Measurement tool to test an antenna as DUT

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

Block diagram

 +--------- 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:

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.