Short note: Microstrips

A microstrip is a planar transmission line made of a thin metal strip of width W on one side of a dielectric substrate (thickness h, relative permittivity εr) with a full ground plane on the other side. It is the most common line for microwave integrated circuits (MICs) and printed RF boards.

        W
     <----->
     #######          <- conducting strip
 ---------------------
 |  dielectric εr    |  h
 ---------------------
 ##################### <- ground plane

Mode of propagation: The field lies partly in the dielectric and partly in the air above it, so a pure TEM mode cannot exist. The wave is quasi-TEM, and it travels as if in a uniform medium of effective permittivity εeff, where 1 < εeff < εr:

• εeff ≈ (εr + 1)/2 + ((εr − 1)/2) · 1/√(1 + 12h/W)
• Phase velocity vp = c/√εeff, guide wavelength λg = λ0/√εeff
• Characteristic impedance depends on W/h: a wide strip (large W/h) gives low Z0, a narrow strip gives high Z0. Typical values are 20–120 Ω; 50 Ω on FR-4 (εr ≈ 4.4) needs W ≈ 1.9h.

Losses: conductor loss (skin effect in strip and ground), dielectric loss (tan δ of substrate) and radiation loss (at bends, open ends and discontinuities; it increases with frequency and with thick, low-εr substrates).

Advantages:

• Small, light, cheap; made by photolithography.
• Easy to mount active devices (transistors, diodes) and chip components on the surface.
• Good for integration of filters, couplers, matching networks and patch antennas.

Disadvantages: higher loss and lower power handling than waveguide or coax, radiation and dispersion at high frequency, and coupling between nearby lines.

Applications: MMICs/MICs, LNAs and power amplifiers, branch-line couplers, Wilkinson dividers, stub filters and patch antenna feeds.