First we have the dictionary definition of radio as a noun, the one that everyone knows. It can usually be summarized as "wireless telegraphy or telephony." But we're also familiar with the noun meaning the device which receives radio broadcasts. In a less common sense it can also refer to the message transmitted by radio. Grammarians will further recognize it as a noun a way to describe a message or signal. But it can also be a verb and here it has a dictionary definition just meaning to transmit a message.
But there is a whole different sense it is used in within evangelism. It is less common now than it was half a century ago but the concept has never made it to the dictionary. In this use, evangelicals are described as "transmitters" and "broadcasters" and the unconverted are "receivers." As radio's star fell in favor of Television, this usage fell out of favor.
These images below are from a children's work book from some kind of Christian vacation Summer camp using this less common, possibly now deprecated definition.