Matching network
An antenna tuner is a matching network. Enter the antenna’s impedance to design the two-part L networks that match it, follow each part’s move on a Smith chart, and see what a tuner at the radio can and cannot fix.
Positive for inductive, negative for capacitive.
Networks that match
The network sees the antenna directly: 200 Ω. Two networks match it to 50 Ω at 14.2 MHz.
Open the chosen network in the circuit bench to sweep it across the band with the antenna.
Try this
Pick an example to set the controls, then watch what changes.
The path on a Smith chart
A series part changes reactance and leaves resistance alone, so it slides the point along a circle of constant resistance (solid grid). A shunt part changes susceptance and leaves conductance alone, so it slides along a circle of constant conductance (dashed grid). Two moves are enough to reach the center.
Starting at the load, the shunt capacitor (97.1 pF) slides the point along a dashed circle of constant conductance until it reaches the solid circle through the center, where the resistance equals the line impedance. The series inductor (971 nH) then slides it along that circle to the center: a perfect match.
- Part next to the load
- Part next to the radio
Away from the design frequency
The parts are exact at one frequency. Tune away and the inductor and capacitor reactances change in opposite directions, so the match drifts and the SWR rises.
Apply it
- An antenna tuner is a matching network. Its job is to match the antenna system to the transmitter’s output impedance, which increases the power transferred from the transmitter to the feed line.
- A tuner at the radio does not change the SWR on the feed line. If the antenna gives 5:1, the coax still runs at 5:1 while the radio sees 1:1.
- High SWR increases loss in a lossy feed line. On a long or lossy run, matching at the antenna, or fixing the antenna, puts more of your power on the air than a tuner in the shack. The SWR tool works out how much.
- Low-pass versions pass the operating frequency and attenuate frequencies above it; high-pass versions do the reverse.
- Transformers and sections of transmission line can also match impedances.
How it works
- An L network has one series part and one shunt part, and there are up to four ways to arrange them. When the load and source are both resistive, the network’s Q is fixed at √(Rhigh / Rlow − 1), so it sets the bandwidth.
- A series part moves the impedance along a circle of constant resistance on the Smith chart; a shunt part moves it along a circle of constant conductance.
- Part values are exact at the frequency entered: inductive reactance is 2πfL and capacitive reactance is 1 / (2πfC).
- Through coax, the network sees the antenna impedance transformed by the line. A matching network at the transmitter end presents 1:1 to the transmitter but leaves the SWR on the feed line unchanged.
- An antenna tuner matches the antenna system to the transceiver’s output impedance, which increases power transfer from the transmitter to the feed line. Transformers, pi networks and lengths of transmission line can also match impedances.
Learn more: Antennas and feed lines
Sources: Microwave and RF Design III: Networks, 6.4 The L Matching Network (NC State University (M. Steer)); Microwave and RF Design III: Networks, 6.7 Impedance Matching Using Smith Charts (NC State University (M. Steer)); Microwave and RF Design III: Networks, 3.4 Smith Chart (NC State University (M. Steer)); Electromagnetics, Vol. 1, 3.15 Input Impedance of a Terminated Lossless Transmission Line (Virginia Tech (S. W. Ellingson)); Electromagnetics, Vol. 1, 3.8 Wave Propagation on a TEM Transmission Line (Virginia Tech (S. W. Ellingson)); Electromagnetics, Vol. 1, 3.10 Coaxial Line (Virginia Tech (S. W. Ellingson)); 8240 (RG-58) technical data (Belden); 9258 (RG-8X) technical data (Belden); 8267 (RG-213) technical data (Belden); 8214 (RG-8, foam) technical data (Belden); 9913 (RG-8 type, semi-solid) technical data (Belden); 7810A (RF-400) technical data (Belden); Radio Mathematics (ARRL Handbook supplement) (ARRL); 2026-2030 Technician question pool (NCVEC Question Pool Committee); 2023-2027 General question pool (NCVEC Question Pool Committee); 2024-2028 Extra question pool (NCVEC Question Pool Committee)
Practice the exam questions: T9 Antennas and Feed Lines (Technician), G4 Amateur Radio Practices (General), G5 Electrical Principles (General), G9 Antennas and Feed Lines (General), E9 Antennas and Transmission Lines (Amateur Extra)
Found something wrong or out of date? Report a problem with this tool on GitHub (a public issue; nothing is sent until you submit it there).