weav said:
KNBR's antenna is not a Franklin; the only real Franklin antennna around here is at KFBK, up around Niclaus north of Sac. along Hwy 99. KNBR's may be sectionalized but is not a Franklin.
Actually, KFBK has TWO Franklins, which comprise the only DA in the US that uses Franklins. AFAIK. there is one other true Franklin in the US--it's the nondirectional daytime antenna of KSTP. KSTP's three-tower night array uses conventional, nonsectionalized towers.
A Franklin is a special kind of sectionalized medium-wave radiator. It is a center-fed dipole comprising two half-wave (180-degree) sections insulated from each other and driven at the center. A Franklin does not require a ground system and is very efficient; KSTP's radiation efficiency is 512 mV/m/kW @ 1 km. To produce the same RMS field, you'd have to put ~82 kW into a (tall) 200-degree conventional tower, ~140 kW into a (typical) 90-degree conventional tower, or more than 165 kW into a 54-degree conventional tower, which is the shortest tower that can produce the minimum Class B/D radiation efficiency of 281.7 mV/m/kW @ 1 km. In addition, the Franklin's vertical radiation pattern does not manifest a high-angle lobe as do the vertical patterns of conventional towers taller than 180 degrees. The high-angle lobe causes interference between the station's groundwave and skywave, producing unpleasant "phasing" in fringe reception areas during critical hours and at night.
But despite the high efficiency and the elegance of the Franklin design, few such towers remain in use. Their great height (360 degrees--plus the height of the insulator at the center) makes tham expensive to construct and maintain. Those problems are compounded by the need to insulate the top half of the tower from the bottom half. By and large, broadcast engineers are not fans of the Franklin design. Several Franklins at major AMs have been taken out of sevice in the last decade or so and have been replaced by shorter and less efficient towers of conventional design.
