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A 50 ohm match can be tricky outside a transmitter to match. One has to deal with ensuring the antenna , coax, and all connectors are all equal or close to the output impedance of the radio.

Let alone the external connections, is impedance matching as tricky when dealing with PCB layout, and internal components?

Does impedance only matter on the final amplifier stage because low power reflections won't make much of a difference in efficiency?

I have found circuits to build simple radio transmitters. If only a schematic is given, and no board layout, or construction notes are provided, how can I ensure the impedances are matched throughout the internal circuitry of the radio, so all the power goes to the external connector?

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If you are building a device from a schematic, then the issue of impedance matching between various parts of the circuit has already been addressed by the designer. Interconnections between circuit components are generally extremely short in terms of the wavelength of the RF energy that they are carrying, especially at HF. Because of this, as long as interconnections in the circuit are kept as short as reasonably possible, there is no need to impedance match the connections between circuit elements.

This is not true when working at very high frequencies like VHF, UHF, and microwave frequencies. In devices designed for those frequencies, printed circuit boards are often designed using special techniques such as striplines or waveguides that do transmit RF energy at a specific impedance.

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