How much does it hurt admissions if I don't take an essential class until after I apply to schools?
I cannot speak directly to antenna engineering, but more generally, having research experience, strong letters of recommendation, a high GPA and a good SOP will almost certainly strengthen your application much more than having simply taken a specific (yet relevant) course. This is true for both masters and PhD programs.

Take a look at this write up of the admissions process decision making for the Computer Science PhD program at Carnegie Mellon University (a very prestigious and difficult to get into institution for Computer Science). Notice that the author doesn't say something like, "The student has taken operating systems. That is an absolute must!" and it should make sense why -- because it's a poor metric of your potential to succeed in a graduate program. If that was the metric they went by then a huge proportion of computer science undergraduates who applied would be accepted. Of course, this is not the case.

On the other hand, having done research and been published is a strong signal of your capacity to succeed in a program. It shows that you have talents and creativity beyond your courses, and that your professors respect you enough to work with you. In addition, having strong letters of recommendation matters significantly more than having taken a single course. If a colleague of theirs speaks highly of your skills and potential, they will likely weight that heavily, given that they know what it takes for a student to succeed in a graduate program as well.

Now, that does not imply that you shouldn't take relevant courses to your discipline (and that failing to take important courses may have an impact on your application). However, when considering the whole spectrum of the admissions process, I believe that not taking a single course will play a much smaller role in comparison to the other factors of your application.