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I'm interested in building an antenna tuner for amateur radio HF frequencies.

On the transceiver side of the tuner circuit, the impedance of the PCB traces can be matched to the 50 Ω of the transceiver.

On the antenna side (after an L-matching network with a variable capacitor and/or inductor) the impedance is unknown since that's what the matching network is supposed to do.

What impedance should be used for the PCB traces on this side of the matching network? Should there just be no ground plane underneath these traces?

Also, if there are books about this type of design I'd be very interested. I'm an electrical engineer, but I've never done any type of RF design.

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  • \$\begingroup\$ At what frequency? What trace length? What impedance range? \$\endgroup\$ Nov 18, 2022 at 20:02
  • \$\begingroup\$ At amateur radio HF frequencies which is from 1.8 MHz for the 160m band to 29.7 MHz for the 10m band. I don't have a trace length yet since I haven't designed the PCB yet. As for the impedance matching range maybe 6 to 1600 ohms on the antenna side. \$\endgroup\$
    – James
    Nov 18, 2022 at 20:18
  • \$\begingroup\$ What's the maximum trace length you could reasonably expect? 30cm? What impedance does 30cm of wire have over ground plane? Through air (chassis wiring)? \$\endgroup\$ Nov 18, 2022 at 20:20
  • \$\begingroup\$ The trace length would probably be longer than that since relays would probably be used to change the inductance. 100cm perhaps on the PCB. As for through the air probably a similar amount going to the antenna. I'm intending to use the tuner at the antenna location. Thanks for your responses BTW. \$\endgroup\$
    – James
    Nov 18, 2022 at 20:32

1 Answer 1

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It depends if you are going to use a feedline to connect the matching network to the antenna, or instead install the tuner right at the antenna element.

If you're using a feedline between the tuner and antenna, you should use a PCB trace (i.e. stripline) of that same characteristic impedance.

If you're mounting this tuner right at the antenna, it's more important to just be sure the trace can handle the current (e.g. 1 kW of power into 6 ohms) and not break down from high voltage (e.g. 1 kW of power into 1600 ohms)

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  • \$\begingroup\$ Yes my intention was to build an antenna tuner for use at the antenna. \$\endgroup\$
    – James
    Nov 18, 2022 at 20:50

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