Antenna matching is a critical element in Radio Frequency (RF) system design, often determining the success of wireless devices and radio equipment. This blog post explains what antenna matching is, why it's important, and how to implement it effectively.
What is antenna matching and why are we doing it?
In an RF system, electromagnetic waves are generated by the transmitter and guided by the transmission line (coaxial cable or PCB trace) to the antenna. The antenna is part of the RF system that transforms the electromagnetic (EM) wave guided through the transmission line into an EM wave traveling through the air. When the EM wave encounters a discontinuity in the medium, it will be totally or partially reflected, which indicates a mismatch in impedance.
When talking about antenna matching, we are referring to the process of adjusting the impedance of the antenna to the characteristic impedance of the RF system to minimize the discontinuity in the medium. This way, we are minimizing the power of the EM waves reflected from the antenna. In RF system design, antenna matching is one of the most important tasks due to the following reasons:
1. Avoiding damage to the transmitter
If the reflected wave constructively interferes with the incident wave, the result is a wave that surpasses the maximum current and/or voltage ratings of the transmitter and causes irreparable damage to the transmitter IC.
2. Avoiding power loss
The energy of the reflected wave will not be radiated by the antenna and from the point of power consumption – it is irretrievably lost. This results in lower system efficiency.
3. Improving the communication range
With the smaller amount of power radiated, the received power decreases and the range of the reliable communication gets smaller.
4. Ensuring optimal performance
There are examples in engineering when the first three reasons don't cause crucial problems. Transmitter ICs often have ratings high enough to avoid being damaged by reflected waves. Many devices are powered by virtually unlimited power supplies, so power loss is not a major issue. In such cases, antenna matching is still very important because the goal of every engineer should be to make things work the best they can!
Reflection coefficient
The matching quality is quantitatively evaluated in the form of the reflection coefficient. The reflection coefficient is the ratio of the reflected wave power and the power of the incident wave generated by the transmitter. The worst case is total reflection – all initial power is reflected, and the reflection coefficient (often called S11) is 1 (0 dB). Ideally, there is no reflected power, and the reflection coefficient is 0 (-∞ dB). In practice, everything below -10 dB is considered excellent matching. In some systems, especially systems with multi-band operation, even -6 dB is considered an acceptable matching.
Mathematically, the reflection coefficient can be calculated as:
where ZL is load impedance, in our case antenna impedance and Z0 is the characteristic impedance of the system. If we show the antenna impedance in a complex plane and normalize it by Z0, the above equation will transform the complex plane into a circular diagram widely used by RF engineers – the famous Smith Chart. It is a very useful tool for designing various RF structures and circuits, whose thorough explanation would require a whole article for itself.
The first step: Antenna impedance measurement
Antenna impedance is measured using a VNA (Vector Network Analyzer). The VNA measures the reflection coefficient and can usually display the measured values on the Smith Chart which is used to read the antenna impedance.
Matching can only be as good as the measurement setup. In the RF frequency domain, the geometrical sizes of the traces and components are comparable with the signal wavelength, therefore the reference plane of the VNA should be moved as close to the antenna feed as possible. This is done by calibration. The best practice in designing the antenna and its matching network is adding the multiple combinations of series and shunt matching components in the initial design near the antenna feed point before calculating their exact value, as shown in Figure 1 in the schematic and the PCB.
Figure 1a: Matching network in schematic view. Picture adapted from Altium designer
Figure 1b: Matching network on the PCB. Picture adapted from Altium designer
These components can be used as VNA calibration points – the coaxial cable from VNA can be soldered across the shunt component pads. Also, these components allow the designer to test the matching quality by soldering the components with calculated values on the prototype and repeating the measurement. If some components in the initial matching network are not required it is easy to omit them – shunt components will not be mounted, and series components will be replaced with 0 Ohm resistors.
Another important thing while measuring the antenna impedance is that the antenna must be in the same environment as it will be in the final application. Because the close environment of the antenna has great impact on its impedance, the device has to be inserted in the housing and mounted in the same environment as the final product while measuring the antenna impedance.
The second step: Calculating matching network components
After we find the antenna impedance in its expected environment we can continue with the second step in antenna matching – adjusting the antenna impedance as close to the characteristic impedance of the system as possible. It can be achieved in two ways. The first is adjusting the antenna geometry. This technique is used by antenna designers to optimize not just the impedance but also the overall antenna performance, but is often limited by additional system requirements such as physical dimensions, efficiency, multi-band operation etc.
On the other hand, circuit designers look at the antenna as a component that cannot be modified but only combined with other components to achieve the best performance. To adjust the antenna impedance to the desired value, circuit designers use matching networks – a various combination of shunt and series lumped passive elements with calculated values.
The old-school way is to mark the measured antenna impedance on the Smith chart and, by adding the lumped reactive components, move the marked point to the centre of the Smith chart, which corresponds to the characteristic impedance of the system. Every component affects the impedance in a unique way in the Smith chart:
1. Shunt capacitor
Increasing the capacitance moves the impedance further along the circles of constant admittance in the direction shown in Figure 2a.
2. Series capacitor
Decreasing the capacitance moves the impedance further along the circles of constant impedance in the direction shown in Figure 2b.
3. Shunt inductor
Decreasing the inductance moves the impedance further along the circles of constant admittance in the direction shown in Figure 2c.
4. Series inductor
Increasing the inductance moves the impedance further along the circles of constant impedance in the direction shown in Figure 2d.
Figure 2: Influence of the lumped passive components on the impedance in the Smith chart.
Picture adapted from: will-kelsey.com
Back in the old days, RF engineers would use the Smith Chart and a geometry set to calculate the exact values of the matching components. Nowadays, there are various software for calculating these values and saving the designer from doing it manually. Furthermore, even the majority of VNAs can calculate the simple matching networks as a built-in feature.
The third step: Evaluating matching quality
The third step is soldering the components with values calculated in the previous step and repeating the measurement. Since the operating frequencies of RF communication are very high, the designer must use components suitable for high-frequency design – huge THT capacitors and inductors will not do the job. SMD 0402 or 0201 components are most used for matching, due to their low values of parasitic resistance, capacitance, and inductance. Also, some component parameters must be checked to ensure the component is suitable for matching purposes and the exact application it is used in.
High-frequency capacitors usually have a Q-factor specified in the datasheets. In matching circuits, higher Q-factor components are better, but you have to be careful when evaluating the capacitors from different vendors. Different manufacturers may specify the measured Q factor at different frequencies. The best way would be to specify the Q factor in the chart with respect to the measured frequency
The same applies to high-frequency inductors, a higher Q factor is better. Datasheets usually specify two more values: DC resistance and SRF (self-resonating frequency). The DC resistance of the inductor for the matching network should be as small as possible. The SRF is a frequency where the inductance resonates with the parasitic capacitance of an inductor. Above these frequencies the component no longer acts as an inductor – it acts as a capacitor – its impedance decreases as frequency increases. When selecting the inductor for matching purposes, make sure that its SRF is significantly higher than your RF system's working frequency.
Still, even if all parameters of the chosen components are carefully evaluated – it is possible that calculated values will not result in optimal matching. It is more of a trial-and-error procedure in which calculated components are used as a starting point.
Conclusion
Antenna matching is a complex procedure that requires many different skills and areas of knowledge, and its quality can easily be subject to errors during the procedure. Mistakes in measurement can make the designer think he has done a great job designing a perfectly matched system, and it is still possible that the system will not work at all. Nevertheless, it is a very important task while designing any wirelessly connected device. Since the amount of such devices is rapidly increasing, it is a very useful skill to learn.
