KNBC's New Franklin Antenna, 1949
KNBC (KPO)'s present 550 ft. tower was completed in 1949.
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 ft. tall, an obvious impracticality. The KNBC tower measures
400 ft. to the midpoint insulator. The upper portion of the tower is shortened
to only 150 ft., and this is compensated for by a 50 ft. 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 left, the old east tower is visible in the background, with the
KNBC-FM antenna on top.
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