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I tend to not think of KNBR as having a big signal. Both KCBS (DA-2) and
KGO (DA-1) are extremely strong down here with little fading; KNBR sometimes
gets as strong but fades more often.
They are using a so called Franklin antenna which is somewhat
unusual. It 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. A traditional Franklin antenna
consists of two half wave antennas stacked end-to-end and fed in phase. At 680
khz. this would be a 1,500 feet!! The KNBR 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.
All three have excellent ground systems.
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