Longwire Match for SW Receivers

Most shortwave receivers for use ‘in the shack’ have a 50-Ohm coaxial input (usually a SO239 socket) which is not directly suitable for the high impedance of a typical longwire antenna. The problem is usually overcome by inserting a balun (balanced to unbalanced) transformer whose primary purpose is to step down the antenna impedance from ‘high’ to to 50 ? and not, as would be expected, to effect a change from balanced to unbalanced (note that a longwire is an unbalanced antenna). Unfortunately, such a balun may be difficult to obtain, make yourself, or both.