the range of temperatures
over which the CTEs for all three axes were tested.
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
Given the availability of modern computer-aidedpower levels can result in variations in electrical per- engineering (CAE) software design tools, antenna deformance. [1] Although the power-handling capabil- signers can now enter many of the material parameters
ity of a printed-circuit antenna is generally limited by mentioned above into a software program to evaluate
the width of the conductive transmission lines and the the influence of different parameters on different printground-plane spacing, another key laminate material ed-circuit antenna designs at different frequencies. Beparameter, the dissipation factor, also provides some cause of the radiated EM field nature of printed-circuit
insight into how signal losses in the dielectric mate- antennas, most designers opt for an electromagnetic
rial can be expected to translate into temperature rises (EM) simulation tool for antenna modeling purposes.
at elevated power levels. The dissipation factor, which
For a printed-circuit antenna designer, an EM simulais also known as a laminate’s loss tangent, is the ratio tor can predict the way that current is distributed in the
of the material’s loss to its capacity. By coupling low conductive metal traces and how much coupling will
loss tangent and smooth foils, one can lower the overall occur between traces. Maxwell’s field equations are
insertion loss of the RF feed as can be seen in Figure the basis for these software tools, which are avail2 comparing RO3730 antenna grade material with con- able as two study planar structures in two dimenventional PTFE/woven glass Dk 3.0 products.
sions (2D), as full three-dimensional (3D) tools or as
Although all laminate materials suffer some dissipa- combinations (2.5D) of the two types of structures.
tive losses, materials intended for antennas with high EM simulation tools use a variety of technologies
transmit power levels should exhibit the lowest values to solve Maxwell’s equations, including the method
possible to ensure adequate power-handling capability of moments (MoM) for planar 2D tools and finiteand less RF energy converted to heat. Of course, the difference, time-domain (FDTD) techniques for full
power-handling capability of a printed-circuit antenna 3D simulators. EM software tools are available from
is often not limited by the laminate material itself but by a large number of suppliers, as both stand-alone
connections to the antenna, such as a stripline feed. The programs and as integrated suites of software tools.
weakest connection point on a printed-circuit antenna, Generally, the tradeoff for EM simulators is accuracy
which is generally also the lowest-temperature point on for computer processing time, since the solution of
the design, such as a feed’s solder joint, is often the main matrices of the field equations that represent a printpoint of concern for reliability.
ed-circuit antenna can be time consuming, even for
Two important laminate parameters that relate to a modern multiple-processor computers.
printed-circuit antenna’s performance at high power
In the end, a printed-circuit antenna designer must
levels are thermal conductivity and coefficient of ther- reconcile a set of requirements when choosing a lamimal expansion. The thermal conductivity is a measure nate material. The requirements, which may include
of the amount of heat that passes through a unit area of weight, thickness, loss, stability of the dielectric cona laminate of unit thickness, with higher values indicat- stant, cost and other factors, will be dictated by the
ing a better capability of conducting heat away from the particular application. By comparing the key matecopper transmission lines and through the dielectric ma- rial parameters of different laminates as they relate
terial. The coefficient of thermal expansion (CTE) de- to printed-circuit-antenna requirements, antenna describes the physical changes that take place to a laminate signers can simplify the selection process and find the
material as a function of changes in temperature. Ideally, best tradeoffs in cost, mechanical requirements, and
the dielectric portion of a laminate material intended for electrical performance for a given application.
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 zaxis, to ensure the reliability of PTH connections.
For example, the RO4500 laminate materials,
available in panel sizes as large as 50 inches by
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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
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Modeling Antennas
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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december/january 2010 Antenna Systems & Technology
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