怎么样探寻句子谓语成分

But the radio waves that we use for these purposes microwaves travel in straight lines.

这句话 that 后边是定语修饰 radio wave， 然后 microwaves travel in straight lines 是什么成分，与这句话的谓语是哪呀？

But the radio waves that we use for these purposes microwaves travel in straight lines.

这位无名网友的解答是正确的。

microwaves是主语the radio waves that we use for these purposes的同位语。