Monday, April 8, 2013
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.
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.
