If you insist on using a valve radio and listening to medium-wave
stations, you have a problem: the existing broadcasters have only a
limited number of records. Here there’s only one remedy, which is to
build your own medium-wave transmitter. After that, you can play your
own CDs via the radio.
The transmitter frequency is stabilised
using a 976-kHz ceramic resonator taken from a TV remote control unit.
Fine tuning is provided by the trimmer capacitor. If there’s another
station in the background, which will probably be weak, you can tune it
to a heterodyne null, such as 981 kHz. As an operator of a medium-wave
transmitter, that’s your obligation with respect to the frequency
allocations. And that’s despite the fact that the range of the
transmitter is quite modest. The small ferrite coil in the transmitter
couples directly into the ferrite rod antenna in the radio.
Medium-Wave Modulator Circuit Diagram
The
modulator is designed as an emitter follower that modulates the supply
voltage of the output amplifier. As the medium-wave band is still mono,
the two input channels are merged. The potentiometer can be adjusted to
obtain the least distortion and the best sound. The RF amplifier stage
has intentionally been kept modest to prevent any undesired radiation.
The quality of the output signal can also be checked using an
oscilloscope. Clean amplitude modulation should be clearly visible.
The
medium-wave modulator can simply be placed on top of the radio. A
signal from a CD player or other source can be fed in via a cable. Now
you have a new, strong station on the radio in the medium-wave band,
which is distinguished by good sound quality and the fact that it always
plays what you want to hear.
Author: Burkhard Kainka - Copyright: Elektor
