How can I explain the Vivaldi antenna operation?

I have experimented with these antennas, though I'm the last person to give a good theoretical explanation.

These are typically considered non-resonant traveling wave antennas. Although there is often some resonant region in frequency, the performance is not solely dependent on this (it extends considerably).


The return currents from the stub normally flow directly under the line in a ground layer when over a solid ground plane, but with the slot and the "cavity" in the way , as you describe, the return currents must flow around the loop and outwards toward the wider opening (in the time domain, the electrons don't know they're walking the plank), which results in radiation.

Until someone smarter explains it, I carry this mental model with me.