Feature: Communications
most effi ciently within a relatively narrow frequency range. T e antenna’s size or length usually
incorporates the wavelength as part of its name, for example, a half-wave dipole. T e wavelength, measured in meters, is the distance a radio wave will cover in one cycle. For example, a wavelength of 1m equates to a frequency of about 300MHz. A half-wave antenna at that frequency would be 0.5m, and a quarter-wave antenna 0.25m. T e higher the frequency, the shorter the wavelength. T e majority of automotive applications
involve transmitters and receivers operating in the very high frequency (VHF) range and above, between 165MHz and 2.4GHz. 5G wireless applications, such as V2X and V2V, push that upper limit toward 6GHz; see Figure 2.
Choosing an antenna When selecting an antenna, there are several essential characteristics to review: • Impedance matching: To achieve an effi cient coupling of RF energy from the transmitter to the antenna, they both must have the same impedance. T is matching also relates to the transmission
Most automotive applications involve transmitters and receivers operating between 165MHz and 2.4GHz, but 5G pushes that toward 6GHz
line used to connect them, for example, a coaxial cable. A 50-Ohm impedance is typical for most RF transmission line components.
• Antenna effi ciency: T is is a measure of how eff ective the antenna is at radiating an RF signal. T e effi ciency is the ratio of power radiated from the antenna compared with the input power, and is expressed in dB.
• Antenna gain and directivity: Some antennas are designed to be more effi cient at radiating a signal in a particular direction. Omnidirectional
antennas radiate equally in all directions, while a directional antenna focuses transmitted energy and exhibits gain in a given direction. Antenna gain is stated in dB against
a standard antenna or an isotropic source – dBi. T e directional antenna’s datasheet will include a radiation pattern indicating the relative fi eld strength measured at the same distance around the antenna.
• Voltage standing wave ratio (VSWR), more commonly abbreviated as SWR, is a measure of how well the antenna is matched to the transmitter at a given frequency. It is expressed as a ratio of the transmitter output power to the amount refl ected back to the transmitter before being radiated. A 1:1 VSWR ratio indicates that the antenna and the transmitter are perfectly matched. For most practical purposes, a VSWR of 2.5:1 or below is acceptable. An ideal antenna match of a VSWR
1:1 is highlighted in Figure 3. No power is refl ected, so the transmitter operates at maximum effi ciency. T e higher the VSWR ratio, the more the transmitter needs to increase its power output to achieve the
Figure 2: Frequency spectrum examples used in automotive ADAS functions
www.electronicsworld.co.uk November 2021 19
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