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Ground conductivity's effect on vertical
"Roy Lewallen" wrote in message news:z8WdnThgM7y9_5fVnZ2dnUVZ_gWdnZ2d@easystreeton line... There are two quite separate ways which ground affects a vertical antenna's performance. The first is loss due to current returning to the antenna base when the antenna is grounded, or induced in the ground under an elevated radial system. To minimize loss, you want as much of the current to flow through radial wires as you can. The power loss is I^2 * R. For a given power input, I is much lower for a half wave bottom fed vertical than a quarter wave bottom fed vertical. So the loss due to the conducted or induced current is much less, and you can get by with a much simpler ground system with the half wave vertical and still have low loss. This ground loss is usually the chief determining factor of a vertical's efficiency. Here we are again forgetting that we are dealing with standing wave circuit and cos/sin current distribution along the elements. Half wave vertical might have low current at the base but quarter wave away it will be max (assuming half wave elevated electrical radial). The radiation pattern is formed between the radiator and radials (and how they are affected by ground under). Radials close to ground couple to it and depending on ground RF quality we are dealing with decent reflecting mirror or "RF eating sponge". The other effect of ground is that the field from the antenna reflects from it some distance from the antenna. The reflected field adds to the directly radiated field to form a net field which is different at each elevation angle. This is a major factor in determining the antenna's elevation pattern. The conductivity and permittivity (dielectric constant) of the ground affect the magnitude and phase of the the reflected field, so the pattern changes with ground quality. In general, the more conductive the ground the better the low angle radiation. However, you can't compensate for this factor when the ground is poor by improving the ground system. The reason is that the reflection takes place much farther from the antenna than nearly any ground system extends. And low angle radiation, where the improvement is most needed, reflects the greatest distance away. The only way to improve the situation is to move the antenna to a location where the ground is better, which usually isn't possible or practical. Dense radial field with electrical length of radials around wavelength has shown remarkable imrpovement in low angle performance over "regular"ground. Because of the two separate effects, the overall field strength might be better or worse as the ground conductivity improves, and it might even be better at some elevation angles and worse at others. Roy Lewallen, W7EL Yuri, K3BU.us Yuri Blanarovich wrote: "Cecil Moore" wrote in message ... Al Lorona wrote: It's funny to think that really terrible ground can have an advantage over pretty good ground. Free space is just about the most terrible "ground" that one can imagine. :-) -- 73, Cecil http://www.w5dxp.com So much disinformation by W8JI School of DC circuitry :-) Modeling various configurations shows benefits of good ground, especially for taller than 1/4 wave radiators. Myth that half wave radiators do not need ground is spreading like snake oil wild fire. They need it but "looking" for it further out, not just at the base. I will anytime trade good ground (mirror) for lossy (RF sponge) ground. Its just where the radiator is "looking" for the mirror, taller one - further out, enhancing signals at lower angles. 3/8 vertical with some 3/8 physical length radials start morphing into far field. Yuri, K3BU.us |
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