i always read RFC (radio frequency coil)
in those schematics, 80m-transmitters
(aka 3.5MHz). But what value do i have to
use? What's the design criteria?
Could anyone give advice, please.

TIA & 73, Oliver, DG2OG
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They are Radio Frequency CHOKE coils. (RFC)

They are required to have a fairly high impedance, like 10,000 or
20,000 ohms, at the lowest frequency of interest and over the widest
range of frequencies as possible.

For the 160m or 80m bands, 1 or 2 milli-Henrys will do.

For the 10m band about 100 uH will suffice.

But values are VERY non-critical,

They should be wound with several fairly close together, but not
touching, banks of turns. This is to minimise self-capacitance across
the coil and reduce the probability of unwanted resonance at much
higher frequencies.
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Reg, G4FGQ