---
title: "Short note: Two-cavity klystron"
subject: "RF and Microwave Engineering"
chapter: 5
key: "semester_7/rf_and_microwave_engineering:two-cavity-klystron-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/two-cavity-klystron-note
site: BCT Engineering
---

# Short note: Two-cavity klystron

RF and Microwave Engineering · Chapter 5: Microwave Generators

- Asked in: 2069 Bhadra (old course), 5 marks
- Video: [Two Cavity Klystron (Basics, Structure, Working, Applegate Diagram & Applications) Explained](https://www.youtube.com/watch?v=yBoAigEL_wY) — Engineering Funda
- 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/two-cavity-klystron-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-5

## Question

Write a short note on two-cavity klystron.

## Short answer

A two-cavity klystron is an O-type amplifier with a buncher and a catcher cavity separated by a drift space. Input RF velocity-modulates the beam; electrons bunch in the drift region and induce amplified output in the catcher. Typical gain is 10–20 dB and efficiency 15–30%.

## Answer

A **two-cavity klystron** is an O-type (linear-beam) microwave **amplifier**. It has two cavities, a **buncher** and a **catcher**, separated by a field-free drift space.

```text
 cathode buncher   drift space L   catcher
 |>==== [ gap 1 ] ============= [ gap 2 ] ==| collector
         ^ RF in   bunches form ->   v RF out
```

**Working**
1. The beam is accelerated by V₀ to v₀ = √(2eV₀/m) ≈ 0.593×10⁶ √V₀ m/s.
2. **Velocity modulation:** the input RF sets the buncher gap voltage V₁ sin ωt. An electron crossing at t₁ leaves with v = v₀[1 + (β_i V₁/2V₀) sin ωt₁].
3. **Bunching:** in the drift space, faster electrons catch up with slower ones that crossed earlier. Bunches form around the electron that crossed when the gap voltage went from negative to positive. The bunching parameter is X = β_i V₁ θ₀ / (2V₀), with θ₀ = ωL/v₀.
4. **Output:** the catcher is placed where the bunches are tightest. The fundamental beam current is 2I₀J₁(X), which is largest at X = 1.841 (J₁ = 0.582). The bunches induce RF current in the catcher, and its retarding gap field takes their kinetic energy, giving amplified RF output.

**Key figures**
- Maximum theoretical efficiency about 58%. Practical about 15–30%.
- Gain about 10–20 dB with narrow bandwidth, set by the cavity Q.
- Can be made into an oscillator by feeding part of the output back to the input.

**Uses:** UHF TV and troposcatter transmitters, radar driver stages, and particle accelerators. More cavities (multi-cavity klystron) are added for higher gain.

## Check yourself

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

1. What are the two cavities of a two-cavity klystron and what is the device?
   A. Catcher then buncher; an oscillator
   B. Buncher and catcher separated by a drift space; an O-type amplifier
   C. Two catchers in parallel; a cross-field amplifier
   D. Buncher and reflector; a reflex oscillator
   Answer: B. The buncher velocity-modulates the beam and the catcher extracts the energy after a field-free drift space, giving amplification. (Key point: Structure and role)

2. How do bunches form in the drift space?
   A. Faster electrons catch up with slower ones that crossed the buncher earlier
   B. Slower electrons are accelerated by the drift space field
   C. The magnetic field squeezes the beam
   D. The catcher gap pulls electrons together
   Answer: A. The buncher gap voltage gives different electrons different velocities, so in the field-free drift space faster ones overtake slower ones. (Key point: Velocity modulation and bunching)

3. At what value of the bunching parameter X is the fundamental beam current 2I0J1(X) largest?
   A. X = 0.582
   B. X = 1.841
   C. X = 3.832
   D. X = 1.0
   Answer: B. J1(X) peaks at X = 1.841, where J1 = 0.582, so the catcher is driven hardest there. (Key point: Optimum bunching)

4. How does the catcher produce amplified output, and what is the typical practical efficiency?
   A. It accelerates the bunches; about 90%
   B. Bunches induce RF current and the retarding gap field takes their kinetic energy; about 15-30% in practice
   C. It reflects the bunches back; about 70%
   D. It debunches the beam to DC; about 5%
   Answer: B. The catcher is placed where bunching is tightest and extracts kinetic energy as RF. Theoretical maximum is about 58% but practical is 15-30%. (Key point: Catcher action and efficiency)

## Related in RF and Microwave Engineering

- [Short note: Multi-cavity klystron amplifier](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amulticavity-klystron-note) — RF and Microwave Engineering, chapter 5, asked 2×
- [Density modulation and multi-cavity klystron oscillator](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Adensity-modulation-klystron-oscillator) — RF and Microwave Engineering, chapter 5
- [Cross-field effect and multicavity klystron amplifier](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Acrossfield-multicavity-klystron) — RF and Microwave Engineering, chapter 5