Symbolism, Imagery, Allegory
Julie doesn't fly alone in the back of Maddie's plane the night she's dropped in France—there are eleven wireless sets with her. Eleven decoy wireless sets, to be exact, designed to trick the Germans into thinking the plane is there to drop off wireless sets and operators. Right off the bat, then, the wireless sets represent deception—instead of serving as clear lines of communication, they are simply a cover story. Julie later uses them for the red herrings they are, convincing her captors that she's a wireless operator and has eleven sets of code to give them.
Julie also tells us that she herself is a wireless set, and that von Linden thinks of her as such. She writes: "the wee Scots wireless set, I mean operator, is still nursing small, hidden, nasty short circuits got during her savagely inhuman interrogation" (1.20.XI.43.42). And when she does, the comparison between herself and a machine highlights the inhuman treatment she's receiving, and also the fact that the Nazis have reduced her to the information they think they can get from her—information that, say, might be transmitted across a wireless. She says plainly:
I'm a wireless set. (1.20.XI.43.71)
And when she says this, we understand just how much information Julie does have and that, positioned as she is between the Allies and the Nazis, she is poised to communicate it. But she doesn't—so she's really more like one of the decoys she came into the country with.
She also admits that she was as much a wireless operator as von Linden is, working on people instead of machines as Eva Seiler:
It is ridiculous that you have not already guessed the nature of my intelligence work, Amadeus von Linden. Like you, I am a wireless operator.
Like you, I am bloody good at it.
Our methods differ. (1.22.XI.43.141-143)
Shifting gears from being the machine to being the machine's operator, Julie speaks to the fact that, as Eva Seiler, she and von Linden perform the same sort of work: getting people to share information. Their methods are quite different, but that they work people, convincing them to communicate things they aren't supposed to, is something these two have in common.