The more I look into it the more I think fleetsubmarine.com got it wrong.
Here is the specification sheet, with installation diagram.
No extendable antenna.
https://maritime.org/tech/radiocat/sj.htm
In every picture I have seen it has been mounted on a horn like that, with what I assume is a support tube leading down from the head to keep it stable while the boat moved to decrease vibration, and sometimes I dont see the support tube.
I have never seen a picture of the SJ extended, ever.
That said, in order to get a heavy wind catcher antenna like that to be mounted on an extendable tube they would need the same hydraulics as the periscope.
Pump up, gravity fed down, and just as thick to keep it from shaking.
Interesting factoid: The SJ radar, since its directional and short range, was used as a telegraph/morse code transmitter to other submarines when traveling in a wolfpack.
Since it cant be heard on radio, and the directionalality and short range means it cant be picked up accidentally it was a perfect way to share secret comms.