The LM338 is a linear regulator so it won't be a noise generator if it's
stable. Use proper RF bypassing to prevent oscillation, taking care to keep
the bypass cap leads leads short.
The same bypass caps will also help keep transmitter RF out of the
regulator. I've had a power supply go out of regulation when transmitting
because of RF pickup even at QRP power levels. Adding bypass caps on the
power supply's input and output leads cured the problem.
>Should I be concerned about any RFI if I were to use a LM338 in a dc-dc
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