Matt BlazeKNBR (AM 680) Antennas, Redwood City, CA, 2024.
All the pixels, with none of the vertigo, at https://www.flickr.com/photos/mattblaze/54131419266
Rodenstock 50mm/4.0 HR Digaron-W lens (@ f/6.3), Phase One IQ4-150 digital back, Cambo 1250 camera (vertically shifted -23mm).
This simple photo pushed the 50mm lens to the limits of its image circle with the large shift required to keep the tall antenna mast fully in the frame while maintaining its geometry. Hard vignetting of the upper corners was visible in the full sensor image, but fortunately the composition benefited from a narrower aspect ratio that cropped out the dark 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. It sits next to the former KGEI site.
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.
The antenna field is in the final approach and takeoff flightpath for SFO airport's runways 28L/R (and 10L/R), and so the site has special markings to warn pilots of a collision hazard. In addition to the usual tower lights and red/white paint, 3-dimensional "HAZ" warnings were installed around the field. These are easily visible in areal photos; see, e.g., https://earth.google.com/web/@37.5471204,-122.23429544,0.73120256a,577.14725587d,35y,0.01179999h,0t,0r/data=CgRCAggBQgIIAEoNCP___________wEQAA
Note, important safety tip: you can get closer to this tower without clearly trespassing or jumping fences than most other 50KW broadcast antennas I've encountered. I measured a field strength of over 80V/m a bit outside the tower fence, which is an incredibly strong signal (though still within OSHA limits at the frequency involved).
Resist any temptation to jump the fence and climb the (energized) tower. You'd be electrocuted as soon as you touched it.
AM broadcast is a technically interesting and somewhat endangered medium. The low frequencies mean that signals routinely travel well beyond their local coverage areas, especially overnight in winter. So there's a bit of mystery in tuning around the dial late at night; you never know what you might pick up.
Sadly, industry consolidation and the growth of higher bandwidth media (FM, satellite, podcasts) has greatly reduced the variety and local focus of programming. But it somehow hangs on.
The rapid decline in recent years of local content on the mediumwave bands has considerably reduced the romantic mystery of tuning around and seeing what you find. It's mostly now a sterile mix of mass-produced, syndicated right wing talk, sports, and so on. But there are still a handful of stubbornly local stations producing their own programming.
Kevin Karhan 
@mattblaze I would rather love some international radio with extras.
- Maybe it's me missing #BFBS Germany on FM or just me thinking that there is need fir more media diversity…