Probability statements and prediction

Last Updated on December 3, 2022 by John Berry

Sporadic E propagation is termed ‘sporadic’ for a reason! The very word means erratic, patchy or random. So, I’d argue that anyone claiming that they can predict when this erratic propagation mode will occur is misguided.

At this point, I’d recommend that you digress and read my page on Chance in radio communications. This page is about how to think about probability statements and prediction.

Is Es predictable?

If you’ve read the previous discussion on Es, you’ll see that I’ve built a picture that suggests that Es would be more E if it wasn’t for erratic disturbances. These erratic disturbances have two sources. The first is changes in the Earth’s magnetic field. The second is tropospheric weather-induced atmospheric gravity waves.

So, the production of densified nodes caused by meteors entering the atmosphere is predictable, but that’s disturbed by erratic phenomena. The result is a sporadic, unpredictable propagation mode.

So, simply, Es is not predictable.

Probability statements

To continue to understand Es, I suggest that a different approach is needed: empirical analysis and statements of probability.

Possibly one of the best illustrations of this is by Mike Whitfield, M0MJW. Mike has developed a graph based on the field strength (in µV/m) received over a given path length for 20% of summer days. He suggests that a field strength of greater than 10µV/m will be received over path lengths of between about 800km and 2,500km (marked a. on the graph).

So, Mike is not trying to predict – he’s observing and stating a result. He’s suggesting that there will be usable Es between say UK and the Mediterranean on 20% of days. This is on 50MHz, and he also develops a probability statement for 144MHz. He suggests that a field strength of greater than 1µV/m will be received over paths of length around 2,000km for 20% of days (marked c. on the graph).

Understanding the science

Mike’s graph is shown here.

Path length to field strength received for summer Es.
Path length to field strength received for summer Es.

I’d suggest that these are quite high field strengths suggesting modest receiving setups – a dipole antenna for example. The point is not to be absolute here. It’s that one needs to discuss Es performance in terms of probability: with signal levels exceeded for percentages of time over given path lengths.

Extending to superior set-ups

Given that the graph suggests a dipole, there are two possibilities when using a superior set-up such as good feeder or low noise amplifier and high-gain Yagi. Firstly, the percentage of days will increase for the given path length range. Secondly, and alternatively, the path length range will increase for the stated percentage of days.

So, for example, for those with superior set-ups, path lengths of around 4,000 and 5,000km will be available for 20% of summer days (marked b. on the graph).

The same would apply when using a data mode like FT8 where the system value is some 23dB better than phone.

Probability statements and prediction

My discussion here illustrates how radio amateurs must talk about Es – in terms of probability. Exciting propagation will occur, just not predictably. But chance and path lengths are increased for those with superior set-ups.

As the adage has it, chance favours the prepared… radio ham.

As I illustrated, meteors give Es a predictable quality, only to have that blitzed by random stuff like solar explosions and surface weather.