Understanding Antenna Standing Wave Ratio (SWR)

What is SWR?
The Standing Wave Ratio (SWR) is a measure of how well an antenna is matched to the transmission line and the transmitter. When an RF signal travels along a transmission line and encounters a mismatch, some of the energy is reflected back, creating a standing wave.

At points where the incident and reflected waves are in phase, their voltages add to form a maximum voltage, called the voltage antinode (Vmax). Where they are out of phase, the voltages subtract, forming a voltage node (Vmin). Other points along the line have voltages between these extremes. This resulting wave pattern is called a standing wave, and the SWR is defined as the ratio of the voltage at an antinode to that at a node:

SWR=Vmin/Vmax

In RF measurements, the SWR measured using a slotted line or other methods can be used to calculate the reflection coefficient and absorption coefficient of materials.

Why SWR Matters
For RF systems, impedance matching is crucial. A low SWR ensures that most of the transmitted power is radiated by the antenna rather than reflected back to the transmitter. In broadband systems, SWR can vary across frequency, so the goal is to achieve good impedance matching across the entire operating range.

Ideal SWR: SWR = 1 means perfect matching; all transmitted power is radiated, with no reflection.
SWR > 1: Indicates some power is reflected back, potentially heating the feedline and, at high levels, damaging the transmitter.
Common SWR Requirements and Measurement

Typical targets: Mobile communications often require SWR < 1.5, while many applications aim for SWR < 2.0.
Measurement tools: SWR meters, vector network analyzers (VNAs), or antenna analyzers are commonly used.
Adjustment methods: Antenna tuning or matching networks can improve impedance matching and reduce SWR.

Why Do Standing Waves Form?

Ideal case: If the transmitter output impedance, transmission line characteristic impedance, and antenna input impedance are all equal (typically 50Ω or 75Ω), all energy flows to the antenna with no reflection. Only a forward traveling waveexists on the feedline.
Practical case: Antenna impedance varies with frequency, environment, and installation, making perfect matching difficult.
Mismatch and reflection: When impedance is mismatched, part of the energy cannot be absorbed by the antenna and is reflected back toward the transmitter.
Formation of standing waves: The reflected wave combines with the incident wave, creating points of maximum and minimum voltage along the line — the “standing wave.”
SWR quantifies the severity of this mismatch and reflection.

How to Calculate SWR
SWR is often expressed as a ratio like 1.5:1 or 2:1 (usually we just say “SWR = 1.5”).

Formula using reflection coefficient Γ:
SWR=1+∣Γ∣/​1−∣Γ∣

where ∣Γ∣ is the voltage reflection coefficient (ratio of reflected voltage to incident voltage), determined by how much the load impedance differs from the line impedance.

Intuitive formula using impedances:
SWR=Zload/Zline or Zline/Zload,take the larger value

Example: If the feedline is 50Ω and the antenna is 100Ω, SWR = 100/50 = 2:1. If the antenna is 25Ω, SWR = 50/25 = 2:1. The further the impedance deviates from the line, the higher the SWR.

How to Improve (Lower) SWR
The key to improving SWR is improving impedance matching:

Use a properly designed antenna: Ensure it operates well within the target frequency range.
Use an antenna tuner: This device inserts a variable matching network between the transmitter and antenna, lowering the overall SWR on the feedline. Note: it does not change the antenna’s inherent SWR but protects the transmitter.
Adjust antenna dimensions: For single-frequency antennas (like dipoles), fine-tune the element length to achieve resonance at the desired frequency, minimizing SWR.
Ensure solid connections: Check connectors for tightness, oxidation, or moisture.
Consider the installation environment: Keep antennas away from metal objects and walls and ensure sufficient height, as surroundings can significantly affect impedance.
Summary
SWR is a key “health indicator” of an antenna system, showing how efficiently energy is transmitted from the transmitter to the antenna. Low SWR means high energy transfer efficiency and a safe, stable system. Measuring and monitoring SWR is fundamental for any RF system, especially for transmitters.