You have probably heard people talk about ionosphere virtual height. What is it and why is it important for HF propagation?
We all know that radio waves propagate by bending or refracting back to earth in the ionosphere. Refraction is a very complex process that covers a great distance and depends on free electron density and frequency. On the other hand, we mere humans are more familiar with the simpler concept of reflection, where waves just bounce back directly.
Virtual height is simply the altitude at which radio waves would reflect, if life worked that way. Fundamentally, it is based on trigonometry as shown above, right. The virtual reflection height makes it easier for a whole bunch of calculations about things like MUF and skip distance between transmitter and receiver. That’s the first reason ionosphere virtual height is important.
Your second reason is because ionosonde instruments provide virtual height of various layers quickly and easily. An ionosonde is a form of pulse radar swept across the sky from 1 to 30 MHz to measure reflections. We have been using ionosonde techniques to measure ionosphere for almost a century.
Above left, you can see these refractions coming back from vertical incidence signals refracted by the E, F1 and F2 layers. These data show the virtual heights of the three layer concentrations (h’E, h’F1 and h’F2) as well as the critical frequencies where refractions stop for each layer (foE, etc.) Note that there are different kinds of wave reflections (ordinary and extraordinary waves) caused by the earth’s magnetic field.
As you recall, the critical frequency is the point where the index of refraction μ approaches zero, and a signal returns to earth. Signals must be either below fc for vertical signals, or less than fc sec(θ) for oblique rays.
What Exactly is Ionosphere Virtual Height
Radio signals travel in space at the speed of light, c. But as they bend in the ionosphere, signals slow down according the refraction index according to μ⋅c. So when you measure the time of return TD for the radar pulse, you get the sum of the time the signal travelled at c in free space plus the bending time at less than the speed of light. When you divide this time by the speed of light, you get the virtual height.
Ionosphere virtual height is simply the height of an imaginary reflection off a perfect electrical conductor. It is important because it allows us to calculate lots of stuff, including how far a signal hops.