That mast is a component of the Shakti EW suite. It is the jammer.
Shakti EW suite is an evolution of the Ellora EW suite which in turn is an evolution of the Ajanta EW suite. The Ajanta suite was first introduced with the Shivalik class frigates. Kolkata class destroyers got Ellora & the Vizag has got the latest Shakti suite.
Now the Vizag's Shakti EW is a bit different from the Vikrant's. Vikrant is a larger & slower ship making it more vulnerable. But the Vikrant's powerplant is more powerful than the Vizag's (88 MW vs 76.5 MW installed power). So, more electricity is available for the EW suite.
The photo below is of a Mid to High band jammer used on the Ellora EW of the Kolkata class. The Vizag class uses the same jammers but at slightly higher power & with improved back end.
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The Vikrant probably uses the same back end as the Vizag but with a significantly more powerful jammer:
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This jammer probably covers a much larger part of the EM spectrum. There are some indications that this jammer would be carrier over to the P-18 class but with GaN HEMTs. Remember the news about India trying to get the "Unicorn" mast from Japan. We don't plan on buying the electronics from the Japanese. We just want to know how they packaged this into a single mast & maybe acquire the radome for testing. Because there are some tradeoffs to having a unified mast. RCS reduces but EWs effectiveness also reduces.