Many thanks to David Giba who shares the following via the Gary J. Cohen’s Shortwave Listeners Global Facebook page:
The Boeing 707 is my favorite commercial jetliner. If one looks at the tail they will see a probe pointing forward off the top of the tail. That is the HF shortwave antenna.
It was matched by an antenna tuner/transmatch much like we have in our HF radios. This is the story of the Univac Coupler. Very interesting.