Range of frequency of electromagnetic waves

[GTrax]

The details of the electric current is a bit complicated. A simple explanation is that electrons are always whizzing about in all directions and current flow is the net flow of electrons in a direction. The frequency of this net flow is the rate of the oscillation in it. The electrons themselves aren't vibrating back and forth at this frequency.
Well, I'd say that in the context of current flow, the signal is the measurement of the voltage or current flow at any particular moment in time, regardless of its properties. The behavior of the signal can be described as wave-like when it behaves a certain way, namely that there is a repeating pattern that a wave equation can be applied to.
That's right. Mathematically, any pulse can be broken down into the waves composing it by using a Fourier Transform.

 

[GTrax]

I believe Drakkith has it right!
All electromagnetic wave propagation I have encountered, (in RF and microwave industry), require that there is at least some rate of change of the strength of either a magnetic field, or an electric field, usually both together, although only one is sufficient, because the other will eventually appear naturally within a couple of wavelengths, such as in screened loop antennas.
-
The fields are locked together, in that changing the value of one is only possible by moving some of the other. We can get up to various contrivances, be they antennas or just electrical transformers, and we see that the only way they function at all is by exchanging energy between magnetic and electric fields in an oscillatory way.
Changes of a pulse nature contains the various sine and cosine components (Ref. Fourier), and they will transmit exactly as expected.
We cannot have "zero" or "negative" frequency, even if the purely mathematical Fourier spectrum representation is allowed to range from -infinity to +infinity.
-
I do like the mechanical analogy - and it brings to mind another. My difficulty in imagining electrons making it from the power station near to the other end of the country at some large(ish) fraction the speed of light was explained by high school teacher who likened it to a line of snooker or pool balls representing electrons. You strike one on the end, and the successive collisions propagate wave-like until the ball at the far end moves. He said the actual progress of any single electron might be quite slow, or not at all, but the net effect of the energy could be felt at the far end very quickly.
I know the story has it's limitations, but I settled for it at the time.

The lower practical limit might be when the wavelength becomes extreme. Despite the inefficiency of propagation through salt water, there have been Very Low Frequency (VLF) schemes used for communication with submarines, using wavelengths expressed in kilometres. Always, it can never be infinite. Frequency, if zero, no longer has meaning, and definitely, no photons can happen because of the lack of d(Phi)/dt.