I attended a panel presented by Kelly Armstrong a few years ago, where she commented on how cellphones were the bane of many urban fantasy writers. They create a lot of "well why don't they just _______" plot holes, requiring the author to get creative in finding ways of preventing the cellphone from working.
I felt cheated. I thought, if the technology of the era you're working in is a problem, why are you writing in that time period?
One of the awesome things about dieselpunk and the other 'punks, is that they're all about the technology (or should be!) The ability to communicate over long distances can be a great plot device that really sets the world apart from medieval fantasy. In contrast with most urban fantasy, you can have one character in one situation, who is in contact with another character in a completely different situation. They can communicate in real time, but not physically help one another. For example, you could have a character in a plane that's on fire and careening into the ground, speaking real time with characters who's heart's are breaking because there's nothing they can do to help.
And on the other hand, the limitations of radio keep it differentiated from urban fantasy. Radio is only good for a certain number of miles, and not so good over rough terrain. Better if you're broadcasting from high up, like in an aeroplane - you can reach much farther then. Less, if you're in the middle of a storm with a lot of electrical activity. Not only that, but there's limitations in the way it works. Only one person can speak at a time on any particular frequency. If one person is speaking, and another person tries to transmit over them, you get static and you're lucky if you can hear either of them.
Incidentally, that's why they say "over" in the movies when they're talking on the radio, and while the rule of saying "over to indicate you're done talking and someone else can talk is still on the books, we pilots don't actually ever say it. Well, there's one guy who does, and we all giggle at him in our respective cockpits. You can generally hear when someone lets go of the transmit button because the background static that's there while they're talking stops.
I wondered about that, years ago. How do people know what frequency to talk on, to talk to a particular person? Do they actually have to set the radio to a specific frequency, and know beforehand what frequency the other person is going to be on in order to contact them? That seemed entirely too inconvenient.
Now of course, I'm a pilot, and part of being a pilot is having a radio license and using a radio regularly. And the above is exactly how it works. There are specific frequencies that are designated for particular purposes. For example, in most countries, 121.5 is the emergency frequency. If you're calling mayday, and you're not already in contact with anyone, you'd call mayday on 121.5. Most airports have their own frequency. The busy ones might have more than one. My home city has five or six, if you include the frequency that's nothing but a continuous loop played over and over of the most current weather report.
If two frequencies are close together, you might hear broadcasts faintly from other frequencies. Or a very strong signal might bleed over all frequencies, like if you're emergency beacon goes off in your plane, it doesn't matter what frequency you're listening to, you'll hear it.
And then there's the cultural things. The frequencies where music would be played, to entertain the masses, or the news. You have radio plays, and with radio, came commercials. And everybody's favourite: propaganda.
Lots of things to bring a dieselpunk world to life, with just one piece of technology.