Impedance matching techniques for antennas are intended to ensure maximum power transfers into the antenna so that the element can radiate strongly.
Antenna impedance matching involves matching the input impedance at the end of the antenna’s feedline to the feedline’s characteristic impedance.
Filter circuits are normally used, as they can be configured to provide specific impedance right at the desired transmission frequency.
Although antennas come in a variety of shapes and sizes, they have one thing in common: they need to have impedance matching enforced at the end of the feedline to ensure maximum power transfer into the load. Impedance matching circuits are rather simple; they act like filters that ensure an antenna’s feedline impedance matches the input impedance at the input port of the antenna. Starting from a filtration perspective is the easiest impedance matching technique in antennas or other RF circuit elements.
Impedance Matching Techniques in Antennas: Defining Impedance Matching Requirements
The need for impedance matching techniques in antennas comes from the fact that antenna impedances are not always 50 Ohms. Some antennas, such as chip antennas, have either low or high impedance when fabricated. For other antennas, such as a printed antenna, it may be difficult to design the antenna to perfectly hit a 50 Ohm target impedance; the traces might be very wide or the antenna may take up board space. The result is that the design needs to be fabricated smaller, which creates an impedance mismatch.
In addition, the antenna and its matching network might connect to a short feedline, so the input impedance of the feedline may not be the line’s characteristic impedance. Instead, the input impedance needs to be matched to the transmitter’s output impedance so that the return loss (S11) can be reduced as much as possible at the antenna input and at the feedline input. Although 50 Ohms on-die termination is often applied or is configurable in devices with an integrated RF transceiver, it doesn’t mean there will be perfect impedance matching.
If taking the circuit-based approach, there are several options for impedance matching techniques in antennas. The goal is to ensure that the (antenna + impedance matching network) equivalent impedance matches the input impedance seen at the inlet of the transmission line. Typical circuit topologies are shown in the table below:
Circuits like a series LC filter are not useful, as they will have a stop band. Higher order RF filters can also be used if very sharp roll-off is needed, although these increase the component count. No matter which type of filter you want to use for impedance matching, the (antenna + impedance matching network) impedance should match the antenna feedline characteristic impedance. This can be evaluated with a SPICE simulation. Once this matching is determined, you can work upstream to the input port of the antenna to ensure further matching.
With the (antenna + impedance matching network) designed to match a target impedance of the feedline, the next step is to ensure the input impedance also matches 50 Ohms. This can be easily done using the antenna’s reflection coefficient at its input with the standard transmission line input impedance equation:
Input impedance of a feedline with a known propagation constant and chosen length. Note that the antenna impedance must be known before calculating the input impedance of the feedline.
Ideally, this value should also reach 50 Ohms. S11 (or return loss) can then be calculated at the feedline input; a typical design goal is no more than 20 dB loss at this input port.
Transmission line stubs can also be implemented as shunt elements, just like the case of a shunt capacitor or inductor. Simply use the input impedance equation above for the stub section to calculate its effect as an equivalent circuit element. The use of stubs is very common in passive RF circuits that use printed circuit elements, so it is also appropriate to use this for impedance matching. Note that, because of the propagating behavior of electromagnetic fields on a transmission line stub, you may be able to match multiple frequencies with an open circuit or short circuit transmission line stub.
Alternatives to Impedance Matching Circuits
In this article, we’ve examined impedance matching with various filter circuits, something which is quite common when connecting to chip antennas, coaxial connectors, and even printed antennas, trace, or slot antennas. Circuit design methods are standard because it is so easy to implement a parametric sweep in a SPICE simulator to examine the input impedance and reflection coefficient at the input port of the antenna. For those who prefer not to use circuit simulations, there are alternative tools for impedance matching in antennas.
Antenna impedance matching can also be performed with a Smith chart. This graphical method of applying impedance matching requires tracing out the impedance of an (antenna + matching network) combination by adding L and C elements in series or shunt placement. There are many guides on Smith chart usage that can be found online, and it’s recommended that newer antenna designers take advantage of these guides to get started with their first attempt at using Smith charts. Make sure to remember series and shunt rules for applying L and C elements to a chart (see below).
Smith chart series and shunt rules are used to move the total impedance of the (antenna + matching network) to a target impedance of 50 Ohms.
The last remaining technique for non-standard radiators is to use an electromagnetic field solver. Finite-difference frequency domain (FDFD) simulations performed in 3D can be used to examine the electromagnetic field emitted from the device, which can then be related back to the voltage and current distribution in the antenna. More sophisticated solvers can extract network parameters from these structures, which can provide a direct calculation of S11 at the input port and at the desired emission frequency/bandwidth. Designers can then use this data to determine the amount of impedance matching for an antenna design using the other techniques outlined here.
The electromagnetic simulation tools in the Clarity 3D Solver from Cadence are ideal for performing impedance matching techniques in antenna design, interconnect design, and waveguide design for unique RF systems. When you’re ready to create your PCB layout, you can use Cadence’s PCB design and analysis software to complete your design and prepare it for signoff.