The transformer is made by "misuse" of  a TDK common mode signal filter. The inductance and especially the mutual inductance of the winding are suitable for tranformer use in HF bands. Measured RF performance is excellent.

TDK signal filter ZJYS51R5-2PB or ZJYS51R5-2PB-01

OBS. TDK signal filter ZJYS51R5-2PB will be soon obsolete because it contains lead. New Rohs compatible type is ZJYS51R5-2PB-01. You can get the transformer (filter) from Farnell which seems to distribute components all around. Farnell part numbers, old 423-0413, new 962-1270 respectively.

If the TDK signal filter is not available you can use a home made transformer, see the instructions

Home made transformer
Twist two 0.25 - 0.35 mm enamelled copper wires together. Wind four turns of this bifilar packet into a Amidon two hore small ferrite bead BN43-2402, see the picture. Finally connect the wires as shown. Check the rigt connection by means of a ohm meter. Be careful not to damage the insulation of the wires. Instead of the two hole ferrite you can also use small ferrite toroid Amidon FT-37-43 which needs 6 turns of the twisted bifilar wire.