Types of Propagation

by Terry R. Dettmann, WX7S
Types of Propagation
How DOES your signal get from your antenna to the antenna of the amateur you're talking to? That's the subject called propagation. 
For the Novice examination, there's only one question on this topic. But, you'll find it important, particularly if you're interested in DX operation. As you go for higher licenses, you'll be expected to know more about propagation. 
In this installment, we're going to review three basic types of propagation, line-of-sight, ground-wave, and sky-wave. When you're working at VHF or higher frequencies, line-of-sight propagation is the mode you'll need to understand. At HF, ground and sky-wave propagation is usually more important. There are other types of propagation, but these will do for now. 
Why should you worry about propagation? After all, once the energy leaves the antenna, there is little that you can do to affect the signal. That's true as far as it goes, but if you DON'T understand propagation, then you may just be wasting your electricity. Once you understand a little about it, then you'll understand why it's important to you. 
Line-of-Sight Propagation
The simplest and most easily understood way in which a signal travels from one antenna to another is by 'line-of-sight' propagation. Line-of-sight propagation requires a path where both antennas are visible to one another and there are no obstructions. VHF and UHF communication typically use this path. 
Unless you are VERY close to your destination, you need to keep the antenna as high as possible. Because radio waves follow a straight-line in this mode, they simply go off into space as the curvature of the earth causes the ground to drop away beneath the radio waves. 
As we elevate the antenna, the distance to the horizon gets further and further away. With enough power to reach the other antenna and a high enough antenna to see it, we can talk without problems. VHF repeaters are usually mounted on high buildings or mountain tops for this very reason. When you are operating with a small VHF hand held, your signal must be able to travel in a straight-line to the repeater or your signal will be lost to someone beyond line-of-sight. 
Ground-Wave Propagation
Ground-wave propagation is NOT a variation of line-of-sight propagation. In fact, ground-waves will travel further because the curved surface of the earth pulls the signal down and keeps it along the ground. You can reach an antenna which is below the horizon with ground-wave propagation, one you can't 'see' by line-of-sight propagation. BUT, since the signal interacts with the ground, it loses a lot of energy as it travels, severely limiting it's range. 
Sky-Wave Propagation
Do you want to get out of the near neighborhood and reach the rest of the country for a Worked All States (WAS) award? Or maybe you want to talk to the rest of the world for a DXCC certificate. Sooner or later you'll HAVE to use sky-wave propagation. In this mode, the signal you send out radiates UP towards the ionosphere, 30 to 250 miles above the surface of the earth. Depending on how ionized it is and the frequency you're using, it will act more or less like a mirror to reflect your signal back to the earth some distance away. Your signal will literally SKIP over a wide section of country and come back down to earth many miles away. At this point, it can be reflected again to bounce a second time (and some times even more) back to earth again. It's like shining a searchlight up and having it reflected back to illuminate the ground many miles from the light. You can only talk to those people in the patch of earth that your antenna is 'illuminating' with it's signal. The place that the signal skipped over is called the skip zone. 
What actually happens in the ionosphere is that the signal is bent (or refracted) by the ionosphere. Below the MUF (Maximum Usable Frequency), there us so much bending that the signal emerges from the bottom of the ionosphere pointed back at the ground. It's not REALLY a mirror and it doesn't REFLECT the signal. 
You can picture the ionosphere like a bunch of glass panes stacked on top of one another. As you probably remember from high school physics, light passing through a different medium (like water or glass) is bent. Imagine that as the signal, arriving at 45 degrees, passes through a layer, it bends over just a little. 
If it bends 10 degrees every time it enters a pane of glass, then after 9 panes, it will have turned by 90 degrees and will be heading down towards the ground at a 45 degree with respect to the glass panes. 
The ionosphere works somewhat similarly. The bending is gradual but increasing as the signal keeps going into the ionosphere until it leaves either at the top going towards outer space (because it wasn't bent enough to come back) or it leaves at the bottom going back to the earth. If it goes out the bottom, then it's a skip signal. 
OK, let's test what you know. There are twelve possible questions on the Technician test covering the types of propagation we've just discussed. There are some other questions related and only ONE question of all of these will be chosen for the test. Try them and see how you do. 

