Handy radio switches to protect one early transmitter (5,4)

I believe the answer is:

tesla coil

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'handy radio' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for tesla coil that I've seen before include "A transformer" , "high voltage generator" .)

I've seen this clue in the Sydney Morning Herald.

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