A recent thread on the ALPB forum outlines a plan for adding several 8-ft ground rods buried in holes in the earth and surrounded by a moistened chemical compound, to improve the r-f ground used with a Part 15 AM antenna. Some thoughts about this ...
- The function of a buried r-f ground used by a monopole radiator is to provide a path for r-f currents flowing in the earth within 1/2 wavelength of the antenna base as a result of its radiation to return to the r-f ground terminal or chassis of the transmitter. Otherwise the monopole will radiate essentially nothing.
- Although the added ground rods described above may have a low resistance path to the potential of the earth immediately around them, the r-f currents flowing in the earth beyond that short radius still must travel through the lossy earth for distances of 1/2 wavelength or more, just to reach those ground rods (wherever they are buried). Therefore the losses in those paths to return r-f earth currents to the transmit system might be higher than expected.
If the site layout and operator risk-taking permit, better operational results might be possible from an effective "r-f ground" or counterpoise using just two 1/4-wave, horizontal radial wires elevated a meter or so above the surface of the earth, running in opposite directions from the base of the monopole, and joined to each other and the transmitter chassis there.
When properly installed, such elevated wires contribute no useful far-field radiation of their own to the transmit system. But note that this antenna system configuration is not explicity permitted by FCC §15.219(b), so it is not without risk of FCC attention.