Total posts : 45366
Electrically short monopoles have a near field that extends to about .1 lambda, regardless of the height of the monopole. [2*(L^2)]/(lambda) does not apply to antennas much shorter than a wavelength. This can be seen from formulas for the electric and magnetic fields from differential current elements. These formulas are in antenna engineering books. I have seen Bode-like graphs of near and far fields for short antennas that show that the radius of the near field is about .1 lambda from the antenna. I could not find a reference that gave the exact factor. I calculated the near field radius as.0983634*(lambda) by finding the radius at which the real and imaginary parts of the vertical electric field in the differential current element formulas were equal. At 1620 kHz, the near field radius is (.0983634)*(185.2) = 18.2 m = 59.7 feet.
A typical urban atmospheric noise level in the AM broadcast band is .15 mV/m. This is a S/N ratio of just over 3 if the signal is at .5 mV/m. This is very noisy. .5 mV/m might be good enough in a rural setting if the receiver is of good quality.
An effective limiter does, indeed, improve noise level in a narrow band FM system. By elimiting amplitude noise, only phase jitter remains. However, the noise suppression of a wideband FM system is much greater than what would be obtained with a limiter alone. By increasing the FM deviation, and therefore the bandwidth of the FM signal, output noise is greatly suppressed. It is this aspect of FM that Armstrong claimed, but Carson did not understand.