Part 15 AM "Ground" Radiation

[rfry]

Statements in the past may have been posted and defended by others to the effect that some grounds used with elevated "Part 15 AM" antenna systems radiate, and some do not.

However by definition, a functional r-f ground cannot, and does not produce useful radiation.

Physics shows that if an unfiltered/unchoked, short "ground lead" of an elevated "Part 15 AM" transmitter (whether or not that transmitter was FCC certified for Part 15 AM) connects to a conducting path of ANY length/configuration such as a "massive ground wire," grounded metal flagpole/tower/mast/billboard frame/building steel/roof mount etc leading to a functional r-f ground buried in the earth, then that entire conducting path will produce very useful radiation. And so can the unfiltered/unchoked DC power and audio leads connected to that elevated transmitter.

There is no may, or may not about this.

Such added. functional, radiating length easily can exceed that permitted by FCC Part 15.219(b).

RF

 

[druidhillsradio]

"Physics shows..."

The science of matter and energy and of interactions between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics, and electromagnetism, as well as in modern extensions including atomic and nuclear physics, cryogenics, solid-state physics, particle physics, and plasma physics.

There you have it.

 

[Ermi Roos]

The 3 m whip that is called the "antenna," and the "ground lead" that is connected to the ground terminal of the transmitter, form the composite antenna of a Part 15 AM system. Labeling the "antenna" and the "ground lead" separately can be deceptive. Unfortunately, the language of Section 15.219(b) mentions the antenna and the ground lead (and also the transmission line) separately, and this enables the deception.

Antenna hoaxers, such as the promoters of the Crossed-Field Antenna (CFA), and the EH Antenna, label only small portions of their actual antenna circuits as the "antenna." For example, the AM broadcast band CFA in Tanta, Egypt is located on top of a building with a flat roof. The "antenna" is mounted over a large copper sheet, and the sheet is grounded through copper straps along the sides of the building. Actually, the copper straps to ground form the major part of the actual antenna system. The promoters of the EH antenna point to the little vertical dipole at the end of a long coaxial cable as their "antenna." Actually, the coax forms the major part of the actual antenna system. The promoters of these bogus antennas use the classical techniques of misdirection used by stage magicians.("Pay no attention to the man behind the curtain.")

There are a couple of different ways of technically describing what are called, "elevated antennas." [Even the term, "elevated antenna" is deceptive, because the actual antenna system extends from the top of the 3 m whip to ground (through the "ground lead").] First, it can be described as a grounded dipole, usually with the dipole arms having unequal length. Second, it can be described as a vertical monopole over ground, not driven at the base, but higher up along the length of the monopole. Both descriptions are equivalent to each other. In the case of "elevated" Part 15 AM antennas, the driving point is about 3 m below the top of the actual antenna system.

When the feed point is 3 m below the top of the antenna, the input capacitance of the antenna is about 30 pF, regardless of the antenna height up to about 1/6 wavelength; but the radiation resistance increases greatly as the antenna length is increased. This is convenient when using a transmitter with an internal loading coil, because the loading coil inductance for resonance does not change much with antenna length. The much higher radiation resistance of long antennas increases range considerably. When approaching 1/4 wavelength, the antenna capacitance increases considerably, and the radiation resistance can be in the kilo-ohms region; however, the antenna is not resonant, and some sort of loading coil is needed for tuning, even at 1/4 wavelength.