Quiz
1. What type of radio-wave propagation occurs when the signal travels in a straight-line from the transmitting antenna to the receiving antenna? 
A. Line-of-sight propagation 
B. Straight-line propagation 
C. Knife-edge diffraction 
D. Tunnel propagation 
2. What path do radio waves usually follow from a transmitting antenna to a receiving antenna at VHF and higher frequencies? 
A. A bent path through the ionosphere 
B. A straight-line 
C. A great circle path over either the north or south pole 
D. A circular path going either east or west from the transmitter 
3. What type of propagation involves radio signals that travel along the surface of the Earth? 
A. Sky-wave propagation 
B. Knife-edge diffraction 
C. E-layer propagation 
D. Ground-wave propagation 
4. What is the meaning of the term ground-wave propagation? 
A. Signals that travel along seismic fault lines 
B. Signals that travel along the surface of the earth 
C. Signals that are radiated from a ground-plane antenna 
D. Signals that are radiated from a ground station to a satellite 
5. Two Amateur Radio stations a few miles apart and separated by a low hill blocking their line-of-sight path are communicating on 3.725 MHz. What type of propagation is probably being used? 
A. Tropospheric ducting 
B. Ground-wave 
C. Meteor scatter 
D. Sporadic E 
6. When compared to sky-wave propagation, what is the usual effective range of ground-wave propagation? 
A. Much smaller 
B. Much greater 
C. The same 
D. Dependent on the weather 
7. What type of propagation uses radio signals refracted back to earth by the ionosphere? 
A. Sky-wave 
B. Earth-moon-earth 
C. Ground-wave 
D. Tropospheric 
8. What is the meaning of the term sky-wave propagation? 
A. Signals reflected from the moon 
B. Signals refracted by the ionosphere 
C. Signals refracted by water-dense cloud formations 
D. Signals retransmitted by a repeater 
9. What does the term skip mean? 
A. Signals are reflected from the moon 
B. Signals are refracted by water-dense cloud formations 
C. Signals are retransmitted by repeaters 
D. Signals are refracted by the ionosphere 
10. What is the area of weak signals between the ranges of ground-waves and the first hop called? 
A. The skip zone 
B. The hysteresis zone 
C. The monitor zone 
D. The transequatorial zone 
11. What is the meaning of the term skip zone? 
A. An area covered by skip propagation 
B. The area where a satellite comes close to the earth, and skips off the ionosphere 
C. An area that is too far for ground-wave propagation, but too close for skip propagation 
D. The area in the atmosphere that causes skip propagation 
12. What type of radio wave propagation makes it possible for amateur stations to communicate long distances? 
A. Direct-inductive propagation 
B. Knife-edge diffraction 
C. Ground-wave propagation 
D. Sky-wave propagation 

Let's go back over the questions one-by-one and see how you did. 
1A. This question sounds like it GIVES you the answer. If the signal travels in a 'straight-line', then it MUST be 'straight-line' propagation. Don't be fooled though. The propagation mode is called 'line-of-sight'! 
2B. VHF and UHF frequencies usually use line-of-sight propagation because of the frequency. It's just too high for the other propagation modes. 
3D. In this case, the question DOES give away the answer: the signal travels 'along the surface of the earth' and hence it MUST be a 'ground-wave'. 
4B. Ground-wave propagation travels along the surface of the earth. Match the two concepts and you'll never be mistaken. 
5B. With a low hill blocking the line-of-sight, it clearly isn't 'Straight-line' and the other modes (which you won't learn about until you go for higher classes of license) are unlikely at 3.735 Mhz (on the Amateur 80 Meter Band). Ground-wave is the only choice left. 
6A. Remember that ground-waves interact with the ground, thereby losing energy. This limits the range of the ground-wave compared to a sky-wave. So the range is 'much smaller'. 
7A. The ionosphere REFRACTS the signals back to the ground in SKY-WAVE propagation. 
8B. Sky-wave propagation uses the Ionosphere to turn the signals back to earth (it REFRACTS them). 
9D. SKIP propagation is simply another name for SKY-WAVE propagation. The signal is refracted back to earth by the ionosphere. 
10A. The area which the signal jumps over in sky-wave propagation is the SKIP ZONE. There will be a weak signal there, but usually not enough for a good communication. 
11C. A SKIP ZONE is the area jumped or skipped over by your signal on it's way up to and back from the ionosphere. 
12D. How far do you want to get? Very close, you'll use straight-line. Over the horizon, you'll use ground-wave. But if you want your DXCC or WAS, you'll HAVE to use sky-wave propagation. 
You've done well this time, haven't you? Basic propagation modes at the beginners level aren't hard. In fact, they're pretty simple once you learn a few terms. 
Transmitted: 95-02-15 09:22:41 EST 

NASA/Glenn Research Center
Educational Programs Officer: Lynn Bondurant
Curator: Bruce Bream {bruce.bream@grc.nasa.gov}
Last Update: 08-November-1995