Fire Control
The antenna mounted on the 4" gun of USS Leary (DD-158) was trained and elevated using the movement of the gun itself; an independently elevating and rotating mount would follow the experiments, as well as a much more powerful antenna to transmit and receive the radio waves, both produced by RCA following a June 1937 presentation. Two test sets resulted - the XAF set, produced according to navy specs, and CXZ, RCA's own higher-frequency design. XAF was installed aboard USS New York (BB-34) above the navigation bridge in December 1938 while CXZ was installed as a mobile deck mount aboard USS Texas (BB-35) either that same month or the following January.
It was tested during the January 1939 fleet exercises. The XAF had better range and was useful for detecting & disrupting night destroyer attacks, while CXZ had better definition of targets; but the CXZ was too delicate for heavy seas and gunfire, thus was disliked and passed over for a production version of the XAF, which became the famous CXAM. In the interim, one of RCA's competitors, Bell Labs, demonstrated CXAS radar - an independently-developed set specifically for main battery fire control, which was modified to be able to transmit two slightly overlapping angular beams, allowing for two received signals to be compared by an operator - much like how the images displayed on a stereo rangefinder overlapped and had to be aligned to bring an image into focus. This technique, called lobe switching, allowed for exceptionally precise target ranging, vastly superior to optical methods.
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Of note is that the XAF trialed aboard USS New York survives to this day and is now on display outside at the Historical Electronics Museum, in Linthicum, Maryland after decades of outdoor display at the National Museum of the US Navy in DC, where its display pedestal still stands vacant with its old signage.
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Photos are public domain from the collections of the US Navy.
