Question

Once the existence of this wave-length had been discovered, it was not long before its use as

the uniquely recognizable broadcasting frequency for interstellar communication was suggested.
请问，这句话我能懂是什么意思，但最后一个 was suggested是什么？从句吗？ 不是已经出现了连接词和2个谓语动词、最后这个 was suggested是什么用法？

Answer 1

这个句子没有错哦
its use（主语） as the uniquely recognizable broadcasting frequency for interstellar communication（修饰use，作为...用）was suggested(谓语)

理解了整句的意思就好理解了：
Once the existence of this wave-length had been discovered,
（一旦这种波长被发现存在）it was not long before its use as the uniquely recognizable broadcasting frequency for interstellar communication was suggested.不久后就被提出它作为独特公认的通信广播频率的用途。

我想你之所以不理解，应该是把its use看成了it's used