Basics of Shortwave Radio Propagation

[KW2P]

 

 

The basics of long-distance / worldwide shortwave radio propagation by reflecting radio waves off the ionosphere can appear complicated. The basics are not complicated but they're not obvious either. There are a few things to know.

The layers of the ionosphere that matter to radio are the D, E, and F-layers.

THE E-LAYER

I'll address the E-layer first and get it out of the way in one paragraph. The E-layer has been known to exist for 100 years but has long been mysterious and not understood. This type of propagation is called "sporadic E" or Es. It comes and goes mysteriously. However, in the past ten years we think we finally understand it. The E-layer is not an ionized layer. The reflectivity of the E-layer comes from clouds of metallic microparticles from meteors. Computer models show that the clouds are formed and moved about by wind shear, which explains the seasonal behaviors of the E-layer and the fact that it's not affected much by UV radiation. Es is also correlated with seasonal meteor showers and the composition of the meteors. It shouldn't be long before we can predict it.

THE D-LAYER AND F-LAYER

The ionized layers of real interest are the D-layer (about 60 miles up) and the F-layer (about 200 miles up). The F-layer reflects radio waves so the stronger its ionization gets the better it reflects. Radio enthusiasts want the F-layer ionized as much as possible. The ionization results from sustained UV and X-ray radiation from the sun. The more the better. The solar number of interest here is the SFI (Solar Flux Index). This ionization is long-lasting. It takes days of sustained radiation to build up the F-layer and the ionization lingers on the night side where the sun isn't shining.

Now comes the D-layer. The D-layer is the spoiler. The D-layer is created by UV and X-ray radiation but instead of reflecting radio waves it absorbs them. Unlike the F-layer, the D-layer responds to radiation immediately. When the sun rises, the D-layer forms immediately. By nightfall, it's gone.

Radio wave absorption by the D-layer is strongest at low radio frequencies (long waves), becoming less and less at higher frequencies (shorter waves). For example, the 80 meter and 40 meter shortwave bands work well for worldwide communication only at night. Not in the daytime. Under typical conditions, the 20 meter and shorter radio bands are not absorbed and are usable worldwide 24 hours a day. (Assuming the F-layer is sufficiently ionized.)  The 20 meter band is the "sweet spot". The D-layer is transparent to it and it's usable even when the F-layer is weakly ionized. For this reason, the 20 meter band is considered the "queen" of all the shortwave bands and it belongs to ham radio operators because we discovered it long ago. The 20 meter band can usually be used for long-range communication, day or night, solar max or solar min.

TRY IT YOURSELF

If you're an AM broadcast radio listener (550 kHz to 1600 kHz, AM) you've likely noticed the D-layer in action. If not, you can check this out yourself. Any AM radio will do. It's best to be outdoors. This band, the 300 meter band or MW (medium wave) is strongly affected by the D-layer. If you tune across the band in the daytime you'll only hear local stations out to about 50 miles. Depending on conditions, maybe 100 miles.

Then, after dark, tune carefully across the band and you'll hear stations from 750 to 1,500 miles away, or farther. During the day, the transmitted signal is totally absorbed by the D-layer and never even reaches the F-layer. At night, the D-layer disappears. The signals can travel up to the F-layer, reflect, and be received 1,500 miles away.

(Note that this activity is difficult in radio markets like Los Angeles where every frequency channel is occupied by a local station. If you have that problem, try it the next time you're on a long drive at night. When driving the empty wasteland of Wyoming at night I listen to radio stations in Seattle, Salt Lake City, San Francisco, and Los Angeles.)

SOLAR MAX

Why the excitement over solar max? Shorter wave bands like 15, 12, and 10 meters require strong ionization of the F-layer. This is what we've been waiting 11 years for. Those bands are capable of breathtaking performance but fully open up worldwide only around solar max.

MORE IS BETTER, EXCEPT SOMETIMES

So far, the above is pretty straightforward. More sustained UV radiation is better. But, there is a case where the radiation is too much: solar flares. A solar flare is usually a very bright source of UV radiation. Yes, this has a positive effect on the F-layer, but remember that the F-layer responds slowly. It takes hours and days of sustained radiation to build up F-layer ionization. A flash of radiation doesn't do much to the F-layer. The D-layer, however, responds immediately to the bright radiation of a solar flare (only on the day-side of the planet, of course). When there's flaring, ionization of the D-layer strengthens rapidly to above-normal levels and the D-layer begins to absorb higher and higher frequency radio waves.

Normal daytime ionization of the D-layer absorbs the 40 meter and 80 meter bands, and all longer wavelengths like the AM broadcast band. 20 meter and shorter radio waves normally pass through the daytime D-layer. But when a solar flare hits, the 20 meter band (14 MHz) will go dead. Stronger ionization and the 15 meter band (21 MHz) also goes dead like it did today. This is a radio blackout. Strong blackouts can extend to 35 MHz, taking out all the HF (shortwave) bands.

SUMMARY

To summarize, radio aficionados want higher sustained UV radiation to get the F-layer as ionized as possible. But we don't want the extreme radiation of a flare because it over-ionizes the D-layer resulting in a radio blackout.

Solar max means more of everything -- higher sustained radiation that benefits the F-layer (good) and also more flares that over-ionize the D-layer (bad). Solar max also means more CMEs and HSS which disrupt radio, but I'll not get into that here. Solar max is a package deal. You have to take the good with the bad.

This covers the basics. I'm skipping the geomagnetic stuff because it complicates the basics.

[hamateur 1953]

May I just add a well done here?  you betcha I will @KW2P 73, Mike/Hagrid. 

[Philalethes]

5 hours ago, KW2P said:

The E-layer has been known to exist for 100 years but has long been mysterious and not understood. This type of propagation is called "sporadic E" or Es. It comes and goes mysteriously.

«Do you know how to go to the Heaviside layer?»

Heh, ever since we performed that musical in elementary school, that line has stuck with me.

Anyway, this is an extremely helpful summary, and answers several of the questions I asked in the other thread; great writeup.