---
title: "Passive components behavior and application protocols"
subject: "RF and Microwave Engineering"
chapter: 1
key: "semester_7/rf_and_microwave_engineering:microwave-components-behavior-protocols"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/microwave-components-behavior-protocols
site: BCT Engineering
---

# Passive components behavior and application protocols

RF and Microwave Engineering · Chapter 1: Introduction

- Asked in: 2082 Bhadra, 4+6 marks
- 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/microwave-components-behavior-protocols
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-1

## Question

Compare the behavior of microwave passive components at low frequency bands against microwave bands. Prepare the detail real-world protocols of any three major applications of microwave communication systems.

## Short answer

At microwave frequencies parasitic lead inductance, stray capacitance and skin effect make resistors, capacitors and inductors self-resonant, so distributed stubs and cavities replace lumped parts. Wi-Fi uses OFDM and CSMA/CA in 2.4/5/6 GHz; cellular uses TDMA/OFDMA with frequency-reuse cells; satellite links use C/Ku/Ka transponders with about 240 ms round-trip delay.

## Answer

#### Behaviour of passive components: low frequency vs microwave
At low frequency a resistor, inductor or capacitor acts as an ideal lumped element. At microwave frequencies its leads and body are a large fraction of λ, so **parasitics** (lead inductance, stray capacitance, skin effect) dominate.

| Component | Low frequency | Microwave band |
|---|---|---|
| Wire | Short circuit | Inductor (about 1 nH/mm) and a transmission line |
| Resistor | Pure R | R with series lead L and shunt C; at high f, the C shorts it |
| Capacitor | Pure C | Series LCR; above its self-resonance it acts as an inductor |
| Inductor | Pure L | Turn-to-turn C causes self-resonance; above it, it acts as a capacitor |
| Realisation | Discrete parts | Distributed: stubs, line sections, cavities, chip parts |

```text
Real resistor at microwave frequency:
      Ls       R       Ls
  o--^^^^---/\/\/---^^^^--o
        |              |
        +------||------+
              Cs (stray)
```

So at microwave frequencies L and C are made from short-circuited or open-circuited stubs, and resonators are made from cavities.

#### Real-world protocols of three microwave communication systems

**1. Wi-Fi (IEEE 802.11 WLAN)**
- Bands: 2.4 GHz ISM (2.400–2.4835 GHz), 5 GHz (5.15–5.85 GHz), and 6 GHz for Wi-Fi 6E.
- Channels: 20/40/80/160 MHz wide. At 2.4 GHz there are 13 channels 5 MHz apart, and only 1, 6 and 11 do not overlap.
- Modulation: OFDM with BPSK up to 1024-QAM (802.11ax). 802.11b used DSSS/CCK.
- Access: CSMA/CA with optional RTS/CTS. Duplexing is TDD.
- Rates: 11 Mbps (b), 54 Mbps (a/g), 600 Mbps (n, MIMO), about 9.6 Gbps (ax).
- Security: WPA2/WPA3 (AES).

**2. Cellular mobile (GSM / LTE / 5G NR)**
- GSM: 900/1800 MHz, 200 kHz carriers, GMSK modulation, TDMA with 8 time slots per carrier, FDD (uplink 890–915 MHz, downlink 935–960 MHz).
- LTE (4G): 700 MHz–2.6 GHz, 1.4–20 MHz bandwidth, OFDMA downlink and SC-FDMA uplink, up to 64-QAM, MIMO, FDD or TDD.
- 5G NR: FR1 below 6 GHz and FR2 at 24–52 GHz (mm-wave), up to 400 MHz bandwidth, flexible numerology, massive MIMO and beamforming.
- Architecture: mobile station, base station (BTS/eNodeB/gNB), core network. Frequency reuse in hexagonal cells, with handover between cells.

**3. Satellite communication**
- Geostationary orbit at 35,786 km. The ground-to-satellite delay is about 120 ms, so one hop (up and down) takes about 240 ms.
- Bands: C band (uplink 5.925–6.425 GHz, downlink 3.7–4.2 GHz, written 6/4 GHz), Ku band (14/12 GHz), Ka band (30/20 GHz).
- A transponder receives the uplink, translates the frequency down, amplifies with a TWTA or SSPA, and retransmits. Typical transponder bandwidth is 36 MHz.
- Access: FDMA, TDMA, CDMA. Example: VSAT networks with QPSK/8PSK and DVB-S2.
- Uplink is at the higher frequency because the ground station can afford more power to overcome the greater loss.

## Check yourself

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

1. How does a real capacitor behave above its self-resonant frequency?
   A. It behaves as a pure capacitor
   B. It behaves as an inductor
   C. It behaves as a resistor
   D. It becomes a short circuit at all frequencies
   Answer: B. Lead inductance in series makes it a series LCR; above self-resonance the inductive reactance dominates. (Key point: Capacitor behaviour)

2. What happens to an inductor above its self-resonant frequency, and why?
   A. It acts as a capacitor because of turn-to-turn capacitance
   B. It acts as an inductor with higher value
   C. It becomes a perfect conductor
   D. It acts as a resistor only
   Answer: A. Turn-to-turn capacitance resonates with L; above this, the part looks capacitive. (Key point: Inductor behaviour)

3. How are L and C realized at microwave frequencies?
   A. Using discrete wire-wound coils
   B. Using short- or open-circuited stubs
   C. Using electrolytic capacitors
   D. Using batteries
   Answer: B. Short/open stubs act as pure L or C; resonators are made from cavities. (Key point: Stub realization)

4. In the 2.4 GHz Wi-Fi band with 13 channels spaced 5 MHz apart, which channels do not overlap?
   A. 1, 2 and 3
   B. 1, 6 and 11
   C. 2, 5 and 8
   D. 4, 8 and 12
   Answer: B. Each 20 MHz channel spans about four channel numbers, so only 1, 6 and 11 are mutually non-overlapping. (Key point: Wi-Fi channels)

## Related in RF and Microwave Engineering

- [Behavior of systems at microwave vs low frequency](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowave-vs-low-frequency-behavior) — RF and Microwave Engineering, chapter 1, asked 3×
- [Transmission line behavior at low and microwave bands](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Atransmission-line-low-vs-microwave) — RF and Microwave Engineering, chapter 1
- [Microwave vs low frequency systems and applications](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowave-vs-low-frequency-systems) — RF and Microwave Engineering, chapter 1, asked 2×
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