Your signals are effected by the Angle of Radiation as they travel from your radio to the other side of the planet. This paper by K2WH explains why.
Angle of Radiation – What is it?
One of the more important performance characteristics of an antenna system is its angle of radiation. Angle of radiation is not built into an antenna, you the amateur make that happen by the placing your antenna at the proper height. Therefore, I thought of this mental exercise of what radiation angle really means.
Angle of radiation when referring to antennas, is simply the take off angle of the RF field when launched from your antenna in relation to the ground (earth). That is, if your dipole antenna is low to the ground (< 1/2 wavelength), in relation to its frequency of operation, the angle of radiation from the dipole will be at or close to 90 degrees – straight up and the dipole will behave as an omni-directional antenna. The higher you place your antenna above ground, the lower the radiation angle. A height is finally reached (depending on installation), when the “Take off” angle (Magic Height) is in the 20 degree range or lower. A radiation angle of 20 degrees or lower is an ideal angle for working long range DX. This means, the major lobe of your RF energy is radiated at an angle of 20 degrees in relation to the horizon. The horizon being zero.
The “Magic” antenna height is generally achieved when your antenna is 1/2 wavelength above ground assuming a perfectly conducting ground (earth). There are many variations in ground conductivity ranging from something very similar to an insulator (sand and broken beer bottles) to salt water which is the best with everything else in between. Different ground types make the 1/2 wavelength rule different depending on where you live. If you live by the ocean, you are very lucky indeed. If you live in the mountains, you’re not so lucky.
How do you know when you are about 1/2 wavelength above your ground? Simple math. Take 468 and divide it by the frequency you intend to operate. Again assuming perfect ground, this number just happens to be the same number you would use to cut a resonant dipole to length.
This begs the question, “Why should I bother to achieve a low angle of radiation”. If you want the strongest signal possible at a distant point, a low angle of radiation is essential.
For instance:
You want to put up a dipole antenna for 40 meters, frequency is 7.250 MHz. Therefore, 468 / 7.250 = 64.55 feet. This height will vary depending on your type of ground, but generally it is the approximate height you would want your feed point to be located for best DX capability. It will give you a low angle of radiation which is very good for working DX. Believe it or not, some angles of radiation are better than others for working different parts of the planet. That’s another story in itself.
Visualize this:
Suppose I have a rubber ball in my hand. I throw it as straight down as possible at the floor. It will rebound and probably hit the ceiling directly above the point where it hit the floor. It will then hit the floor again and then rebound to hit the ceiling again close to the same spot again, all the while losing energy in the bouncing process from floor to ceiling. This will continue until all the energy is used up. Notice the ball did not travel very far from the origin point.
The floor in this example, can be looked upon as your ground, the ceiling the reflecting medium or, the ionosphere. This is how your signal travels from a low dipole delivering a very strong local signal because most of the RF energy is expended locally. The signal (ball) goes straight up and straight down for the most part. The result is many hops losing energy as it moves forward very little.
Now if I throw the same ball at the floor so it rebounds at about a 20 degree angle from the floor, the ball travels much further on the first bounce before it hits the ceiling. On the second bounce, it has moved quite far from its origin point. This is how your signal travels from a dipole when it is 1/2 wavelength above perfect ground. It travels much further between hops and loses much less energy before arriving at its final destination – the DX station. Remember however, your signal has now become directional so don’t point the ends of your high dipole to the part of the world you want to talk to. No free lunch here.
With a 20 degree take off angle, your local signal is much less powerful because most of the RF energy is passing overhead and not being reflected straight down. Local stations hear your ground wave signal which is good for about 50 miles and not very strong beyond that. You receive mediocre signal reports from your 1/2 wavelength high dipole vs your low dipole. This is why a vertical is considered a superior antenna for DX vs a dipole because they have an inherent low angle of radiation when installed properly and are not directional but usually require lots of radials to work properly.
Now if you are a DX hunter, a low angle of radiation is a good thing. A low angle of radiation equals a good DX antenna but if you want a strong signal locally, then a low dipole with a high angle of radiation is better.
As we can see, to really do your best with the DX and still have a good strong signal locally, you really need 2 antennas that you can switch between. You need one antenna with a high angle of radiation and one antenna with a low angle of radiation. Quite possibly, a low dipole and a good vertical.
I hope this helps readers out there to grasp the meaning of “Angle of Radiation”.
K2WH
73, Lee ZL2AL