Picking PCB Material For A Patch Antenna
Patch antennas are a mainstay of many wireless applications, including in Global Positioning System (GPS) receivers and in wireless local area network (WLAN) routers. They feature the low profile that allows them to be mounted unobtrusively to a flat surface and they are relatively simple to fabricate. They can be as simple as a rectangular conductive patch that is one-half wavelength long at a center frequency of interest, mounted above a slightly larger ground plane, with dielectric material in between. Of course, the type of dielectric material can play a strong hand in the performance of a patch antenna.
A patch antenna, which is also known as a microstrip antenna, can be fabricated with standard printed-circuit-board (PCB) processes using high-frequency laminate materials. An antenna can be as simple as a rectangular patch above a ground plane or as elegant as a complex array of patches, customized for a specific radiation pattern. As with other printed circuits, the choice of circuit material can greatly impact the performance possible from the final antenna design. That choice should be guided by a clear understanding of how a circuit material’s electrical and mechanical properties relate to the performance of a patch antenna.
A laminate’s relative dielectric constant, denoted as εr or Dk, is a good starting point when considering different circuit materials for a patch antenna. Since the dielectric material in a circuit laminate isolates the circuit traces from the ground plane, it is usually specified in terms of Dk value in the thickness or z direction, with z-axis values from about 2 to 10 commonly used in RF/microwave circuits. Laminates with the lowest Dk values are typically used for patch antennas. These lower Dk values typically support circuits, such as patch antennas, with higher center frequencies, enabling effective RF/microwave operation. But other material characteristics, such as thickness, can affect final performance.
The Dk of a dielectric circuit material has a part in determining the physical dimensions of a patch antenna needed for resonance at a particular wavelength and, thus, frequency. The transmission lines carrying signals to and from the patch antenna, or the feed lines, are also affected by the laminate’s Dk value, with higher Dk values resulting in smaller physical dimensions for a given wavelength or frequency for both the feedlines and the antenna patch. The consistency or Dk tolerance of a circuit material is also important to consider because variability in the Dk value results in variability of the center frequency of a patch antenna formed on that material. In general, a tight Dk tolerance is to be preferred for consistent patch antenna center-frequency performance.
But Dk is not the only circuit material parameter to consider when choosing laminates for patch antennas. While it may be possible to shrink an antenna’s dimensions by using a laminate with higher Dk value, other material parameters, such as dissipation factor (Df) and laminate thickness, may point to tradeoffs in performance. A laminate’s Df, which provides an indication of the loss or gain that can be expected from a patch antenna designed on that material, can increase with increasing Dk value. As a result, a microstrip antenna with smaller patch on higher-Dk material will provide less gain and less efficiency than a larger patch antenna on lower-Dk material, with both operating at the same center frequency. So, the savings in size with the higher-Dk material can result in a sacrifice in performance.
Laminate thickness must also be weighed when selecting a circuit material for a patch antenna. The design of any patch antenna is really a compromise between the conflicting requirements of the resonant patch and the microstrip feedlines. The goal for the patch is highly efficient radiation of microwave energy (in transmission), which calls for a thicker circuit substrate. But a thicker substrate will also mean increased radiation loss for the feedline to the antenna patch. Ideally, the feedline would use a thinner laminate with high Dk value for reduced microstrip radiation losses, while the patch would be formed on a thick substrate with low Dk value for high radiation efficiency. While it is possible to address this compromise with a multilayer design using two different circuit materials, any patch antenna with feedlines fabricated on a single circuit material requires a choice in laminate that helps balance the needs of the patch and the feedline.
For patch antenna designers, another laminate parameter to consider is how changes in temperature can affect a circuit material’s Dk value. This parameter, known as the temperature coefficient of dielectric constant, or TcDk, should be carefully considered for any antenna expected to endure a wide range of operating conditions, with smaller values resulting in lower center-frequency variations. An ideal material would exhibit no change in Dk value as a function of temperature, or 0 ppm/°C. Real materials, however, can suffer significant shifts in Dk with temperature, exhibited as unwanted changes in center frequency with changes in temperature.
What are examples of real circuit materials engineered for patch antennas?
Three examples of high-performance circuit materials are the RO4500™, RO4730™LoPro™, and RT/duroid® 5880LZ laminates from Rogers Corporation (www.rogerscorp.com). RO4500 material is an antenna-grade laminate available with Dk values between 3.3 to 3.5 in the z-direction at 10 GHz. These ceramic-filled, glass-reinforced hydrocarbon materials feature excellent dimensional stability. The RO4730 LoPro laminates offer low-profile copper foils with Dk value of 3.0 and Df of only 0.0023 at 2.5 GHz. Both of these RoHS-compatible circuit materials are designed for use with standard FR-4 PCB fabrication methods and high-temperature lead-free processing. RT/duroid 5880LZ laminate is based on PTFE dielectric material with a unique filler, resulting in a low-density, lightweight circuit material ideally suited for portable and airborne antenna applications. It has a very low Dk value of 1.96 at 10 GHz in the z-axis, with low Df of 0.0019, also at 10 GHz. This RoHS-compliant material is usable for antennas to Ku-band frequencies and higher.
These three materials share attributes that make them attractive for patch antennas: low Dk values, low Df and good thermal conductivity for handling relative high power levels when necessary. They are certainly not the only circuit materials suitable for patch antennas, but they illustrate the type of circuit material characteristics that are preferred for antennas. For those who would like to know more about circuit materials for antennas, and will be attending the upcoming Mobile Antenna Systems 2012 show (www.antennasonline.com/conferences) September 18-19, 2012, at the Hyatt Denver Tech Center, Denver, CO, drop by Booth #35 and ask the representatives from Rogers Corp. how these three materials, as well as some of their other laminates, may help your antenna design.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.