KNBR (AM 680) Antennas, Redwood City, CA, 2024.
All the pixels, none of the RF exposure, at https://www.flickr.com/photos/mattblaze/54131419266
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this post was submitted on 21 Jan 2025
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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.
This distinctive stacked dual antenna arrangement is used to lower the radiation angle of the antenna, concentrating transmitted power to the "ground wave" and reducing energy that would otherwise be sent upward into the sky.
The smaller (300 foot) freestanding mast in the background left is not in current use. It can be used as an emergency spare antenna for KNBR during maintenance of the taller main antenna.
@[email protected] Interesting enough, another result is that the audio frequency bandwidth is a bit restricted. When Susquehanna owned KNBR, they tried to broadcast HD on it but found they couldn't because of HD's bandwidth requirements. Most listeners wouldn't notice the restricted bandwidth, since the NRSC standard limits bandwidth to ± 10 kHz and almost no modern AM receivers can go that far up anyway.