Watching an old archive film on Operation Dominic I saw something rather odd.
At around 4:20 the film shows some, I presume, radar dishes; the odd thing is the thingamabob in the center that appears to be either vibrating randomly or spinning around at high speed, I know a thing or two about different types of antennas, but I’m puzzled at what that could possibly be.
Any ideas?
Got it in one.
It appears to be a Mark 25 radar on a USN destroyer. The view is of the forward part of the destroyer, the Mk 25 antenna seen atop its Mark 37 fire control director not visible below, main air search radar visible above and behind on the mast.
As mentioned, the Mk.25 used a mechanical conical scan technique. The rotating thing is the feed which as it rotates and reflects its electromagentic energy off the antenna itself creates a beam slightly off the axis of the antenna. When the radar is pointed other than directly at its target the return signal therefore varies in strength, but does not when the radar is pointed directly at the target, the target then being the same amount slightly off the beam’s cardinal direction at every point of the rotation of the feed.
The purpose of this particular radar was to provide tracking data of one target at a time for the director controlling the ship’s main 5" gun battery. It would be ‘cued’ by data from the air or surface search radars (latter is a smaller antenna barely visible through the framework of the air search antenna) approximately where to look for the target. The Mk.37 director was developed in the 1930’s. At first it used only optical sights/ rangefinder, then early in WWII the Mark 4 radar, then later in the war the similar looking Mark 22 radar, a distinctive appearance feature of US DD’s during the war. Mark 25 replaced Mark 22 from 1948. Later on, Mark 25’s had a solid dish, and the lattice ones also varied slightly in appearance (the one in linked photo is slightly different than the one in video) but ones in books I have look like the one in the video and I’m pretty sure that’s what it is.
Mark 37 director with Mark 25 radar
with eariler Mark 4 radar
http://navalhistory.flixco.info/H/266380/8330/a0.htm
Very informative, thank you.