HF Antennas
Modern HF antennas and couplers in commercial aircraft are quite different from those described above. By modern, I mean 1968 and newer.
All the modern airliners use current-fed notch/slot antennas. They amount to a battery cable attached to the dorsal or tail fin; very low voltage, very high current. The whole plane becomes the antenna. Yes, if designed and maintained correctly, it is highly efficient.
The best commercial antenna flying still today was on the first DC-10, and continued on the MD-11. It was designed by Ben Hornby, an ex-pat Brit whose learning was on the VC-10 or something. The DAC tri-jet has the ideal location for the shunt fed antenna, just below the #2 engine, for really good coupling. The L-1011 was not so good, and Hornby had to help them make it work.
Boeing did not switch to the leading edge shunt antenna until pressured by airlines some years later. Their first was in the 727, which design they bought from Eastern Airlines. Boeing still did not understand shunt antennas a dozen years later, so the 767 had terrible HF for years, maybe still.
Airbus adopted shunt antennas in the fin from the beginning. I don't know how well they understand the dynamics, but feeding hundreds of amps into a carbon fiber fin makes me wary. That said, I have no direct experience with A/B HF antennae. AA587 and prior events it highlighted really had me wondering.
Airline HF is 400W P-P, BTW, and always USB, upper sideband. The antenna slot in most airliners is no more than a meter long.
Is this pertinent to AF447? Only regarding SelCal, I guess.
GB