Bearcat 210 Scanner Conversion for Weather Satellites
By Steve Warley / K1LJL swarley@zoo.uvm.edu
Modifications to the 210 will provide the following:
1) Reception of 137 Mhz Range.
2) Increased bandwidth of the IF.
3) Low Level audio output
Modification to the Bearcat 210 scanner for reception of
137 Mhz FM is a fairly simple process. These receivers
operate from 146 to 170 Mhz using low side oscillator
injection, with a non-standard (10.8) mhz Intermediate
Frequency. By utilizing High side injection with the
oscillator above the input frequency we can receive
137Mhz simply by selecting a frequency 21.6 mhz above
137.xx Mhz. Examples below are frequencies for NOAA and METEOR satellites.
BC210 Weather Sat Freqs (F+21.6)
Channel Scanner Freq Satellite Sat Freq
1 158.9 MET 2-17 137.3 (137.3+21.6=158.9)
2 159.1 NOAA 10 137.5
2 159.1 NOAA 12 137.5
3 159.22 NOAA 14 137.62
3 159.22 NOAA 11 137.62
4 159.45 MET 2-20 137.85
4 159.45 MET 3-5 137.85
Of course the rf amplifier is now tuned to 158 mhz and
needs to be re-tuned to 137 Mhz.
The Bearcat receiver design utilizes "track-tuning" using
varactor diodes on the rf amplifier stages. At 158 Mhz
the Voltage on the diodes is approximately 9 V. At 137
Mhz the voltage needs to be about 3 V. This voltage isn't
critical since the Rf stages are fairly broad.
Make a simple voltage divider using a 100K pot in series
with a 10K resistor connected to the internal 10 V power
Supply. The CT of the pot is tied to the control diodes
after cutting the circuit land on the circuit board going
to the VCO Tuning Voltage.
Where to cut the circuit land and install the CT wire?
Look at the component side of the circuit board near the
Antenna input to the scanner. Locate Coil labeled L9.
Just to the right of this label is a 47 K ohm Resistor
connected to a land connection with two traces. The CT of
the pot issoldered to the resistor at the land. The trace going to
the IC is cut. The 10 V can be obtained from regulator
Q8.If your board is not labeled with component numbers
you can identify Coil L9 as follows:
__ Ext.
Blue->O Blue->O I__I Antenna
* L9 YELLOW-->O Red->O Input
L9 47 Kohm
10K 100K POT O /-0--/\/\/\/\-
10 V 0-\/\/\/--/\/\/\/-Gnd / |^\Cut
^CT------------/ | \ TO IC
The Receiver is now modified for tuning the 137 mhz
range. The receiver should be sensitive enough to hear
satellites once the pot is adjusted to 3V. Once a
satellite is heard, fine tuning of the pot for the
strongest signal is recommended, however stay as close to 3 V as possible
to detune the Rf amps away from the 158-159 Mhz range.
The IF on the scanner is +/-5 KC for Narrow band FM
reception .This will give marginal APT pictures.
To increase the bandwidth to +/-40 KC, 1 of the two
10.8 MHZ filters must be removed and replaced with a
jumper. Narrow bandwidth was achieved using 2 filters in
series. This crystal filter is located between Y1 and Y2
next to R3 labeled on the board. Cut the traces going to
the filter and install a jumper in place of the filter.
The filter can be left in place soldered to the board.
Note: I removed both filters from the circuit board and
remountedone filter to the foil side of the board. This is an
alternative but not necessary. This completes the
conversion to the scanner.
The scanner should have a GASFET preamp installed in
order to get "state of the art" low noise figure
performance. I am using a Hamtronics $29 preamp with
excellent results. I use a linearly polarized 7 Element 2
Mtr Beam AZ/EL and get noise free APT pictures with the
satellites 2-4 degrees above the horizon. The only
possible problem that will be encountered is reception of
commercial signals on the 158-159 Mhz frequencies since
the receiver is tuned to the image freq. I am using a
military filter at the antenna input to attenuate the 1
local signal I have on NOAA 9 (159.22) freq. This is a
F-193/U Army Bandpass filter with a frequency range of
121-142 MHZ which i found at a hamfest.
Low level audio output can be obtained by soldering a
wire to the high side of the volume control. This wire can be brought out to a
phono connector on the back panel. This will allow constant audio amplitude
to the demodulator board . I am using the MultiFAX demodulator that has
its own level control.
I would like to acknowledge the "Weather Satellite
Handbook"by Taggart for the ideas in this conversion.
Have fun and good APT reception.