So the cheap devices he used only worked inches away. A more powerful device might work up to 20 meters away. Now, I assume a more powerful antennae is going to mean a bigger one. Isn't this going to stand out? I would hope that there is someone in charge that would notice a foot long antennae being pointed at voting areas.
You can secure the machine itself, but if you don't have real people doing their part, it doesn't matter how secure your voting machine is.