How much does it hurt admissions if I don't take an essential class until after I apply to schools? I've asked a related question about dealing with two courses that I need that are scheduled at the same time. I'm not sure if I'll be able to get it resolved yet, and so it's been on my mind. 

One of the courses I need to take is very important for my desired field of graduate study (as in, it's important for the specific field of research), and it's possible that I won't be able to take it until spring of my last year, which means it wouldn't be on my transcript when I apply for graduate schools. Will this hurt my application if I say I want to do work in this field while not having taken the necessary course yet? For reference, I've got plenty of research experience, I should have great letters of recommendation, and potentially a 4.0 GPA, and I will have taken many upper-level courses in my major, so while my application may be strong in those areas, I'm wondering how detrimental not taking this course early on will be (particularly when applying to top 15 schools). Of course, I will have taken the course by the time I graduate, but perhaps that wouldn't mean much to admissions committees (who wouldn't know my grade in the course).

For further reference, my interest is in antenna engineering, and the course I'm referring to is a standard engineering electromagnetics course. Also, if it makes a difference, I plan on applying to masters programs, and so if there are differences between what is looked for in masters vs PhD admissions, please keep that in mind.
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.