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Weather FAX

Homemade > FAX

Weather FAX

2-meter FM receiver with synthesizer for Satellite

Source of the kit: Sigfried Hari - 63 488 Seligenstadt

Die technischen Daten des Empfängers:

 

Frequency Range
Capture Range of AFC
Frequency Raster
Sensitivity
Receiver Noise
IF-Bandwidth
Channel Selection

136.000 - 138,550 MHz
+/- 4kHz
10 kHz
0,4 pV bei 20 dB S/S+N
2 dB
30 kHz
>45 dB im 60 kHz distance

Image Ratio
IF-Bandwidth
Distortion Factor
AF-Output Power
Power Requirement
Power InputDimension

>45 dB
50 ... 5000 kHz bei -3 dB
<3% bei 8kHz Hub
0,5 Watt an 8 Ohm bei Ub=9 V
7... 13 Volt
30 mA stdby, 100 mA max. AF
70 x 100 mm / 2,75" x 3,94"

The principle

The meteorological satellites of the type NOAA or METEOR circles on relatively low orbits around the earth (LEO, low earth orbit), their period duration is situated in such a way around the 100 minutes. The course mostly leads across the pole and has with each circulation a misalignment opposite the earth's surface, so that in the course of the time the entire earth, strip for strips, is illustrated. The imaging occurs in such a way that a photo item (no camera) behind an appropriate optics sits, as now the satellite turns around its axle, scans it per revolution character row. The line space results from the path speed, since the satellite further -moved with each new revolution again somewhat. The picture is sent, contrary to Meteosat, in an unfinished way, it has thus no start and no end. One sees the strips the satellite shows between up and fall.

The signals, those the satellites will supply within the range between 136 and 138 MHz radiated, preferably on the frequencies 137 .500 and 137.620 MHz. The polarization is clockwise rotationally circular letter. The carrier is FM-modulated, i.e. one needs a FM recipient for the receipt. However the demodulated signal consists again of a 2400 Hz subcarrier, which is FM-modulated with the actual picture content. The max. modulating frequency is 2400 Hz, so that the AF­bandwidth of 4800 Hz amounts to (with TO: Bandwidth = 2 * Fmod). Because of the large deviation of approx. 8 kHz on the FM page occupies sending a bandwidth of almost 30 kHz (with FM: Bandwidth = 2 x (deviation + Fmod)). This is during the interpretation of the recipient to regard.

The transmitter frequency is afflicted because of the high rate of the satellite of 8km/s with a Doppler shift. If the satellite comes up moves it towards the recipient, the frequency is higher, with the fall is the movement of the recipient opposite and therefore lower. The max. deviation is with 4 kHz.

Structure
The structure of the circuit takes place on a on both sides copper-covered printed circuit board from 70 x the 100 mm size. The control elements, sockets, potentiometer etc. In the front and backing plate of the housing are accommodated. The frequency becomes over 8 switches in digital form within the range of 136.000. 138,550 MHz adjusted.

Antenna
The satellite signals from all directions to come there know, are an antenna with as more favourable a omnidirectional antenna as possible. Omnidirectional means not only the horizontal, like in the 2m-Band separate the entire sky hemisphere. Therefore usual 2m-omnidirectional antenna as still as possible such is suitable with gain, conceivably badly for the satellite reception. To be used should a two element cross dipole, which is so wired by a phase line that it indicates clockwise rotationally circular letters polarization. Thus one can write most satellite passages without noise. Of the north coast of Africa to high into the north over Norway.

Some pictures of the device (Year 1994)

[ CLICK ==> ENLARGE ]

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