Hi Christiaan,
On the 737NG the HF aerials are hidden away inside the leading edge of the fin, the tech site has a nice schematic of where most of the aerials are located..
Communications and just scroll to the bottom of the page.
As you can see on this pic from !!!!!!!!!!!!!! the HF set up isn't visible...
Photos: Boeing 737-86N Aircraft Pictures | !!!!!!!!!!!!!!
I'd be quite interested if anyone has a diagram (with accompanying pilot-friendly description!) of how the HF system is set up inside
Hope that helps!