Total posts : 45366
Ermi about a suggested Standard Part 15 AM Antenna: The Standard Antenna is so conservatively designed that there can be little doubt about its legality. Of course, there is no actual guarantee of its legality, because the design has not been reviewed or approved by the FCC. However, care has been taken to remove as many possible objections as could be reasonably anticipated. What follows is an edited version of Prof. Trainotti’s report:
“…The Part 15 AM monopole, operating at 1.7 MHz, 3 meters in height, can be made from an aluminum tube, 50 mm (about 2″) in diameter. Three or four dacron ropes can be guys. An Artificial Ground Plane mounted below the base of the monopole, 3 meters in diameter, is made of metal, like chicken wire or copper. There is a Natural Ground Plane extending a half wavelength, 88.17 meters, in all directions from the base of the monopole.” …
While in the last paragraph Dr Trainotti states that the metal box with the coil, and the transmitter can be connected “directly” to the metallic ground plane, he does not state the physical location of those components with respect to that ground plane.
From the description given for this Standard Antenna, the 3-meter monopole, coil and transmitter could be mounted an arbitrary distance above the Artificial Ground Plane, and connect to it directly using a long conducting path (short ground lead plus “massive” ground wire/metal pole/flagpole/billboard steel, etc). That path, of course, becomes a significant radiating component of the antenna, which can have a great affect on the radiation efficiency of the functional r-f system (see note below)
For example, 25 milliwatts of applied power, a coil Q of 200, and 3.47 ohm r-f ground resistance in the Standard Antenna system produce a 1.24 millivolt/meter groundwave field at 100 meters, according to the quoted post. This is a reasonable field value for those conditions if the base of the Standard Antenna, and the Artificial Ground Plane are mounted within a few inches of the physical earth.
However if the transmitter, coil and antenna are elevated some 20 feet or more above the Artificial Ground Plane, then radiation from the extended length of the conductor(s) connecting the transmitter chassis to that ground plane could increase the field strength at 100 meters by a factor of 5 or more. This is the equivalent of applying 25 X or more transmitter power to the antenna system when the base of the monopole is within a few inches of the physical earth, when using that Artificial Ground Plane either on, or buried in the earth.
If for the assumed average conditions (coil Q of 200, 3.47 ohm r-f ground loss, and 25 milliwatts of applied power), a 1.24 millivolt/meter field at 100 meters can be considered a “benchmark” for a Standard Antenna on 1.7 MHz, then the inverse distance groundwave field at 1 mile would be about 80 microvolts/meter (excluding earth conductivity losses). Of interest then is that this field would not provide good service to a typical indoor AM radio in an urban environment, which even in a non-urban area with low electrical noise can require several hundred microvolts/meter for (barely) acceptable performance.
This tends to show that Part 15 AM transmit systems claiming/producing a useful coverage radius of several miles would be using systems that could not be “reasonably anticipated” to meet the parameters of this Standard system — for which “little doubt about its legality” is claimed in the post.
Of course everyone is free to install and use whatever system they choose, but maybe the information here will be useful in making such decisions.
Note: The paper at http://filebay1.home.comcast.net/~filebay1/Elevated_Part_15_AM_Antennas.pdf shows why this is true for elevated antennas.