Hi David B, Buzz here. I'm used to seeing the High Freq. radio antenna on the wingtips of the older 747's. Let's see, the old 707's had it pointing forward from the vertical fin...... so it could spear birds? (grin)
On 757's, 737's and 767's it's built into the leading edge of the vertical fin.
On DC-8's it was built into the top 5 feet of the vertical fin. I can't remember where it was on a DC-10.
Why use HF? because it's a (relatively) low frequency (2,5 -- 20 hz) and it bounces off the ionosphere for long range communication. The normal VHF radios are line-of-sight only, once you get 150 miles away you're out of sight....... that is if the Earth really is round.
HF radios are not easy to talk / listen on. Lots of things interfere with the signal you want to listen to. It's like listening to AM radio at night sometimes. But from the Pacific Northwest (USA) i can hear the Aussie BBC, and time signals from Ft Collins Colorado.