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Selecting Substrates For Printed-Circuit Antennas

October 13, 2010
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October 13, 2010


Printed-circuit antennas must provide big performance in small packages, especially for modern fixed and mobile wireless devices. In some cases, they must provide high gain, or light weight, or handle high power levels. The choice of circuit-board laminate material plays a key role in the size and performance of a printed-circuit antenna, such as achieving maximum gain at RF and microwave frequencies. When selecting a circuit-board laminate for a printed-circuit antenna, it helps to understand how laminate material parameters relate to antenna performance.

Printed-circuit antennas are fabricated on laminates comprised of a dielectric material with copper conductor layer on one or both sides. The dielectric may incorporate glass, ceramic, or other filler for improved electrical and mechanical stability. Antennas can be formed on one or both sides of the laminate or by using multiple layers of laminate materials. When more than one side is used, conductive layers are usually connected electrically by plated through holes (PTHs) in the insulator layer. An antenna array consists of multiple antenna elements, each with its own feed. By adjusting the phase between elements, an antenna array can be steered electronically, without having to physically move the antenna system.

How does one go about choosing a laminate for an antenna? The answer requires an understanding not only of the requirements of an application, in terms of electrical performance, size, and weight, but of how to compare different materials by their performance parameters. Substrate material parameters include dielectric constant, dissipation factor, and coefficient of thermal expansion (CTE). A previous blog offered a brief summary of relative dielectric constant (εr) and how it relates to circuit-board materials. The relative dielectric constants of substrates for printed-circuit antennas are usually in the range of about 3.0 to 10.3, as measured in the z-axis or thickness of the material.

In brief, using a low-dielectric-constant material for an antenna usually also means using a low-loss material which can deliver higher antenna gain. But the low dielectric constant can also affect the size of the antenna. An antenna operates at a resonant frequency, usually determined by fabricating a microstrip or stripline resonant structure at some fraction of the wavelength of a frequency of interest. The size of the resonant structure as it corresponds to a fractional wavelength is related to the dielectric constant of the substrate material. Materials with higher values of relative dielectric constant allow the use of smaller resonant structures for a given frequency. However, the cost of higher dielectric constant is usually higher loss or dissipation factor, resulting in lower antenna gain. Higher loss can also be concerns for antenna designs that must handle high power levels, typically in transmit applications.

For larger antennas, the dielectric constant across a panel of substrate material should be as consistent as possible, since variations in dielectric constant directly affect the impedance, amplitude, and phase of antenna across its operating bandwidth. A material’s dielectric constant can also be evaluated for consistency as a function of temperature, using a parameter called thermal coefficient of dielectric constant. Materials suppliers may typically blend a good dielectric material, such as polytetrafluoroethylene (PTFE), with reinforcing materials such as glass to not only stabilize the dielectric constant with temperature but also improve the mechanical integrity of the material.

The CTE and another material parameter, thermal conductivity, are usually of interest to circuit designers dealing with high power levels. As with many other substrate material parameters, CTE is measured in all three axes. Since typically a copper conductor layer is laminated on the substrate dielectric layer, its CTE values in the x and y axes should be as close as possible to that of copper, which is around 17 ppm/ºC. In the z axis or thickness plane, the CTE should be as low as possible, to ensure the reliability of PTH connections in multilayer antenna constructions. The thermal conductivity can be used to compare the capability of different laminate materials to transfer and dissipate heat from a circuit to a housing, heat sink, or other heat-dissipating structure.

How do these materials parameters relate to real-world circuit-board laminates for printed-circuit antennas? Low-loss, low-dielectric-constant materials, such as PTFE-based laminates, are often used for their high-gain capabilities at microwave frequencies, although PTFE requires more extensive processing of structures such as PTHs than when using lower-cost (but lower performance) materials such as FR-4. As an example of a PTFE composite material for antennas, RT/duroid® 6002 PTFE composite from Rogers Corporation has a relative dielectric constant of 2.94 ± 0.04 at 10 GHz with a dissipation factor of  0.0012 at 10 GHz and low CTE of 24 ppm/°C in the z axis. The thermal conductivity is 0.60 W/m/K. Although capable of excellent electrical performance, the low dielectric constant of this material results in larger printed-circuit antennas than when using materials with higher dielectric constants.

In contrast, Rogers RO3200™ series materials are lower-cost ceramic-filled laminates reinforced with woven fiberglass and available with three different dielectric constants, 3.02, 6.15, and 10.2  at 10 GHz (and tight tolerances of ±0.040, ±0.15, and ±0.50, respectively) to provide antenna designers with choices in terms of final circuit size and other performance tradeoffs.

For portable or mobile applications, antenna weight can also be an issue. RO4730™ LoPro™ antenna grade laminates from Rogers Corp. have been developed to allow antenna designers the benefits of a low-dielectric-constant material without the associated weight of PTFE-based laminates. With a dielectric constant of 3.0, the RO4730 LoPro material incorporates hollow glass microspheres as the filler material for about a 30-percent reduction in weight compared to glass-reinforced PTFE laminate materials. They also feature a low-profile conductive copper foil with excellent low-loss and low-distortion qualities.

 These low-density thermoset resin materials excel in a material parameter that is of growing important to antenna designers: passive intermodulation (PIM) performance. PIM is essentially the amount of spurious signal generation caused by the mixing and modulation affects of the types of multiple-signal carriers used in modern wireless communications signals. High-performance antenna materials will yield extremely low PIM numbers. The RO4730 LoPro laminates have demonstrated PIM performance of better than -154 dBc for two-tone testing performed at 1900 MHz with +43-dBm signals. The laminates achieve such outstanding performance by their use of reverse-treated electrodeposited copper foils and a proprietary surface modifier to bond these foils to the thermoset dielectric with strong adhesion.

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