---
title: "Measurement tool to test an antenna as DUT"
subject: "RF and Microwave Engineering"
chapter: 8
key: "semester_7/rf_and_microwave_engineering:antenna-dut-measurement-tool"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/antenna-dut-measurement-tool
site: BCT Engineering
---

# Measurement tool to test an antenna as DUT

RF and Microwave Engineering · Chapter 8: RF and Microwave Measurements

- Asked in: 2074 Bhadra, 8 marks
- Video: [Cable and Antenna Analyzer Training Video | FieldFox Handheld Analyzers | Keysight](https://www.youtube.com/watch?v=I4gk2DLS3U4) — Keysight Handhelds
- 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/antenna-dut-measurement-tool
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-8

## Question

Choose a proper microwave measurement tool to test an antenna as a DUT; and explain its working principles.

## Short answer

A Vector Network Analyzer (VNA) tests an antenna. It measures S11 magnitude and phase using directional couplers to separate incident and reflected waves, digitizing after down‑conversion. Calibration yields return loss, VSWR, and input impedance. With a reference antenna on port 2, S21 gives gain and radiation pattern.

## Answer

The proper tool to test an antenna as a device under test (DUT) is a **Vector Network Analyzer (VNA)**. It measures S-parameters (magnitude and phase) over a frequency sweep, giving the antenna's return loss, VSWR, input impedance and bandwidth; with a second antenna in an anechoic chamber, it also gives gain and radiation pattern.

#### What is measured on the antenna

| Antenna parameter | VNA quantity |
|---|---|
| Return loss, VSWR, Γ | S11 |
| Input impedance | S11 on Smith chart |
| Resonant frequency, bandwidth | S11 vs frequency (e.g. S11 < −10 dB) |
| Gain, pattern, polarization | S21 between AUT and reference antenna |

#### Block diagram

```text
 +--------- Vector Network Analyzer ----------+
 | Swept  ->  Directional  ->  Port 1 --------+--> AUT
 | source     couplers                         |
 |   |          |      |                       |
 |   |      ref (a1) refl (b1)                 |
 |   v          v      v                       |
 |  LO -> Mixers/IF -> ADC -> DSP -> display   |
 |                                Port 2 <-----+-- Ref.
 +---------------------------------------------+  antenna
```

#### Working principle
1. A **synthesized swept source** produces a signal over the required band (e.g. 1–6 GHz).
2. **Directional couplers** (or bridges) separate the incident wave a1 from the wave b1 reflected by the antenna.
3. Each signal is down-converted to an IF by mixers driven by a common LO, then digitized. Because a common reference is used, both **amplitude and phase** are measured.
4. The processor computes the ratio S11 = b1/a1 at each frequency. From it:

```text
Return loss = −20 log|S11|  dB
VSWR = (1 + |S11|) / (1 − |S11|)
Zin = Z0 (1 + S11) / (1 − S11)
```

5. **Calibration** (Short–Open–Load, or SOLT for two ports) is done at the cable end before measurement. It removes systematic errors (directivity, source match, tracking), so the result refers to the antenna's feed point.
6. For radiation tests, the antenna under test is placed in an **anechoic chamber** with a reference horn on port 2. S21 is recorded while the AUT rotates on a positioner, giving the pattern; gain follows from the Friis equation or by comparison with a standard gain horn.

#### Why a VNA
- Broadband, fast swept measurement.
- Gives phase, so impedance and matching networks can be designed.
- Error correction gives high accuracy compared with a slotted line.

Simpler alternatives are a **scalar network analyzer**, which sweeps frequency but gives only magnitude (return loss, VSWR), or a slotted line with a VSWR meter, which gives only magnitude at one frequency at a time.

## Check yourself

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

1. Which tool is proper for testing an antenna as a DUT?
   A. Bolometer bridge
   B. Vector network analyzer
   C. Cavity wavemeter
   D. Static calorimeter
   Answer: B. A VNA measures S-parameters (magnitude and phase) over a frequency sweep, giving return loss, VSWR and impedance. (Key point: Choice of tool)

2. What does the VNA compute for reflection measurement?
   A. S11 = a1 x b1
   B. S11 = a1 - b1
   C. S11 = b1 / a1, the reflected over incident wave
   D. S11 = a1 / b1
   Answer: C. Directional couplers separate incident a1 and reflected b1; their ratio is S11. (Key point: S11 computation)

3. Why does the VNA measure both amplitude and phase?
   A. Because mixers use a common LO and reference, giving complex S11 for impedance and matching design
   B. To avoid using a source
   C. Because phase gives the power in watts
   D. To skip calibration
   Answer: A. A common LO allows phase, so Zin = Z0(1+S11)/(1-S11) and matching networks can be derived. (Key point: Why phase measurement)

4. What does SOL/SOLT calibration at the cable end achieve?
   A. It increases source power
   B. It removes antenna radiation
   C. It converts S11 to gain
   D. It removes systematic errors so results refer to the antenna feed point
   Answer: D. Calibration corrects directivity, source match and tracking errors, moving the reference plane to the feed. (Key point: Calibration)

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