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. Ideally, this value should also reach 50 Ohms. Note that the antenna impedance must be known before calculating the input impedance of the feedline. Input impedance of a feedline with a known propagation constant and chosen length. This can be easily done using the antenna’s reflection coefficient at its input with the standard transmission line input impedance equation: 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. Once this matching is determined, you can work upstream to the input port of the antenna to ensure further matching. This can be evaluated with a SPICE simulation. 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. Higher order RF filters can also be used if very sharp roll-off is needed, although these increase the component count.
Input impedance patch antenna series#
Parallel LC, in parallel with the antennaĬircuits like a series LC filter are not useful, as they will have a stop band. Typical circuit topologies are shown in the table below: 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. If taking the circuit-based approach, there are several options for impedance matching techniques in antennas. 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. 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. 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. The result is that the design needs to be fabricated smaller, which creates an impedance mismatch. 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. Some antennas, such as chip antennas, have either low or high impedance when fabricated. The need for impedance matching techniques in antennas comes from the fact that antenna impedances are not always 50 Ohms. Impedance Matching Techniques in Antennas: Defining Impedance Matching Requirements Starting from a filtration perspective is the easiest impedance matching technique in antennas or other RF circuit elements. 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. Impedance matching techniques for antennas are intended to ensure maximum power transfers into the antenna so that the element can radiate strongly.Īntenna impedance matching involves matching the input impedance at the end of the antenna’s feedline to the feedline’s characteristic impedance.įilter circuits are normally used, as they can be configured to provide specific impedance right at the desired transmission frequency.Īlthough 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.
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