As already mentioned above, some aircraft have "slot" or "notch" HF antennas in the vertical tail. Not overly efficient, HF-wise, but they have the advantage of not causing drag.
Usually they're painted over, so you can't see them, and it's difficult to imagine what they look like.
Here's an exception, the British Concorde prototype 002.
Concorde had two separate HF transmitter-receivers, hence the two separate notches (plastic-covered) in the forward part of the vertical tail..
History doesn't tell why on Concorde 002 they were not painted until shortly before the end of its career.... on all other Concordes they were painted over.
Us aficionados apreciate it.... it often makes it possible to identify 002 in a photo, even in the absence of other clues.
A closely related tale....
As already mentioned too, notch antennas are very low-impedance, hence very high HF currents are induced in the structure around it.
During early flight tests it was noted that whenever somebody talked on the HF...... Concorde slightly wagged its tail!
After having some long and hard looks at the flight test recordings, we traced it to the yaw rate gyros (mounted in the tail) of the autostab system. The HF currents in the area were such, that the gyros picked it up and produced a spurious yaw rate signal.
Once we'd found the source, some additional RFI filtering took care of the phenomenon.
CJ