0

Dornier DO-228 indicating string with an arrow[![ c130j [![usaf c130j ]2 228]3]3

Usually I got to notice a string attached between front fuselage to rear tail section of the transport aircraft's, what exactly might be the purpose or function of it? But comparatively, I don't find to see on commercial aircraft like Boeing 747, 737, A380 etc. why is that so!

Scramble shravan
  • 825
  • 1
  • 11
  • 15
  • Also Why did some Caravelles have a very long dorsal fin? (about HF antennas in particular) – mins Dec 07 '18 at 19:08
  • If you see one of these on an old Piper or Cessna it is an automatic direction finder (ADF) antenna. – JScarry Dec 07 '18 at 20:05
  • @JScarry: The ADF principle is to rotate the antenna to find the direction of the beacon. How can a fixed antenna do that? – mins Dec 08 '18 at 21:41
  • @mins “All ADF systems have loop and sense antennas. With the older ADFs, they are two separate antennas. The loop antenna is a flat antenna usually located on the bottom of the aircraft, while the sense antenna is a long wire that often runs from top of the tail to the top of the of the cabin. (On larger aircraft, the sense antenna may be located on the bottom of the aircraft.)” https://www.avweb.com/news/avionics/183233-1.html – JScarry Dec 09 '18 at 05:18
  • @JScarry: The directional antenna (loop or wire) must rotate per principle to find the null. The sense antenna to remove the ambiguity between two possible directions must be omnidirectional (hence usually it is vertical). See more. So I don't see how the directional long wire could automatically find the direction, unless the aircraft makes turn in the sky. This looks like more a communication antenna, and if you look carefully you will see there are actually two separate antennas. – mins Dec 09 '18 at 11:26
  • @mins I”m not commenting on the photo. Just pointing out that on old GA aircraft, like my 1968 Piper, they had a sense antenna running from the top ot the vertical stabilizer to the front of the fuselage. The loop antenna was on the bottom of the plane. My 1963 Cessna has the two antennas combined into one. At one point I could actually sort of fly an ADF approach, and even though it may have looked like I was rotating the plane to rotate the antenna, it wasn’t necessary—just turn the dial to set the course and adjust your track for the wind. – JScarry Dec 09 '18 at 15:57
  • 1
    @JScarry: Got it! Here is what you mean. So in this system, the sense antenna was definitely not omnidirectional, but as it's only to remove the ambiguity, it was obviously sufficient. I must apologize for my previous insistence. – mins Dec 09 '18 at 17:33

1 Answers1

6

That is an end-fed long wire antenna for HF radio use.

niels nielsen
  • 21,023
  • 2
  • 40
  • 74
  • +1, but there are actually two antennas (see the insulators at the tail end), used by two HF transceivers. – mins Dec 09 '18 at 11:05
  • I missed the second one. why two wires? – niels nielsen Dec 09 '18 at 19:49
  • An antenna can be used for reception while the other is used for transmission. That's what military call "Simultaneous Operations" (SIMOP). See this paper. Too bad the question was closed as this is a particularity of military aircraft not covered in the other linked question. – mins Dec 09 '18 at 20:11