---
title: "Short note: Probe coupling against loop coupling"
subject: "RF and Microwave Engineering"
chapter: 4
key: "semester_7/rf_and_microwave_engineering:probe-vs-loop-coupling-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/probe-vs-loop-coupling-note
site: BCT Engineering
---

# Short note: Probe coupling against loop coupling

RF and Microwave Engineering · Chapter 4: RF and Microwave Components and Devices

- Asked in: 2069 Bhadra (old course), 5 marks
- Video: [coupling probes and loops in microwave](https://www.youtube.com/watch?v=Zu9JZkLzjA4) — Ravi Teja Creative Catchers !!
- 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/probe-vs-loop-coupling-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-4

## Question

Write a short note on PROBE-coupling against LOOP-coupling.

## Short answer

Probe coupling uses a short antenna inserted through the broad wall at the E-field maximum (approx λg/4 from a short) to couple electrically. Loop coupling employs a small loop placed at the H-field maximum near a short wall, oriented perpendicular to the magnetic field; coupling is adjusted by rotating the loop.

## Answer

Energy is fed into or taken out of a waveguide or cavity resonator with **probe (electric) coupling** or **loop (magnetic) coupling**. A coaxial line is usually used to launch the wanted mode.

**Probe coupling**
- The inner conductor of the coaxial line is extended into the guide as a short antenna (probe) of about λ/4 length. It goes through the **broad wall**.
- The probe is placed where the **E-field is maximum**, parallel to the E-lines. For TE₁₀ this is the centre of the broad wall, about λg/4 from the shorted end.
- It works like a monopole antenna. Coupling is varied by changing the probe depth or position.

**Loop coupling**
- The inner conductor forms a small loop whose end is joined to the guide wall.
- The loop is placed where the **H-field is maximum** (for example near the shorted end wall), with the plane of the loop **perpendicular to the H-lines** so that the most magnetic flux links it.
- Coupling is varied by rotating the loop: rotating it by 90° reduces coupling to almost zero.

```text
 Probe (E-field)        Loop (H-field)
   coax                    coax
    ||                      ||
 ===||=========          ===||====|
    |  <- probe             (_)   | <- short
 E-lines ||||           loop near short wall
```

| Point | Probe coupling | Loop coupling |
|---|---|---|
| Field coupled | Electric (E) | Magnetic (H) |
| Position | E-max, ~λg/4 from short | H-max, near short wall |
| Orientation | Parallel to E-lines | Loop plane ⟂ H-lines |
| Adjustment | Depth or position | Rotation of loop |
| Acts like | Monopole antenna | Small magnetic dipole |
| Typical use | Coax-to-waveguide adapters | Cavities, klystron and magnetron outputs |

Both can also excite higher modes when placed at those modes' field maxima. A third method is **aperture (slot) coupling** through a hole in the common wall.

## Check yourself

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

1. Probe coupling couples which field and where is the probe placed?
   A. H field, at the H maximum
   B. E field, at the E maximum parallel to E-lines
   C. H field, at the broad wall corner
   D. E field, perpendicular to E-lines
   Answer: B. A probe acts like a monopole and picks up E parallel to its length. (Key point: Probe coupling field)

2. How is a loop oriented for maximum coupling?
   A. Plane parallel to H-lines
   B. Plane perpendicular to H-lines at the H maximum so flux links it
   C. At the E maximum
   D. Anywhere
   Answer: B. Maximum magnetic flux must link the loop. (Key point: Loop orientation)

3. How is loop coupling adjusted, and what happens at 90° rotation?
   A. By depth; rises
   B. By rotating the loop; coupling falls almost to zero
   C. By length; doubles
   D. It cannot be adjusted
   Answer: B. At 90° no flux links the loop. (Key point: Adjusting loop coupling)

4. For TE10, where is a probe placed?
   A. Centre of the broad wall, about λg/4 from the shorted end
   B. At the narrow wall corner
   C. At the shorted end wall
   D. Anywhere on the narrow wall
   Answer: A. E is maximum at the centre of the broad wall and λg/4 from the short. (Key point: Probe position for TE10)

## Related in other subjects

- [Preventing ground loops; electromagnetic coupling](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aground-loop-em-coupling) — Instrumentation II, chapter 6
- [Tower top potential and discs against back-flashover](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atower-top-potential-back-flashover) — High Voltage Engineering, chapter 3