KPJL FM wrote:So, if I was constructing a new 190 degree AM tower, would I still use only 1/4 length radials, because there is not enogh advantage to matching radial length to tower (electrical) length? Or is there a magic formula for figuring the best radial length for a tower length?
A study of the BL&E paper shows that it is important to collect the r-f currents flowing in the earth within about 1/2 wavelength of a monopole, regardless of the electrical height of the monopole. Those earth currents are produced when displacement currents resulting from the capacitance of the monopole with the earth are converted to conduction currents at, and just below the surface of the earth in that area. They need to be collected by the ground plane and returned to the transmit system in order to complete the path needed for r-f current to flow along the monopole.
If those currents are forced to travel more than a short distance through the lossy earth to reach a buried radial, then proportionally more of the transmitter power is dissipated as heat in the ground plane, rather than being radiated as EM energy by the monopole.
If the best length and number of buried radials is related to the highest radiation efficiency for any monopole antenna system (especially short ones with low radiation resistance), then 120 radials of at least 0.4 wavelengths each probably is wise. Economics may dictate otherwise.
I believe that one of the Class I AM stations in Chicago with a single, ~1/2-wave tower uses 360 buried radials, each 1/2-wave long (free space).