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!
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asked Dec 7, 2018 at 17:44
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$\begingroup$ Also Why did some Caravelles have a very long dorsal fin? (about HF antennas in particular) $\endgroup$– minsCommented Dec 7, 2018 at 19:08
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$\begingroup$ If you see one of these on an old Piper or Cessna it is an automatic direction finder (ADF) antenna. $\endgroup$– JScarryCommented Dec 7, 2018 at 20:05
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$\begingroup$ @JScarry: The ADF principle is to rotate the antenna to find the direction of the beacon. How can a fixed antenna do that? $\endgroup$– minsCommented Dec 8, 2018 at 21:41
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$\begingroup$ @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.)” avweb.com/news/avionics/183233-1.html $\endgroup$– JScarryCommented Dec 9, 2018 at 5:18
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1$\begingroup$ @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. $\endgroup$– minsCommented Dec 9, 2018 at 17:33
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That is an end-fed long wire antenna for HF radio use.
answered Dec 7, 2018 at 17:46
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$\begingroup$ +1, but there are actually two antennas (see the insulators at the tail end), used by two HF transceivers. $\endgroup$– minsCommented Dec 9, 2018 at 11:05
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$\begingroup$ I missed the second one. why two wires? $\endgroup$ Commented Dec 9, 2018 at 19:49
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$\begingroup$ 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. $\endgroup$– minsCommented Dec 9, 2018 at 20:11