Lesson 7- Atmospheric Propogation

There is no subject that I have more difficulty understanding than Propagation in terms of Amateur Radio. I've been trying to explain it to a Ham-in-training, but I've had some difficulty, so I'm going to take the time to learn about it and explain it to you all.

First of all, I need to explain the different layers of the atmosphere. There exists a good image at Amateur Radio Wiki, which I have included below.

The first thing that you will notice is that there are two layers which essentially go away during night, the D and the F2 layer (Which really merges with the F1 layer). Each of these layers reacts very differently to the various types of radio signals, so I'm going to take them one at a time. In general, the higher layer a wave is reflected off of, the further it will go.

UHF and VHF signals (23cm-6m bands) typically just go right through the atmosphere. Occasionally, VHF signals will reflect off of the Sporadic E layer, especially in the spring/summertime, but that is the only major exception. They can also do something called Tropospheric Ducting, which basically means that the radio waves can get trapped in the troposphere and bent, in a similar manner to how fiber optics trap light. The way this happens is when there is a layer of air which is significantly different than the air above and below it. This is called temperature inversion. Other than one of these two phenomena, usually VHF and UHF are line of site only, meaning they can only talk until the curvature of the earth will block them. They will travel slightly further than direct line of site, essentially because the Earth doesn't seem to curve as much for a radio wave.

Now for the fun part, HF. First of all, let me say that I can only give you general trends. I should add that Amateur Radio operators discovered this, but it quickly came to be used by many others. Here are some of the key trends:

During the day:

The D region absorbs most of the lower frequency (40m-160m), but will sometimes reflect them, allowing for "local" communication. The smaller wavelengths get through the D layer, and tend to get reflected off of one of the highest layers. Ocassionally, there are large pockets called "Sporadic E" that will allow reflection of signals in the HF in particular, but occasionally including the 2m band as well.

During the night:

The D region goes away, and the larger wavelengths open up (40m-160m). The atmosphere tends to not reflect the largest waves (10-17m) as much, and thus much of the capability is lost to communicate on these bands.

Special Situations:

Occasionally there are special phenomena which allow for communication where you couldn't normally get it otherwise. The two in particular are the Northern and Southern lights, and meteor showers.

Meteor showers will create a hot pocket of gas that will reflect radio waves. These tend to work best on 6m, and work well on VHF in general. They allow for short communication bursts to happen where it wouldn't normally occur otherwise.

Auroras are a period of extremely ionic particles in the atmosphere. They will allow for more connectivity than normal, similar to Sporadic E previously mentioned.

Bottom line:

The larger the wavelength, the less likely that the signal will propagate far in the day, but the more likely it will at night. 17m and 20m are always a good choice. Sometimes, depending on the exact propagation, you can get signals all the way down to 2m.