How to Measure Antenna Impedance
Key Takeaways
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The impedance measured at the terminals of an antenna can be defined as antenna impedance.
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At resonance, the magnitude of antenna impedance is maximum.
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Antenna impedance measurements can be obtained using a vector network analyzer, impedance bridge method, or the slotted line method.
Matching the antenna impedance with the system impedance is critical for maximum power transfer
Antenna impedance is a significant parameter influencing the performance of communication systems. Matching the antenna impedance with the system impedance is critical for maximum power transfer, as an antenna transmits or receives maximum power when the impedance is matched. The bandwidth of the antenna is also dependent on the antenna impedance. It is important to measure the antenna impedance to ensure the impedance matches the feedline impedance using matching networks. In this article, we will discuss antenna impedance, resonance frequency and impedance bandwith, and how to measure the antenna impedance.
Antenna Impedance
Antenna impedance is the impedance measured at the terminals of an antenna. This impedance can be determined as the ratio of voltage to current at the antenna terminals. The ratio of appropriate components of electric and magnetic fields at a point also present antenna impedance.
Consider an antenna with terminals a and b with no load connected. The ratio of voltage to current at terminals a and b give the antenna impedance Za :
Ra is the resistance of the antenna and Xa is the reactance of the antenna. The real part of the antenna Ra is due to heat dissipation and radiation loss. The antenna resistance Ra can be resolved as presented:
Rl is the resistance of the antenna due to heat dissipation and Rr is the resistance of the antenna depicting radiation loss. The radiation efficiency is also dependent on the radiation and loss
The antenna reactance indicates the stored energy around the antenna, the energy that is present in the near zone. When the stored magnetic and electrical energy are equal in magnitude, it leads to a resonance condition in the antenna. At resonance, the antenna reactance is equal to zero.
Resonance Frequency and the Impedance Bandwidth of an Antenna
At resonance, the magnitude of antenna impedance is maximum. The frequency at which the resonance occurs is called resonant frequency, fr. Antenna operation at resonance is desirable, as it is easy to match the antenna impedance with the system impedance. As the characteristic impedance is real at resonance, impedance matching can be easily achieved in input transmission lines.
The antenna impedance magnitude falls below half of its resonant value at a frequency band called impedance bandwidth of the antenna, denoted by Δfr. Within the impedance bandwidth, the antenna is useful. Outside the impedance bandwidth, the antenna reflects the input power due to impedance mismatch and results in poor radiation.
To achieve optimal antenna performance, it is important to know how to measure antenna impedance. Antenna impedance measurement results are useful for designing the matching networks for impedance matching. There are various techniques that can be used to measure antenna impedance, and we will discuss a few in the upcoming section.
How to Measure Antenna Impedance
The antenna impedance is dependent on factors such as frequency, method of excitation, geometry, and proximity to surrounding objects. Due to the dependence on the environment where it is intended to function, antenna impedance measurements are usually performed in situ. Some ways to measure antenna impedance include using a vector network analyzer, impedance bridge method, or slotted line method.
Vector Network Analyzer
A vector network analyzer (VNA) is a type of measurement equipment appropriate for measuring antenna impedance in the end-user environment. VNAs measure the antenna input impedance as a function of frequency.
Impedance Bridge Method
The impedance bridge method is more suitable for frequencies below 30 MHz. Typically, the Wheatstone bridge is used for measuring the antenna impedance in the impedance bridge method. The Wheatstone bridge is calibrated by shorting or open circuiting the unknown impedance terminals along with three known impedances. An antenna of unknown impedance is placed at the short or open terminal, and the bridge is re-calibrated. The bridge is balanced for magnitude as well as phase. Using the impedance equation, the unknown antenna impedance is determined.
Slotted Line Method
For frequencies above 1000 MHz, the slotted line method is used to measure antenna impedance. The slotted line method, also known as the standing wave or standing wave ration method, determines impedance by the voltage or current standing wave ratio and the distance between the voltage or current minimum and the reference point of impedance.
How one measures antenna impedance between 30 MHz and 1000 MHz depends on the level of convenience and equipment available. Cadence’s suite of design and analysis tools can help measure antenna impedance as well as assist designers with impedance matching. Subscribe to our newsletter for the latest updates. If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.