Five Eighths Wavelength or 5/8 Lambda Vertical Antenna

Five Eighths Wavelength or 5/8 Lambda Vertical Antenna is said to have highest gain among the vertical antennas with 3 dB gain over a quarter wave vertical antenna. Paper published in 1924 by Ballantine S who introduced the 5/8 wavelength vertical antenna had shown that for a broadcast antenna over perfect earth with feedpoint at ground level and assuming sinusoidal current distribution, maximum ground wave radiation was for an antenna length of 225 degrees corresponding to 5/8 wavelength. 5/8 lambda antennas are widely used at VHF and UHF ranges and sometimes for HF. Even my CP22E vertical VHF antenna is based on 5/8 lambda, though it has two vertical elements with a phasing coil in between. Being a vertical antenna, 5/8 lambda antenna has an omnidirectional radiation pattern.

It is often used as a mobile antenna where the vehicle body acts as the groundplane. A small loading coil at the base and a capacitor to cancel out any remaining reactive element is needed for good matching of 5/8 lambda vertical antenna with the radio. This makes 5/8 lamba have an impedance like a 3/4 lambda antenna which has better matching. On HF frequencies, the low angle of radiation is useful in long distance communications through the ionosphere. In mobile application, the loading coil may reduce the rigidity of the antenna during movement causing flutter and changes in VSWR, which if too much could result in automatic reduction of power output of modern radios.