If you have operated on the HF bands, you have probably heard of the "ground wave." You probably know that the ground wave gets weak to the point of disappearing over a rather short distance. And you know that the sky wave travels up and bounces off the ionosphere, and returns to earth very far away.
But did you know that there is a ground wave at VHF?
Just what is a ground wave, and why does it have so very much attenuation over short distances? Envision a radio wave traveling parallel to the ground. The part of the wave that is actually in contact with the ground interacts with the the ground (and the objects sitting on the ground). This energy see a dielectric constant which is much greater than that of the air. A radio wave traveling through a high dielectric constant travels slower than through a low dielectric constant. As a result, the energy near the ground travel slower than that which is far away from the ground (up high). The wave front which is radiating out from the transmitting antenna is in effect dragging its feet along the ground. This causes a bending of the wave front, where that part which is near the ground is "leaning" forward in the direction of travel.
Now it turns out (if you take graduate level university courses in electromagnetic theory!) that the energy in a radio wave front ALWAYS flows in a direction perpendicular to the wave front. Thus the energy at the foot of the wave front dragging along the ground is traveling at a slant, down towards the ground. This means that much of the energy near the ground will be absorbed by the ground. So the wave front near the ground is depleted of its energy. Hence the loss for the ground wave is very much more than that for a wave traveling in free space. The HF signal drops into the noise after traveling only a short distance. (A distance much too short to be accounted for by the curvature of the earth.)
So of what importance is this at VHF? At VHF the effect is very pronounced. The strength of a signal only 20 feet above the local ground level can be very much stronger than that 6 feet above ground level. This effect was hammered home to the writer by observing his Doppler while driving on the freeway, with the signal coming from straight ahead. On open parts of the road, the bearing was consistently dead ahead. But every time we went under a large overhead sign (across the freeway), the bearing flipped to dead to the rear! You see, the energy bouncing down upon us from the overhead sign was substantially stronger that the energy coming from in front of us at a height of 6 feet off the ground. In order for a bounce like that to be stronger, the energy level of the signal hitting the sign must have been rather significantly stronger than the "ground" signal.
There is another example of this known to a lot of Southern California hunters. There is a stretch of freeway which passes by a single very tall building about a block off the freeway. The freeway is often used by T-hunters while hunting. On two meters, as you come near this building, all bearings will be towards the building. More than one T-hunter has been "pulled off the freeway" at this point. Of course, what was happening was that the upper part of the building was seeing very much stronger RF wave front that the hunter in his car. The building scatters the energy in all directions. When you are close enough, the energy coming down from the building overpowers the "direct" ground wave.
If you know about this effect you are not as likely to be fooled by it!
George Russ Andrews
President
George R. Andrews (Russ, K6BMG)
BMG Engineering, Inc.
9935 Garibaldi Avenue
Temple City, CA
91780, USA
Voice 1(626)285-6963
Fax 1(626)285-1684 (24 hour automatic)
Web:
http://members.aol.com/bmgenginc
(1 Jun 1997)
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