Total posts : 45366
[quote:d810f920f7=”PhilB”]Even in the case of the external loading coil, very little of the overall signal is radiated by the coil (Rich may want to comment here).
I did a quick NEC-2 model of a 50-turn, air core, linear coil about 5″ high and 4″ in diameter to see what its 1.7 MHz free space radiation patterns would be, all by itself. Driving this calculated impedance of 4×10^-14 +j2x10^-6 is a practical impossibility, but this can be ignored because radiation patterns/gains don’t depend on the impedance of a radiator. The peak gain of the coil pattern for the vertically-polarized field in the horizontal plane was about -65dBi — which would add insignificantly to the field produced by a ~ 3-meter radiator, if it was attached. The coil also radiates some horizontally-polarized field, at a peak gain of
-125dBi at an elevation angle of about 40 degrees, and nulling in the horizontal plane (meaningless).
So the bottom line is that the loading coil for a 3-meter Part 15 AM vertical adds virtually nothing by itself to the radiation of the antenna system. Its only purpose is to reduce or cancel the capacitive reactance of the electrically short radiator, which when used with other matching techiques then can produce a reasonably good match to a transmitter expecting a 50 ohm, non-reactive load.
[quote:d810f920f7]A long, skinny loading coil would require the antenna to be shortened to stay within the 3-meter limit and would not perform as well.[/quote:d810f920f7]
True, but probably it is not all that important. A 1/2″ OD, 98″ copper pipe has a radiation resistance of about 0.08 ohms at 1.7 MHz. An 88″ version has about 0.067 ohms. These two lengths would allow for a loading coil external to the transmitter, or internally — plus 10 to 12 inches for the interconnect and ground lead.
If in both cases the loading coil resistance is 10 ohms, and the resistance of the path to Earth ground is also 10 ohms, then the radiation efficiency of the longer antenna is 0.398%, and for the shorter one 0.334%. This is a difference of 0.76 dB in radiated power, which converts to about a 6% difference in the radial distance to a given value of field strength. Not a huge benefit, there.