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
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this post was submitted on 23 Jun 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.
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.
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 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.
@mattblaze@federate.social I keep meaning to write something to decode the phase modulated signal on top of the audio, but I've never got around to it. Maybe they'll delay it long enough for me to retire and have enough time to try...
@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 on the road in Canada, often CBC's mediumwave broadcast is the only thing that works. And definitely not mass produced right wing nonsense.
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 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 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.
@pirateradiomap@toot.community I can regularly (in summer, overnight) receive identifiable stations on all but 11 of the MW slots. A few gems in there, but also a lot of monotony. And yes, the local ads are often the best part!
@mattblaze "produce a glorious mush" that is
@mattblaze Also I miss the clear channel (the band allocation, not the company) overnight Trucker shows. The Road Gang from WWL New Orleans was my favorite.
@pirateradiomap and WWVA’s overnight trucking show.
@mattblaze@federate.social
With Big John Trimble! My favorite though was Big John Parker, the weekend guy on the Road Gang...he was what I like call to a "cornball nihilist" He'd come up with these crazy pranks for the truckers to do...like look for the missing hour during the change back to standard time. And he would record the truckers who would call in and then he'd edit the tape to make them say weird things.
@mattblaze And he was a collector of vintage country music on Sunday mornings between 4-6 AM he had a specialty show called "Country Music the Way It Used to Was." I"d stay up all night to tape it and would switch over the WRVA to catch The Silver Star Quartet an old school Black Gospel harmony group doing request sfor people who were getting up for church. Caught a few of those on tape.
@mattblaze
I've probably got about 100 hours worth of the Road Gang on cassette...a digitization project I should get to post haste given I recorded them in the mid-late 80s.
@pirateradiomap Oh, that definitely needs to b e online!
@mattblaze@federate.social Yeah! I've got some digitizied...I'll try to put them up soon.
@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
indeed
@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 In case anyone is morbidly curious what happens when you touch a AM tower, demonstrated via sausage: https://www.youtube.com/watch?v=WerKkrkuwHg
@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)
@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!)
@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.