High frequency Antenna PCB material

For high performance radio frequency designs where the dielectric constant of the substrate is important, and low levels of loss are needed, then PTFE based printed circuit boards can be used, although they are far more difficult to work with. A patch antenna, which is also known as a microstrip antenna, can be fabricated with standard printed-circuit-board (PCB) processes by using high-frequency laminate materials. 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.

PTFE PCB

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.

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 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.

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