Normally we expect man portable radios to use a long pole aerial in order to get the radio signal they need to operate. Whilst modern aerials can be very short, in World War II a long aerial was essential to get even the most modest of signal, especially in valleys or urban areas. A long pole aerial was not always practical- it made the radio operator an obvious target and could get tangled on trees and overhanging vegetation. An alternative was the trailing aerial which was a long length of wire that could trail along behind the radio operator and despite its lack of height, the long length of wire allowed a reasonable radio signal to be sent and received. These aerials were usually 25 feet in length and were issued wound onto a wooden spool that was carried in the radio haversack alongside more conventional sectional pole aerials:

The wire is covered in black plastic coating and the designation of this aerial is stencilled onto the spool, here it is a 25ft Aerial No3:

The opposite end of the spool has the stores code, here ZA26801:

I have been unable to get a positive ID on exactly which radio this aerial would have been used with, although I have been told it was for the WS38 MkIII set so it is currently with that radio in my collection until I hear otherwise.