What is the exact purpose of the string attached between front fuselage & the tail section of the transport aircraft's? ]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!
That is an end-fed long wire antenna for HF radio use.