Why H.F. Propagation is Weird

We've gotten fairly good at predicting radio propagation, at least when compared to predicting weather. This is quite surprising, since Man has been contending with weather for his entire existence, while he's only been contending with the ionosphere for the last century or so.

Most of us have a moderate understanding of the connection between solar activity and radio propagation. “Sunspots good, no sunspots bad” has served us pretty well as a rule of thumb. The critical frequency and electron density profiles are, for the most part, as regular as clockwork, quite literally, as indicated by the Digisonde Scaled Data from various ionosondes. (Example here: http://maestro.haarp.alaska.edu/cgi-bin/digisonde/scaled.cgi).

DXers watch the solar indices like vultures scanning the desert for roadkill, because, for the most part it WORKS.

The fact that so much of ionospheric propagation IS so predictable makes the weird propagation we sometimes see all the more mystifying. What are we to make of those times when all the numbers are right, but radio just doesn't seem to work? The following should give us some insights.

There are three major assumptions most of us make when it comes to the ionosphere.

None of these conditions can be true as long as the Earth has a magnetic field. But for the sake of argument, let's imagine an Earth with no magnetic field. What would radio propagation look like?

Here is a short list.

Now, because we DO have a terrestrial magnetic field, a lot happens to disturb this rosy picture. Let's look at the effect of some of these false assumptions.

Most explanations of ionospheric propagation show the ionosphere as being a flat layer or layers, absolutely uniform over large geographical areas. In reality, the magnetic fields of the Earth cause the ionosphere to pucker, fold, lump and warp. Radio propagation can have EXTREMELY localized conditions. You only have to see the Northern Lights (Aurora Borealis) once to realize just how wrinkled the ionosphere is. The “curtaining” effect isn't just an optical phenomena…it can be tracked precisely with H.F. radar. And you don't have to live in Fairbanks, Alaska to see a wrinkled ionosphere, although it's MORE wrinkled here than anywhere else. The only place the ionosphere is nearly flat is along the magnetic equator.

Let's look at reciprocity. A uniform non-magnetized, horizontal ionosphere should present us with totally reciprocal propagation. For decades, us Alaskans who complain about “one way” propagation have been reviled by Lower 48ers who insist it's a myth, that there is no physical mechanism for one way propagation.

This is TRUE, assuming the ionosphere is HORIZONTAL. In highly magnetized regions, such as Alaska, the ionosphere is NOT horizontal, however. It can have tilts exceeding 60 degrees, over large regions and even MORE over smaller regions. It's EXTREMELY easy to explain why one-way propagation would be quite normal for a tilted ionosphere….the angle of attack, hence the critical frequency, would be much higher going one direction than the other. Many signals arriving into Fairbanks from Europe arrive essentially from DIRECTLY OVERHEAD! Any long time DXer here will tell you about times where he can spin his high gain 20 meter yagi clear around the compass with NO change in signal strength from over-the-pole propagation!

Finally, we not only have a warped and wrinkled ionosphere, but we have one that doesn't stay put. With a Doppler Ionosonde, you can measure how fast the ionosphere is advancing or retreating. Near sundown, you can often observe the ionosphere receding at over 700 meters per second! But even more astonishing, it doesn't all recede at the same time. From the same ionosonde, you can see parts of the ionosphere ADVANCING at the same rate. Grab a hold of a throw rug and flap it up and down vigorously, observing the waves that form along the fabric. This is a more realistic model of the ionosphere than your normal static model, especially near nightfall.

The bottom line is, the ionosphere is a moving target, and, believe me, it can move FAST!

Although the Sun is the prime mover, it's not the only thing happening, when it comes to propagation. One thing that's astonishing is how LITTLE material is doing the job for us, when it comes to the ionosphere. The ionosphere is comprised of almost NOTHING, when you compare it to the atmosphere at the surface of the Earth. The fact that a relative handful of electrons way out there can have any effect on radio waves, much less a total reflector at times, is quite remarkable.

We shouldn't be surprised when the ionosphere acts a little weird at times. The fact that it works at ALL is nothing short of miraculous.

73 de KL7AJ