Longwire Match For SW Receivers

Most shortwave receivers for use ‘in the shack’ have a 50-coaxial input (usually a SO239 socket) which is not directly suitable for the high impedance of a typical long-wire 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 50Ω and not, as would be expected, to effect a change from balanced to unbalanced (note that a long-wire is an unbalanced antenna). Unfortunately, such a balun may be difficult to obtain, make yourself, or both. The circuit shown here is a transistorized (i.e., inductor-free) equivalent of the wire balun. The grounded-collector configuration is used because a relatively high input impedance (the long-wire antenna) has to be stepped down to 50 Ω (the receiver input impedance).

Voltage amplification is not required here. The two anti-parallel diodes at the antenna input prevent damage to the circuit as a result of static discharges or extremely strong signals. Like an active antenna, the circuit receives its supply voltage (in this case, 9V) via the down-lead coax cable. Current consumption will be of the order of 20mA. The coax cable should be earthed at the receiver side. The length of the antenna wire will depend on local conditions and what you hope to be able to receive. For most SW broadcast service and amateur radio listening, a wire of about 3m will be sufficient but bear in mind that the long-wire antenna is prone to pick up electrical interference.