New Franklin Antenna
KNBR’s present 550-foot transmitter tower was completed in 1949 when the station was known as KNBC. (It had been KPO until 1947.)
This type of antenna is called a Franklin antenna, a design used by only a few AM stations in the U.S. The antenna is easily recognized by the porcelain insulator in the middle of the structure.
A traditional Franklin antenna consists of two half wave antennas stacked end-to-end and fed in phase. At 680 kc, this would require a tower 1,500-feet tall, an obvious impracticality. The KNBC tower measures 400 feet to the midpoint insulator.
The upper portion of the tower is shortened to only 150 feet, and this is compensated for by a fifty-foot diameter capacitive top loading “hat” at the top of the structure.
The top section is fed from a shunt tap on the lower section. There is a copper tube that runs up from the tap on insulated stand-offs, where it connects to the upper section just above the insulator.
The Franklin Antenna is used as a means to lower the radiation angle of the signal. This increases the ground wave coverage of the station, and reduces the night time interference at the fringes of the ground wave signal, caused by sky wave signals originating from the same tower.
In the photo at right, the old east tower is visible in the background, with the KNBC-FM antenna on top.
All articles copyright © 1997-2006 by John F. Schneider. All rights reserved.
Reprinted with the generous permission of the author.