When an electrical signal propagates through media, a portion of the signal is reflected at the interface between sections with differing impedance.
It is equal to the ratio of the amplitude of the reflected wave to the incident wave.
Figure 1
Taking note of Figure 1, it can be shown that the reflection coefficient of a load is determined by its impedance ZL and the impedance to the source Zs and is calculated as following:
The Standing Wave Ratio (SWR) is introduced by modifying equation 1 as:
The SWR does not measure the actual impedance of the load, but quantifies the magnitude of impedance mismatch by performing a measurement on the transmitter side of the transmission line.
Impedance matching was an issue faced mainly by people specialized in high speed designs like DRAMs board designers. But in a world where everything is connected, the development of low power wireless communication, like Bluetooth low energy, design of matching impedance is very common. In wireless applications impedance matching is paramount. A bad matching is translated in no communication or a degraded signal that decreases dramatically the scope of the signal.
For instance, the 47948 2.4GHz SMD ground antenna is the smallest on-ground antenna. It is suitable for any application using Bluetooth, WiFi , ZIGBEE and other wireless standards. IoT always requires smaller boards. This Molex antenna measures only 3x3mm. It main advantage is in the PCB layout. It does not need any ground clearance, leaving the ground layer intact while freeing space on the reverse side of the PCB for other components. This simplifies the job for board makers and saves significant PCB space.
For optimized performance, this antenna needs 50 Ohm input impedance.
The following parameters have an impact on the resistance of the PCB:
• Dielectric value
• Thickness of the dielectric
• Thickness of copper line
• Distance between trace and ground plane
• Width of the trace
Figure 2
Impedance is also calculated at a given frequency. Some parameters, like the copper thickness is standard. For instance, a 1 oz copper thickness is equal to 1.37 mils. Dielectric and board thickness must be determined from your PCB manufacturer as these parameters can change from supplier to supplier. Besides, if for thermal dissipation reasons, you decide to switch from a 1 oz to a 2 oz copper board, it’s important to relay the board. Indeed, in the example of Figure 2, changing the thickness parameter T from 1.4 to 2.8 mils, changes the impedance from 50.0 Ohm to 47.1 Ohm.
In the end, for optimization, it’s recommended to design a matching circuit and measure the exact impedance of the board. Indeed, all the equations for simulation are not 100% accurate for representing the tricky nature of RF physics.