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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Rodenstock 50mm/4.0 HR Digaron-W lens (@ f/6.3), Phase One IQ4-150 digital back, Cambo 1250 camera (vertically shifted -23mm).
The large amount of shift required to keep the tall antenna mast fully in the frame while maintaining its geometry pushed the 50mm Rodenstock lens to the very limits of its image circle, Hard vignetting of the upper corners is visible in the full sensor image, but fortunately the composition benefited from a narrower aspect ratio that cropped out the blackened corners.
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.
@mattblaze
And quite often it seems if they can get an AM broadcast tower just above some swampy saltwater, they will. I suspect that this gets better performance out of the radials that surround the vertical element, but I've never gotten around to asking an engineer about it. We have quite a few low-lying marshy areas with towers just inside in the Bay Area. The easiest ones to see are around the Bay Bridge and San Mateo bridge, but there's also a directional set of 3 right next to 101 in San Mateo for KZDG 1550 AM.
@spatula Yes, the ground resistance is actually the most important factor in determining mediumwave antenna efficiency. Part of the license process involves getting an extensive site survey for this.