It could be HF antenna, but I think more likely a LF antenna for range station navigation that was in use at the time. LF is just below the broadcast band from about 190 kHz to 550 kHz. Range stations broadcast a beacon that gave a steady tone if you were on course a dit da or a da dit depending on what side of the course you were on.
I have navigated by them back in the 50’s but they no longer exist.
Also I am pretty sure that VHF communication for aircraft was in common use during WWII. HF was usually used only for longer ranges and would not be needed in a trainer like the T-6.
I believe the wire in question is an HF antenna.
In late 1941 the FCC had notified American radio “hams” of the scheduled re-assignement of the 75meter band ,i.e. several hundred kilocycyles in the 3.75 mc range to be used for aircraft crew training purposes.
There was a nice little ARC-5 radio set that operated in that range.They were simple to operate,reliable and used standard tubes.
Later the military did employ the much more complicated and expensive SCR-522 aircraft set in the 100-156 Mc range.
Reliable VHF tubes were a requirement and a development of the war;only hardy experimenters were working much above 60 Mc. Radios in the 100-156 commonly use a one to two foot rod antenna.
Magnetrons were expensive and not suited for voice communication;the magnetron and other radar tubes send out very short pulses of high power but the tube is off much more than it is on.
The LF direction-finding antenna would more likely have been a small loop that could be swiveled;the 180 degree ambiguity of simple directional antennas is thought to have lead to the loss of the training flight off the Florida coast(Bermuda Triangle) during WW2.