That is exactly what it is...an NVIS HF loop antenna.

Where you will find it in this link is "complicated." It is a long discussion. At one point, a poster to the thread starts presenting a series of posts on NVIS antenna, with attached photos. His fifth series post deals with the Sentinel class, and his photos indicate that is exactly what the antenna is. You must simply read, there is no getting past it. You are looking for a paragraph--well down into it--which begins:
"This post is the fifth of a short series relating to:- [SIZE="4"]Near Vertical Incidence Skywave (NVIS) Loop Antenna Arrays[/SIZE] [SIZE="3"]- (continued)[/SIZE] Also, folllowing on from the previous commitment in post #635......"

https://www.worldnavalships.com/forums/thread.php?threadid=9725&page=13

His fifth of the series has 6 attachments.

Here are the two best:

https://www.worldnavalships.com/forums/attachments/1/1/7/1/9/156533.attach

https://www.worldnavalships.com/forums/attachments/1/1/7/1/9/156531.attach

Here is the catalog with the part...tenth item down:

https://fsg-59-electrical-electronics.parttarget.com/fsc-5985-antennas-waveguides-5985-01-614-5760_5985-01-614-8137.html