Feature: IIoT
Figure 2: Function of the ground clearance area of a loop antenna
VSWR-Return Loss T e voltage standing wave ratio (VSWR) and return loss, also called the refl ection coeffi cient or S11 from the scattering (S) parameters, are two diff erent ways of showing the same information, see Figure 1. T e resonant frequency and impedance
of the antenna are something you can easily measure using a vector network analyser (VNA). For gain and effi ciency, you need the anechoic chamber to do the measurement, but for VSWR a VNA is suffi cient.
Frequency and wavelength T e number of periods or cycles per second is called the frequency, which is measured in Hertz (Hz). In wireless communication, the common frequencies used range from around 3 kHz to 300 GHz. Wavelength is the time required to complete one full cycle of the wave, from peak to peak or trough to trough – it is dependent on the wave speed (the rate at which the wave moved through the medium like air) and frequency. To calculate the wavelength, you divide the speed of light in air (3x10ex10 cm/s) by the frequency. So, as shown below, for a Wi-Fi or Bluetooth antenna working at 2.4 GHz, you would need an antenna with a 12.5 cm length.
So, if you have a smartwatch with a 4x4cm bracket and still
want to have a fully functional Wi-Fi antenna – how do you fi t in the antenna?
Figure 3: The difference between a matched and mismatched antenna
www.electronicsworld.co.uk March 2025 23
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