KNBR (AM 680) Antenna, Redwood City, CA, 2024
Very tall pixels at https://www.flickr.com/photos/mattblaze/54131419266
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this post was submitted on 13 Nov 2024
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KNBR is a 50KW "Class A" (formerly "clear channel") mediumwave (AM) rado station broadcasting on 680 KHz, serving the San Francisco Bay area (and, at night, most of the west coast of the US). Opened in 1922, It was originally known as KPO, (later KNBC, and still later KNBR), and soon became the flagship station for the National Broadcasting Company (NBC)'s new western radio network. It is currently owned by Cumulus Media and now broadcasts a sports format.
Mediumwave (AM) broadcast radio uses lower frequencies than other modern broadcasting and so requires much larger antennas (generally getting larger and larger as the frequency gets lower on the dial). This often entails highly customized antenna designs engineered for the particular site and station frequencies. For most radio stations (FM, TV, etc), the towers are there simply to get the relatively small antennas up high, but for AM stations like KNBR, the towers generally ARE the antennas.
The taller tower (550 feet) at right is the main KNBR antenna, built in 1949. It employs an unusual "pseudo-Franklin" design; it's actually an array of two antennas stacked atop one another. The 400 foot lower section is insulated from the ground. The upper 150 foot section is insulated from the lower section. The large (50 foot) diameter "capacitance hat" at the top (reminiscent of the Parachute Jump at Coney Island) electrically lengthens the top section, saving 250 feet of additional height.
@[email protected] how does the capacitance hat work?
@[email protected] Oh god, it's complicated.
The basic idea is that the radiated current in a vertical antenna approaches zero at the end, and so is not fully efficient. By extending radials outward, you're allowing additional current to flow to the end of the vertical section by providing a capacitive path to ground (between the radials and the ground).
Modeling this is not for faint of heart.
@[email protected] that was a surprisingly intuitive description of something I definitely am not prepared to understand.