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Prof. Valentin Trainotti has recently published a paper about calculating the ground loss of a ground plane of a vertical MW antenna in the December, 2007 issue of the IEEE Antennas and Propagation Magazine. The name of the article is “Accurate Evaluation of Magnetic and Electric Field Losses in Ground Systems.” The “accurate evaluation” mentioned in the title is possible by a reader of Prof. Trainotti’s article only after a lot of labor. The integrals in Trainotti’s results cannot be evaluated in closed form, but must be numerically calculated. A young doctorate student is named as the co-author of the article. I don’t know for sure, but knowing about how things are usually done, I would guess that the graduate student did the computer programming necessary for the numerical calculations, but Trainotti developed the general theory of the problem in mathematical form.
When I learned about Trainotti’s recent article, I saw that the subject matter relates directly to Part 15 AM. I saw this to be an opportunity to ask Prof. Trainotti to apply his powerful intellect to the subject of Part 15 AM. I asked him to provide a numerical value of the ground resistance of the gound plane described in the post by Dan Jackson. Prof. Trainotti’s response was disappointing. He said, in effect, that the whole point of Part 15 regulations, and similar regulations in other countries, is to limit coverage area. So, the problem of poor range with Part 15 AM is mostly regulatory, and not technical. Consequently, Prof. Trainotti was disinclined to help with the technical aspects of Part 15 AM.
I think that Prof. Trainotti is only partially right. There can be no doubt that the FCC intends to limit the range of Part 15 stations. In the case of TIS stations, which use a lot more power than Part 15 stations, the FCC requires that the TIS station must be outside the .5 mV/m contour of any licensed broadcast stations on the same frequency.
I had failed to point out to Prof. Trainotti how poor the state of the art of Part 15 AM actually is. In a recent NOUO, a supposedly Part 15 station was cited for using excessive input power to the final stage, even though The FCC’s field strength reading indicated that that the effective radiated power was only about 22 uW. 22 uW, and more, could have been obtained from a perfectly legal Part 15 AM installation. After all, 22 uW represents an overall system efficiency of only .022% in a legal system.
Although Prof. Trainotti does not wish to directly contribute to the technology of Part 15 AM, the information in his articles can be very useful. He supplies a great deal of information about the resistance of ground planes. Anybody is free to use the information contained in his published material.
Here is the information Trainotti has published about a ground plane with 180 radials, 1.8 m in radius, operating at 1.7 MHz:
For seawater, with a conductivity of 5 S/m, and a relative permittivity of 80, the ground resistance is .4 ohms.
For wet soil, with a conductivity of 30 mS/m, and a relative permittivity of 20, the ground resistance is 5 ohms.
For normal soil, with a conductivity of 10 ms/m, and a relative permittivity of 10, the ground resistance is 10 ohms.
For dry soil, with a conductivity of 1 mS/m, and a relative permittivity of 4, the ground resistance is 35 ohms.
These results, while disappointing for licensed operators, look pretty encouraging for Part 15 AM operators. Dan Jackson’s ground plane should give pretty good results, also.
The 180 radials specified by Trainotti are pretty dense, but the radius of the ground plane is less than 6 feet. So, laying out a solid metal radius of about six feet radius should give about equivalent results. For regulatory reasons, I think that solid metal is better than radials. This is because the FCC could possibly include the total length of the radials toward the allowable length of the ground lead.
Remember that the total radius of the ground plane is about a half wavelength, although, in this example, the metal portion of the ground plane reaches a radius of only about six feet.
Trainotti said that, because of the low radiation resistance of the antenna, as much ground plane and top loading as possible should be used. My guess is that the FCC will not tolerate a very big ground plane, and, especially, not a lot of top loading. So, I think that a ground plane and top loading should be used with moderation.