---
title: "Short note: Microstrips"
subject: "RF and Microwave Engineering"
chapter: 2
key: "semester_7/rf_and_microwave_engineering:microstrips-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/microstrips-note
site: BCT Engineering
---

# Short note: Microstrips

RF and Microwave Engineering · Chapter 2: RF and Microwave Transmission Lines

- Asked in: 2069 Bhadra (old course), 5 marks
- Video: [Microwave Engineering ( Microstrip )](https://www.youtube.com/watch?v=Qkqt3CmmHTk) — Taghreed Al -Erani
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/microstrips-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-2

## Question

Write a short note on microstrips.

## Short answer

A microstrip is a planar transmission line consisting of a metal strip on a dielectric substrate backed by a ground plane. It supports a quasi-TEM mode with effective permittivity εeff and characteristic impedance controlled by strip width. It is widely used in microwave integrated circuits due to ease of fabrication and component mounting.

## Answer

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.

```text
        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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does a microstrip carry a quasi-TEM wave rather than a pure TEM wave?
   A. The strip is too thin to carry current
   B. The field lies partly in the dielectric and partly in the air above, so there is no single uniform medium
   C. The ground plane is only partly covering the substrate
   D. The dielectric has no loss
   Answer: B. Because the field is split between the substrate and air, a pure TEM mode cannot exist. The wave is treated as travelling in a medium of effective permittivity. (Key point: Quasi-TEM mode)

2. How does the characteristic impedance of a microstrip depend on strip width?
   A. A wider strip (large W/h) gives lower Z0
   B. A wider strip gives higher Z0
   C. Z0 does not depend on W/h
   D. Z0 depends only on frequency
   Answer: A. Z0 depends on W/h. A wide strip gives a low Z0 and a narrow strip a high Z0, with typical values of 20-120 ohm. (Key point: Impedance versus W/h)

3. If eps_eff of a microstrip is 3.2, what is the guide wavelength relation?
   A. lambda_g = lambda_0 x sqrt(3.2)
   B. lambda_g = lambda_0 / sqrt(3.2)
   C. lambda_g = lambda_0 / 3.2^2
   D. lambda_g = lambda_0
   Answer: B. The wave travels with vp = c/sqrt(eps_eff), so lambda_g = lambda_0/sqrt(eps_eff), which is shorter than free-space wavelength. (Key point: Effective permittivity and velocity)

4. Where and when is radiation loss in a microstrip greatest?
   A. Along straight sections, at low frequency, on thin high-eps substrates
   B. At bends, open ends and discontinuities, increasing with frequency and thick low-eps substrates
   C. Only inside the ground plane
   D. Only at DC
   Answer: B. Radiation happens at bends, open ends and discontinuities and grows with frequency and with thick, low-permittivity substrates. (Key point: Radiation loss)