Hamilton said:
So why not allow a water pipe or other massive metal that from a reasonable engineering prospective can be called ground be “ground”
But a reasonable engineering perspective cannot ignore the principles of physics, which show that anything that radiates cannot be considered as an r-f ground (no matter how massive). AM broadcast towers are massive, yet they radiate very well indeed. Calling something a ground doesn't make it one.
Here is a good definition of ground as applied to antennas (paraphrased from a website):
<em>An electrical connection to earth as a reference potential for radio frequency antenna signals. As high frequency signals can flow to earth through capacitance, capacitance to ground is an important factor in effectiveness of signal grounds. Because of this a complex system of buried rods and wires can be effective. An ideal signal ground maintains zero voltage regardless of how much electrical current flows into ground or out of ground.</em>
The last sentence in the paragraph above is very important, especially as concerns the use of "massive wires" considered by some (even Part 15 tx OEMs) as providing a "ground" to an elevated transmitter installation (on a roof top, flag pole, mast etc). The ONLY part of such a ground wire approximating the true electrical characteristics of the earth at radio frequencies is the part that is IN the earth. Once that conductor exits the earth, it behaves as an antenna for radio frequencies. The top of such a "massive wire" has greatly different r-f properties than the part that is in the earth, and in fact, that ground wire often contributes the largest part of the total radiation from such an elevated Part 15 AM system.
To illustrate the point, there is a type of monopole radiator used in commercial AM broadcast consisting of an uninsulated tower directly connected to the earth at its base via ~120 buried radials. It is driven by applying tx power to it at some physical height above the earth. This is possible, because the impedance of the tower at radio frequencies is a function of height along the tower face. This tower has essentially zero impedance when measured on the structure where it enters the earth, but when properly driven still functions as a very efficient radiator. This application and the elevated Part 15 tx+whip used with a long conducting path to something buried in the earth are electrically identical.
In some applications it is impractical to use the earth as the reference potential for a vertical monopole antenna. In such cases a properly designed network of horizontal, non-radiating, elevated conductors can be used as that reference, but the better term for such would be a counterpoise, rather than "ground." And their use for Part 15 AM systems might take some convincing for FCC inspectors.
Lastly on this subject of radiating ground wires, the Rangemaster website allegedly states that running such a ground wire up to the elevated tx via a metal conduit that is connected to the earth only at the bottom will inhibit radiation from the ground wire. But just as with the bare ground wire itself, that conduit would have the electrical properties of the earth only for the part of it that is physically buried in the earth. And like the ground wire, that conduit will behave as an antenna once it exits the earth -- so there is no real benefit in using it. This was confirmed rather easily using NEC.
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