KNBR (AM 680) Antenna, Redwood City, CA, 2024
Very tall pixels at https://www.flickr.com/photos/mattblaze/54131419266
#photography
this post was submitted on 13 Nov 2024
10 points (100.0% liked)
Photography
1 readers
38 users here now
All things photography. Share your own original photos, your questions, your inspiration.
Rules
Share your own original photography. No NSFW images. Be Nice.
founded 1 year ago
MODERATORS
you are viewing a single comment's thread
view the rest of the comments
view the rest of the comments
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.
@[email protected] how does the capacitance hat work?
@[email protected] 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.
@[email protected] that was a surprisingly intuitive description of something I definitely am not prepared to understand.