Weather Satellite Converter
This little converter is easy to build and use a standard FM radio as receiver.
This circuit converts the signal from the polar orbiting satellite ( 137,300 to 137,850 Mhz ) to a signal which a standard FM radio
can receive ( about 99 Mhz ).
The converter is mounted between the antenna and the FM radio, the speaker output from the FM radio is then connected
to the line input connector on the soundcard.
The principle of that converter is simple.
It contains a filter at the antenna input, which is tuned to 137,500 Mhz, a mixer and a crystal oscillator
( NE 612 ) which mix the crystal oscillator frequency with the antenna frequency.
The output from the mixer is connected to the FM radio antenna input ( 87 to 108 Mhz ).
The mixer will subtract crystal oscillator frequency from the antenna frequency and the result frequency
is feed into the FM radio antenna input.
Example: 137,500 Mhz ( Antenna frequency ) - 38,050 Mhz ( crystal frequency ) = 99,450 Mhz ( Output frequency ).
The crystal frequency is not critical, but the output frequency must be inside the range of the FM radio ( 87 to 108 Mhz ).
I choose a crystal with frequency 38,050 Mhz and the output frequency is listed in table below.
It is important that the output frequency is not the same as the frequency of other strong radio station, these stations
will disturb the signal from the converter.
The inductors L2 and L3 are made of copper wire ( 1 mm. in diameter ), the inside diameter of the inductor
is 5 mm. and it's 5 turn.
The inductors L1 and L4 is not critical, it is made of insulated copper wire ( about 0,5 mm. in diameter ).
It is about 5 turn whirled around a little ferrite core.
You don't need L1 if you don't have an amplifier in the antenna, this inductor feeds power up to the amplifier
through the antenna cable, the amplifier is recommended.
All the conductors in this circuit must be as short as possible.
It is practical to use a car radio as receiver, because it has a good antenna input connector,
and it is powered by 12 V.
This coaxial cable is used between the converter and the car radio.