The Restricted Radiotelephone Operator's Licence requires a written and/or practical (or oral) test in many countries in the world. Radio basics theory, English language communications, communications rules, etc...
So those of you required to get one (as our friend Jhooper mentions, to operate an aircraft radio while outside the USA), consider yourself lucky that you do not have any test to obtain it. Just send an application by mail, and a check for the FCC... You may be required, while overseas, to show your pilot license AND
your radio license...
While VHF radios are quite easy to use, I would recommend, should you be required to use a HF
radio, to learn basic radio basic theory, propagation etc. Some radio amateur stores sell little booklets for that subject. HF
radio in aircraft are equivalent to Ham "Shortwave" equipment, operating between 2,000 and 30,000 kHz. Collins Radio, in Cedar City, Iowa, can probably send you a small operator booklet for the model "618T" which is common airline HF
radio equipment. Maybe even for free.