Earlier references to spread spectrum than Starship Troopers?

So, I’m re-reading Heinlein’s Starship Troopers, for the first time in many years, and I noticed that he describes something very much like spread spectrum radio:

The frequency control to maintain tactical security is very complex, at least two frequencies for each circuit, both of which are necessary for any signal at all and each of which wobbles under the control of a cesium clock timed to a micromicrosecond with the other end—but all this is no problem of yours.

Now Starship Troopers was first printed in 1959, decades after Hedy Lamarr first patented the idea of spread spectrum, but I don’t recall any earlier mentioned in fiction.

Anyone know of any?