Thanks for that. I was foolishly thinking that the capacitance across the narrow gap would prevent entry, but if you think about it, the voltage is along the gap, not across it, so the capacitance is neither here nor there. What I have taken to heart is that RF is like plumbing (and often looks like it); if you don't take the utmost care, you get leaks, and it's a lot easier to prevent leaks than to fix them after they've occurred.