( ( ( Radio waves ) ( at ( these frequencies ) ) ) ( ( have ( been ( ( found useful ) ( for ( long-range communications ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NNP Radio) (NNS waves)) (PP (IN at) (NP (DT these) (NNS frequencies)))) (VP (VBP have) (VP (VBN been) (VP (VBN found) (ADJP (JJ useful)) (PP (IN for) (NP (JJ long-range) (NNS communications)))))) (. .)))
Radio waves at these frequencies have been found useful for long-range communications.
(ROOT (S (NP (NNP Am) (NN radio) (NNS waves)) (VP (VBP exist) (PRT (RP in)) (NP (DT the) (ADJP (QP (CD 540) (TO to) (CD 1600))) (NN khz) (NN frequency) (NN range))) (. .)))