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54 The PCB Design Magazine • December 2016 A large variety of PCB-based antenna struc- tures 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 frequen- cy, which is related to the ½ wavelength circuit feature size. Wavelength is associated with fre- quency 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 ma- terial 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. An- other common attribute for antenna designs using PCB technology is the use of thicker lami- nates. A thicker microstrip circuit will radiate by John Coonrod ROGERS CORPORATION PCB-Based Antennas and PIM Concerns LIGHTNING SPEED LAMINATES

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