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Have you ever seen this mysterious line-shaped thing on the vertical stabilizer of a Boeing 707 or 720? What purpose does this thing do?

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    $\begingroup$ Could you maybe provide a picture that is a reasonable size? $\endgroup$
    – Ron Beyer
    May 7 '20 at 16:25

It is the high-frequency shortwave antenna of a 707-321B, specifically Clipper Yankee Ranger N418PA. Also present on the KC-135.

You can read about the Antenna Coupler Program from the people involved in its development.

You might want to look at What radio frequency bands are associated with the long wire antennas, why do some aircraft have them and others don't? for more information.

Pan-Am 707 source

707 HF antenna source

  • 3
    $\begingroup$ Surely UNIVAC Coupler isn't the mainframe? $\endgroup$
    – zymhan
    May 7 '20 at 17:37
  • 5
    $\begingroup$ The Univac division of Remington Rand Corp were the creators? developers? designers? of the UNIVAC computer. A few years later they developed electrical couplers for military aircraft antennae, which were later made available to civilian airlines. $\endgroup$
    – CGCampbell
    May 7 '20 at 21:42

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