Elevated Radials for Part 15 AM Systems

Rich · 2015-01-04T13:58:19Z

There has been some discussion here about the use of elevated radials in Part 15 AM setups.Some think that the lengths of those conductors need to be included in the limit given in FCC §15.219(b).Others think that, because they produce very little far-field radiation, they should be exempt from §15.219(b).The official position of the FCC on this matter is not widely known, if known at all.Below is a post I made on another board (ham-related) that gives some background on the subject.It might be of some interest to Part 15 AMers, where an elevated "source" (transmitter) is located at the base of a ~3-meter whip, with one or more elevated horizontal radials connected to the r-f ground terminal of the transmitter (no "ground lead").Again -- we don't know how the FCC would view this, so potential users might want to factor that into their thinking/planning.--------------------In a radiating dipole, the current that flows from the 2-terminal source of the dipole onto one arm of the dipole originates on the other arm of the dipole (see graphic below).When current flows toward the source it is 180 degrees out of phase with the current that flows away from that source. Yet the fields radiated by both arms of the dipole reinforce each other in space.This is not the case for horizontal radials used in a ground plane antenna. Those single-conductor radials are driven at their physical center by ONE terminal of the source, which source is located at the bottom of the vertical conductor. Therefore the current flowing onto each half of that radial has the same relative phase as it flows along each half to the open ends of that conductor.However the current on one half of the radial is flowing in a physical direction opposite to an equal current and phase flowing on the other half of the radial. So the radiation patterns from the two halves of that radial are out of phase, and (very nearly) cancel each other in the far field.

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