Caravelle question
 

Looking at various pictures of Caravelles some have a ridge extending from the base of tail fin along the top of the fuselage to a point level with the wings. On others this is missing.

What was it for and why was it only on certain aircraft?

Excellent question and one that has exercised my mind in the past...

Apparently, it was a fairing for the HF antenna. Not all Caravelles were fitted with an HF antenna, and those that weren't didn't have the fairing.

I'd post a link to a page with more information, but I don't have 10 posts yet...

Thanks for that. It seems the mystery is solved.

That's an informative link, thanks. It should be noted that not all HF antennas used the wire. Later HF antennas were built into the leading edge of the vertical stab (727) or the stinger version (707/720), The later being more common. Then of course there was the pointer version as seen on the 707/720 series.

Apparently the French like to do it that way.
https://i.pinimg.com/originals/21/b5...cf709cb89b.jpg

Not on the A300.
The HF antenna was a "Notch" type in the leading edge of the Vertical Stab.
The fairing on the top of the fuselage covered the ADF antennas.