Vertical Antenna efficiency

Efficiency

by Lou Rummel KE4UYP

Traditional monopole verticals rely heavily on having large quantities of ground mounted radial wires to achieve high efficiency. With 120 ground mounted radial wires and a 1/4 wavelength tall vertical radiator you can achieve the highest possible efficiency for a vertical on 80m.

Unfortunately for the average amateur radio operator this is not practical considering the size of the average back yard. On 80 m you would need a clear area in your backyard of 135 feet square and the vertical radiator would be approximately 66 feet tall. For decades antenna manufacturers have attempted to miniaturized the classic monopole vertical, but here is the first problem, if you reduce the amount of radials from 120 down to say eight or less the ground loss goes up dramatically.

Ground loss is the No. 1 contributor to low efficiency of low band antennas. The closer any antenna is to the ground the higher the loss regardless of whether it is vertically or horizontally polarized. The second biggest contributor to efficiency deficit is when the vertical radiator is less than 1/4 wavelength long, and of course most if not all commercially manufactured 80m verticals fit into this category.

Radiation Resistance
On the opposite end of the spectrum from ground loss is radiation resistance, unlike ground loss the more radiation resistance you have the better off you are. A textbook perfect vertical would have no ground lost or resistive loss just radiation resistance. The length of the antenna determines the amount of radiation resistance it will have. The classic monopole vertical only develops radiation from the vertical element. The ground mounted radial wires contribute no radiation in fact, they do not even contribute to the radiation resistance. For any part of the antenna to contribute to radiation resistance it must produce RF radiation.