I found a first answer here :
http://www.dai.ed.ac.uk/homes/cam/gadgets/SW100.html

Extract :
"If you want to use a Datong or other active/passive aerial plugged into the
external aerial socket then insert a .01uF capacitor in the centre lead to block
the power (this remark may not apply to the post-1997 SW100)."

As far as I know : 0.01 uF = 10 nF, so IF the lead-in center connector of the
DE/KA31 jack is protected by C1 in the schematic, then the antenna will work
with SW100... according to what is said on Chris Malcom's web page.

But is it ????

Charly