Both Charlie Newton G2FKZ (in discussing radio auroras) and Jim Bacon G3YLA (in discussing sporadic E) talk of patches. It’s a good analogy – trouser patches in the E Region of the ionosphere about 110km up that reflect radio waves. If those patches occur at a useful location and at a useful time, useful E Region propagation will result by one of several possible mechanisms.
Whether the patches are useful depends on several factors, but the path geometry and frequency of the transmission are all-important.
The following shows a view of E Region geometry roughly to scale.
The nearer the launch angle (of the radio wave) comes to the horizon, the greater the distance possible between the two stations (assuming the system value allows). But the actual launch angle is seldom anywhere near zero degrees. Ground effects cause the antenna vertical response to be tilted upwards.
The result is that a practical horizontally polarised amateur station antenna some two to four wavelengths from the ground typically achieves a launch angle of greater than 10 degrees.
Simple geometry dictates therefore that for a single hop Es (via mid-latitude E Region patch) in a normal atmosphere, the maximum distance between the two practical stations is about 1,200km – or from central UK to central Spain.
You can test this by approximate calculation using trigonometry:
The horizontal distance to the patch centre = (110km/tan 10) = 623km.
The full path distance is therefore around 1,200km. It’s approximate since the figure comprises one straight line (the height to the patch) and two arcs (the flight of the radio wave refracted through the atmosphere and the curvature of the Earth between the two stations). And spherical trig is just too complicated to worry about getting an accurate answer! In any case, this simple calculation assumes that the height of the patch is indeed 110km, and that height is not fixed.
The geometry shows that higher launch angles achieve lower maximum distances. A practical angle of 20 degrees will only achieve 300km to the path centre and about 600km overall. That limits the communications for such a UK station to northern France and the Netherlands.
The maximum distance will in all cases also be slightly longer since atmospheric refraction (bending the wave towards the Earth) increases the distance to the patch. The degree to which this happens depends on complex mechanisms including atmospheric pressure (which reduces with height above the Earth).
So, the patch will be useful if it occurs over a mid-point location occurring between countries where there are active, suitably equipped radio amateurs.
If the patch occurs somewhere over the Atlantic, it’s less likely to be useful to an amateur in the UK since the Great Circle distance to America, the nearest country, is around 4,000km. That would demand beneficial atmospheric refraction (extending the distance to the reflection point) and two Es hops – both possible but rare.
The patch will also be useful if the incident radio wave is at the right frequency.
Very broadly: too high a frequency and the wave goes straight through the patch and is lost to space; too low a frequency (too high a wavelength) and the patch won’t support the reflection.
The patches will also be useful if the angle of incidence between the radio wave and the patch is right. Too steep an angle and the wave goes straight through and is lost to space.
We know these effects from experiments. Most E Region patches occurring sporadically support transmission from time to time at 50MHz. Some, of different density, support 144MHz. And some support 28MHz. Es occurs mainly at 50MHz but is also observed on the 28MHz, 70MHz and 144MHz amateur bands.
Northern hemisphere mid-latitude patches are interesting to European amateurs. These Es patches can occur on late spring and early summer days, in the morning and afternoon, and sometimes in January. They are set up by a complex mechanism but typically occur at a point orthogonal to the Sun’s rays. That means that from a European perspective, patches supporting Es move with the Earth’s rotation from being, for example, over Romania earlier, to over France later.