What is this mysterious thing on the vertical stabilizer of the Boeing 707?
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&#39;t? for more information.

 
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