HF vs high frequency

HF and high frequency both are nouns.

Word	Noun	Adjective	Verb	Adverb
HF	Yes	No	No	No
high frequency	Yes	No	No	No

As nouns, HF and high frequency are synonyms defined as:

HF (noun)	high frequency (noun)
3 to 30 megahertz	a pitch that is perceived as above other pitches
	3 to 30 megahertz