Meaning
Planar transmission line fabricated on a printed circuit board guides high-frequency electromagnetic waves between components. An open microstrip structure consists of a single conductor strip separated from a ground plane by a dielectric substrate. This layout allows for easy mounting of surface-mount rf components during the circuit board assembly process.
The boundary of this transmission line type ends where shielding becomes mandatory, which requires a transition to a strip-line or co-planar waveguide.
Impedance Control
Conductor width and dielectric thickness together determine the characteristic impedance of the transmission line. To maintain a standard fifty-ohm impedance, the microstrip dimensions must be calculated with high precision using the dielectric constant of the substrate material. Manufacturers must maintain tight tolerances on copper etching to prevent signal reflections at component boundaries.
Even a minor variation in trace width can degrade the return loss of the finished transceiver module. This accuracy is typically verified using time-domain reflectometry during the quality assurance phase of production.
Substrate Selection
Dielectric material choice affects both the velocity of the signal and the thermal dissipation of the high-frequency circuit. Choosing a low-loss laminate enables the microstrip to transmit signals with minimal attenuation at microwave frequencies. This selection determines the thermal limits of the RF front end when operating at high duty cycles.
Radiation Loss
Unshielded traces tend to radiate energy into the surrounding air when the operating frequency rises. Because a microstrip has an open structure, it suffers from higher radiation losses than enclosed transmission lines. This radiated energy can cause electromagnetic interference within the device enclosure.
Developers solve this by placing metal shielding cans over the sensitive RF sections during the final assembly of the product.
