"My understanding is that electromagnetic waves can have just about any frequency."

For the most part, yes. There is an upper limit when the wavelength of gamma rays become so short the photons start interacting with vacuum fluctuations, which if I recall correctly is approaching the plank length.

"They can be basically electric such as radio waves that can be detected with the apppriate type of radio receiver."

Radio waves are fully electromagnetic. EM waves are composed of both an electric and magnetic field. The two fields are orthogonal to each other and propagate longitudinally.

"Or they can be basically magnetic. And those are generally more difficult to detect."

See above.

As far as I am aware, only at DC (zero hertz) is it possible to separate the electric and magnetic fields. Examples are static electric fields and bar magnets. However, the moment either field begins to fluctuate that field induces the other, and you have an EM field.

Unless... Maybe you're talking about "scalar electromagnetics" and using devices along the lines of those developed by Larry Park?

Brian