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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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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.
@mattblaze@federate.social
that's awesome
@mattblaze@federate.social how does the capacitance hat work?
@iris@neuromatch.social Oh god, it's complicated.
The basic idea is that the radiated current in a vertical antenna approaches zero at the end, and so is not fully efficient. By extending radials outward, you're allowing additional current to flow to the end of the vertical section by providing a capacitive path to ground (between the radials and the ground).
Modeling this is not for faint of heart.
@mattblaze@federate.social that was a surprisingly intuitive description of something I definitely am not prepared to understand.
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.
@mattblaze@federate.social
So what you're saying is that the smaller tower is in alternating current use?
I'll get my coat
The antenna 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
Also, important safety tip: you can get closer to this tower without clearly trespassing or jumping fences than any other 50KW broadcast antenna I've ever seen. I measured a field strength of over 80V/m a bit outside the antenna fence, which is an incredibly strong signal (though still within the OSHA limits at the frequency involved).
Resist any temptation to jump the fence and climb the (energized) tower. You'll be electrocuted as soon as you touch it.
@mattblaze@federate.social Great thread, thanks!
@mattblaze@federate.social With towers over 200ft, the FAA requires lights on the top, and these towers can make the systems that power and monitor the lights act very funky. I never had the pleasure, but I worked with people who did.
@Zoarial94@infosec.exchange yeah, they make special tower light monitoring systems specifically for AM radio stations. Also, maintaining the lights is definitely a job I Do Not Want.
@mattblaze@federate.social I was remote lighting support for the field, and I loved it. The field techs, however, really have it rough.
@mattblaze@federate.social incredibly interesting. Thank you
@mattblaze@federate.social oh that's a funky antenna! Most AM antennas aren't that intricate.
Apparently that's a "pseudo Franklin" design (I'd call it more like a collinear) with a capacity hat.
KFBK Sacramento has the only true Franklin antenna design on an AM station in the USA and it's reeeeeally funky looking. It does with two towers what some more conventional AM directionals take four to EIGHT to accomplish. It's kinda boss.
@vxo@digipres.club Yeah, KFBK is on 1530, making it at least possible to build a full Franklin antenna (2 half wave verticals stacked atop each other - a full wavelength!)
@mattblaze@federate.social I wonder if the original Ampliphase development/test transmitter is still there. It's my understanding that their daily driver is a Nautel
@vxo@digipres.club Wait, which station? KNBR is (according to their license) a Gates.
@mattblaze@federate.social oh I was talking about KFBK. At least as of 2009, a Nautel (not sure what model but the front panel reminds me of the V series?), a Harris DX50, and the RCA Ampliphase TX appeared to live there.
https://www.youtube.com/watch?v=97F9TO0yBNU
@vxo@digipres.club Neat!