Vector network analyzer antenna measurement

Measuring antenna impedance

While simulations and modeling can predict antenna performance, real-world factors often introduce variations. As such, physical measurements under real-world conditions are necessary to verify antenna performance.

There are two types of antenna measurements:

• Radiation measurements, which measure parameters such as gain, directivity, beamwidth and efficiency
• Impedance measurements, which measure the (complex) impedance of the antenna

The impedance of an antenna determines how much of the input or transmit power is absorbed or radiated by the antenna, and how much is returned to the transmitter. This is determined by injecting a signal into the antenna and then measuring the magnitude and phase of the reflected signal.

There are several different methods and tools that can be used for measuring antenna impedance, but the preferred method is using a vector network analyzer (VNA) to perform a reflection (S11) measurement.

VNAs are versatile instruments capable of performing scalar and vector measurements of forward and reflected power. They have a high accuracy over a wide frequency range, and they also have a high dynamic range. These instruments can also be used for other RF tasks, such as measuring cable loss or testing filters and amplifiers.

How to measure antenna impedance

There are three key steps in antenna measurements:

1. Connecting the antenna
2. Configuring the VNA
3. Calibrating the setup

Let’s begin with connecting the antenna. Antennas are often connected to VNAs via a feedline. This feedline may be directly connected to the VNA port, or an additional device under test (DUT) cable may be used to connect the VNA to the cable under test. If a DUT cable is employed, calibration at the end of the cable ensures accurate measurements.

There are three aspects to consider when configuring the VNA:

• Configuring the tracking generator, which provides a swept stimulus signal that is used both as the input to the antenna and as a reference when looking at the amount of signal reflected from the antenna.
• Specifying the sweep frequency range over which the tracking generator is swept.
• Setting the number of measurement points; increasing this number beyond the analyzer’s default settings will provide greater detail, but more frequency points will also increase the amount of time needed to make a single sweep.

Calibration standards for accurate antenna impedance measurements

In addition to configuring the VNA, one-port calibration is necessary for accurate antenna impedance measurements. The calibration process involves sequentially attaching calibration standards to the location where the antenna will be connected.

The three calibration standards are:

• Open
• Short
• Match (or load)

Electronic calibration units can be an alternative to these manually attached standards. These units automatically switch their internal standards and are controlled by the attached VNA. They tend to be much faster than using manual standards.

If the antenna or feedline will be directly attached to the VNA port, the calibration standards should also be attached directly to this port. If a DUT cable will be used, then the calibration standards should also be attached to the end of a DUT cable.

Antenna impedance measurement formats

VNAs provide several formats for analyzing antenna impedance:

• Standing wave ratio (SWR): SWR indicates the ratio of forward to reflected power, with an ideal value of 1:1. It’s often plotted against frequency to determine bandwidth.
• Return loss: Expressed in dB, return loss represents the ratio of reflected to forward power on a logarithmic scale. Higher values indicate better antenna performance.
• Complex impedance and Smith charts: Beyond scalar values, VNAs can measure complex impedance, providing insight into real and imaginary components. Results are displayed on a Smith chart , offering a comprehensive view of impedance behavior over frequency.