Specifically, an HF Radio Antenna. Long-range communications, and in any case higher frequencies for comm weren’t all that easy prior to the development of semiconductor technology. . .
Not true, even magnetron were used for radar that generated microwave. The magnetron IS a vacuum tube and is still being used in many airborne radar systems.
It could be HF antenna, but I think more likely a LF antenna for range station navigation that was in use at the time. LF is just below the broadcast band from about 190 kHz to 550 kHz. Range stations broadcast a beacon that gave a steady tone if you were on course a dit da or a da dit depending on what side of the course you were on.
I have navigated by them back in the 50’s but they no longer exist.