There’s huge ambiguity across the radio amateur books, magazines, and sites when it comes to describing how it is that the ionosphere returns radio waves to Earth. This ambiguity is also prevalent in writing about other media such as meteor trails and the aurora.
The ambiguity is this. Many writers routinely talk of reflection. Others often call it scattering. And some refer to the mechanism as refraction. So, which is it?
First, we need some context.
An antenna has a broad response in both the vertical and horizontal plane. As a result, the transmission from an antenna is not a single pencil beam. The wave emits from all parts of the response. And it spreads and hence can be considered in a ray-tracing sense to be multiple pencil beams – within the main lobe and the various side lobes. This occurs in three dimensions. So, it’s a complicated picture.
And the signal received at the receiving antenna travels via many paths. Those conceptual pencil beams from the transmitter are acted on in the ionosphere. Many will arrive at the receiver. Some will be attenuated by the poor receiving antenna response at their angles of arrival. The resulting energy at the receiver is the vector sum of all discrete rays – of all energy received from all paths. Receiving is equally complicated.
The waves transmitted along all the various paths from transmitter to receiver are acted on by the ionosphere.
Let’s investigate each possible mechanism – reflection, refraction, and scattering.
Considering reflection, and using a single ray-tracing model, but accepting the complicated multiple ray model described above, the wave is launched from Station A towards the ionosphere (below).
The wave front aa-bb advances toward the ionosphere. In specular reflection, Snell’s law is obeyed, and the angle of arrival equals the angle of departure. The situation is described below. Snell’s law is obeyed for each point on the wave front and each point is turned such that the wave front departs as described by the reflected wave front cc-dd. One of the many rays is shown dashed.
This would require the ionosphere to be a flat reflecting surface like a lake. We do know that the ionosphere is a region of ionised gases. It’s acted on by the Sun and atmospheric winds and tides. It seems unlikely that its lower boundary (shown red in the diagram) could be a surface like a lake.
Reflection would therefore seem to be an over-simplification.
So, what about refraction?
We do know that the ionosphere varies in density. It generally reduces in density with height. Refraction is where the wave front is turned as it propagates the ionosphere. The lower part of the wave front is slowed relative to the upper part. As a result, the wave front enters the ionosphere as aa-bb and exits as cc-dd, turned and heading back to Earth, as described in the diagram below.
The refraction idea seems to fit the make-up of the ionosphere. As a theory, there’s no need for a flat surface, just a region of gas of reducing density.
That brings us to scattering.
Many hams use scattering to describe the way in which a wave is returned from patches of the ionosphere, from meteor trails and from the aurora. So, is that correct?
Scattering is the case where discrete rays are turned off at another angle. Backscatter would describe the case where the wave is turned back in the direction from which it came. Forward scatter describes the case where the wave propagates forward in the same general direction as it was travelling before experiencing scatter. This state is described in the figure below.
For scattering, the wave collides with some material object such as an ionospheric particle. The mechanism is like one billiard ball hitting another on a table.
As John Worsnop, G4BAO, commented in his GHZ Bands regular in RadCom (Volume 97, Number 1, October 2021), the empirical limit of scatter is where the size of particle doing the scattering is about one tenth of a wavelength across. Above that wavelength (below that in frequency) the scattering efficiency is just too low. John comments that for rain drops between 0.5µm and 3µm diameter, the optimum frequencies are the 5.7GHz and 10GHz amateur bands.
Simply, the tiny particles found in the ionosphere won’t scatter HF, VHF, UHF or SHF waves. To talk of scatter is wrong.
So, what mechanism describes the turning of waves incident upon the ionosphere and other ionised media?
There’s no surface associated with the ionosphere to support reflection. HF, VHF, UHF and SHF signals are of too large a wavelength to be scattered. That leaves us with refraction as the only viable mechanism.
The best description is therefore a plurality of refractions, arising across the patch of the ionosphere (or other ionised medium) reacting to the signal. The received signal is the vector sum of all arrivals at the receiver as a result of these refractions.