- cross-posted to:
- photography@fedia.io
- photography@fedia.io
- cross-posted to:
- photography@fedia.io
- photography@fedia.io
KNBR (AM 680) Antennas, Redwood City, CA, 2024
All the pixels, less risk of electrocution or falling, at https://www.flickr.com/photos/mattblaze/54131419266
#photography
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.
@mattblaze@federate.social I was surprised that they operate in the 441m wavelength space, meaning the antenna does have to be huge. For some reason I thought commercial AM operated in a shorter band.
@mikewas Way down there: 540-1700 kHz.
@mattblaze@federate.social Yeah. I should know better but never really thought about it.
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.
@mattblaze@federate.social I’m fascinated by the cap hat, specifically why we don’t see more of them. I’d have thought saving tower height would always be welcome. I guess it’s a tradeoff and the hats only make sense, given some limitation or other, at particularly large wavelengths?
@gilester45@twit.social The hats aren’t as efficient as a proper length antenna; they serve mostly to increase the current flowing at the top end. And they’re BIG (this antenna is so tall that you can’t easily tell that the top hat is almost 50 feet across!)
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 touch it.
@mattblaze@federate.social In case anyone is morbidly curious what happens when you touch a AM tower, demonstrated via sausage: https://www.youtube.com/watch?v=WerKkrkuwHg
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.
@mattblaze@federate.social
just tell me you’ll pay shipping (from Pa) and it’s yours.
@cpm@spore.social very kind, and I’m drooling! But sadly, space.
@mattblaze@federate.social Yeah, I remember very well growing up (and into my early adulthood) being able to pick up far away AM stations when the conditions were right. It was fun back then to see what you’d find!
@mattblaze@federate.social Sadly, the Irish AM LW transmitter was shut down in 2023. I see there is a plan to shut down the BBC Radio 4 on LW in the UK, but there seems to efforts to stall that.
@dwmalone@mastodon.ie Yeah, the BBC station has been “about to go off the air” for at least 10 years now. You can pick it up on the east coast of the US on winter nights sometimes.
The rapid decline 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.
@mattblaze@federate.social I used to enjoy listening to radio stations from over a thousand miles away when I was a kid. They still played music on AM back then.
@mattblaze@federate.social The graveyard still produce a glorious at night…and this past winter I spent some time Sunset Skipping…and can still hear some local daytime stations in Virginina/Tennesee/North Carolina playing country gospel with local small business ads from my QTH in Brooklyn…but not like it used to be.
@mattblaze@federate.social agreed, and many of those unique, local broadcasters are operating at relatively low power levels, while the QRM from poorly shielded switching electronics gets louder every day.
kind of feels like MW AM isn’t the only communication medium suffering this kind of SNR degradation lately.
@mattblaze@federate.social Oh good. My fear of heights is saving me from another previously-unknown grisly fate.
Thanks ancestors who watched my non-ancestors fall out of trees!
@mattblaze@federate.social LOL having once been unable to start my car at Mt. Wilson, broadcast radio towers are something else… (had to shift it into neutral and roll downhill out of RF range – it was jamming my third party ignition cutoff burglar alarm)