• Matt Blaze@federate.socialOP
    1·
    7 days ago

    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 blackened corners.

    • Matt Blaze@federate.socialOP
      1·
      7 days ago

      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.

      • Matt Blaze@federate.socialOP
        1·
        7 days ago

        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.

        • Matt Blaze@federate.socialOP
          1·
          7 days ago

          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.

          • Matt Blaze@federate.socialOP
            1·
            7 days ago

            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.