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Lightning Speed Laminates: PCB-Based Antennas and PIM Concerns

A large variety of PCB-based antenna structures are used at microwave frequencies, and some are used at higher frequencies. A common PCB antenna structure is a microstrip patch antenna. A microstrip structure is a two-layer copper circuit with a signal plane and a ground plane, but it is more common for this type of circuit to be the outer layers of a multilayer circuit.

The size of the copper feature or patch, for a microstrip patch antenna, has to do with a fraction of a wavelength, usually ½ wavelength. The patch will radiate (transmit) or will be very sensitive to receive energy at a specific frequency, which is related to the ½ wavelength circuit feature size. Wavelength is associated with frequency as well as the dielectric constant (Dk) of the circuit material. Just for reference, a higher frequency will translate to a shorter wavelength and a smaller patch. Also, using a circuit material with a higher Dk will also decrease the wavelength and make a smaller patch. As a general statement, the circuit materials used for PCB-based antenna applications typically have a lower Dk and commonly have a Dk value in the range of 3-4.

Additionally, circuit materials with higher Dk will cause the electric fields to concentrate more between the signal plane and the ground plane of the circuit. The field concentration will reduce radiated energy and accordingly, PCBs with antenna radiating elements will often use a material with a relatively low Dk value. Another common attribute for antenna designs using PCB technology is the use of thicker laminates. A thicker microstrip circuit will radiate energy better and it is common for microstrip patch antenna designs to use thicker material (30 mils or thicker).

As with most engineering issues, there are tradeoffs. The combination of a thicker circuit material with a low Dk is good for antenna radiation but may not be good for the feedline properties. The feedline is typically a 50-ohm transmission line which brings energy to and from the radiating elements of the antenna circuit. A microstrip transmission line using a thick material can be limited by RF performance due to natural resonances that can occur between the signal plane and the ground plane or across the width of the signal conductor itself. These resonances can interfere with the clarity of the energy being transferred on the feedline to the radiating elements. If the energy is not cleanly transitioned to the radiating elements, less energy can be transmitted or the reception of the energy is altered. Multilayer antenna PCBs have a buried feedline and are often a stripline structure. This type of feedline offers the benefit of energy transitioning cleanly in the circuit and then transitioning to the radiating elements on the outer layer of the PCB using plated through-hole vias. 

To read this entire article, which appeared in the December 2016 issue of The PCB Design Magazine, click here.