HF Propagation

Radio Signals don’t Travel in Straight Lines by Onno VK6FLAB

Reposted by Layne, AE1N

The other day a friend of mine asked a really silly question. How come when I point my YAGI at a direction for a station using the great circle, the signal is there but weak, but when I point it in a different direction, say 20 degrees away from the great circle, the signal improves?   Being a good little Amateur, I responded with the logical explanation. Well, two things come to mind, one being that you’re not pointing where you think you’re pointing, that is, North on your antenna isn’t North in reality, so when you point at the other station, it’s not actually where you’re pointing, and when you adjust, the antenna ends up in the correct direction.

Another explanation I came up with is that the pattern of their YAGI isn’t what they expect. There might be local factors that influence the pattern, putting weird distortions into their foot-print and making for “interesting” nulls where there should be a signal, and vice-versa. That, in turn, started a whole conversation about directions and where stations are.

Leaving aside the difference between long-path and short-path, which I should probably talk about at some point, an antenna should get a signal from the direction in which you point it, right?   So, what if I told you that the antenna was, in fact, pointing correctly and there were no distortions in the antenna pattern, what then?

Turns out that the Ionosphere isn’t uniform – who’d have predicted that – in case you’re wondering, that’s a joke – the Ionosphere isn’t uniform, it takes in many and varied influences, from the earth’s magnetic field to heating by the sun, to solar storms, coronal mass ejections, and any number of factors that we as a species are only just beginning to discover.   If you imagine for a moment a radio-wave coming up from your antenna, bouncing against the Ionosphere, back to earth, then bouncing back up, then doing the same thing again, you’ll quickly understand that because the Ionosphere is variable, the height and angles at which this bouncing is occurring varies along the path.

But here’s a shocker, who said that the signal had to bounce up and down vertically, what if the same variability of the Ionosphere height caused a signal to bounce in some other weird direction, like at an angle, or sideways. Would the path of the signal from your station to the other end follow a great circle line?   Turns out that this silly question wasn’t silly at all and I learned something unexpected, my radio signal isn’t a straight line, something which I confess, did come as a surprise, but now, looking back, seems pretty obvious.   I love silly questions, they often turn into an opportunity to learn.

Onno VK6FLAB

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