MARITIME RECEIVER
(1985)
KLIK HIER VOOR DE NEDERLANDSE VERSIE
The maritime receiver for the reception of weather forecasts.
The maritime receiver for reception of coast stations.
Reception of weather forecast and wind warnings transmitted by coast stations is very important when sailing on sea. I wanted a small and simple receiver that could be easily tuned to the desired frequency and with good stability for the Single Side Band signals. There were two coast stations in my sailing area, so only a few frequencies were needed.
This simple receiver with 5 crystals can easily be tuned to the desired frequency. And because of the crystals, frequency stability for SSB reception is very good. Even 18 years after its last use and without the use of the fine tuning, it was exactly on the frequency of 2182 kHz for SSB reception when switched on for fun.
| Frequencies and coast stations |
Switch
position |
Frequency
(kHz) |
Station |
| 0 |
None |
Off |
| 1 |
2182 |
International calling and distress MF frequency |
| 2 |
1862 |
Scheveningen radio north |
| 3 |
1890 |
Scheveningen radio west |
| 4 |
2614 |
Norddeich radio |
| 5 |
2775 |
Norddeich radio |
Explanation
This receiver is a simple direct conversion receiver. The local oscillator works on the reception frequency. There is no IF, the output of the mixer is the LF signal. A disadvantage is that both sidebands are received, but in practice this was never a problem. There were never signals on the other side band when receiving SSB signals of a coast station.
RF amplifier
At the input, an LC circuit is tuned to the reception frequency with the variable mica capacitor. The receiver is tuned to the maximum noise level. The diodes do protect the FET againsg high peak voltages. The purpose of the selective input filter is to suppress all kinds of strong signals on other frequencies than the reception frequency. This LC circuit is followed by an amplifier with a FET BF245. This FET is not a voltage amplifier but a current amplifier or an impedance converter: the input has a high impedance, the output has a low impedance.
After the FET, you can find a low pass filter with a cut off frequency of 4 MHz. The receiver is also sensitive for signals on 2x and 3x the reception frequency. This filter suppresses the strong signals above 4 MHz on these frequencies.
Circuit diagram
Mixer and Local oscillator
The IC SO41P contains the mixer and also the local oscillator. Because of the expensive crystals, it was not a cheap receiver. In the diagram, only 2 crystals are shown, but the receiver has 5 crystals and a 2x6 position switch. The 6th position of the second section is the OFF position of the receiver. The crystal frequency is equal to the frequency of the coast station. Fine tuning for SSB reception can be done by a second variable mica capacitor. The receiver was supplied from a 12V accu that also was used for the depth sounder and for the lights.
Audio frequency characteristic.
LF filters and amplifier
The LF signal at the output of the mixer is filtered and amplified by two stages with a BC547 transistor. The volume control can be found between the two stages. A simple final LF amplifier with a LM386 drives a small loudspeaker. An 7808 IC stabilizes the 8V for the whole circuit, except the final LF amplifier. The audio frequency characteristic does not look so very good. But it compensates the frequency characteristic of the small internal loudspeaker, that is not so sensitive for lower frequencies.
Inside view
Results
This simple receiver did an excellent job. When sailing on a moving boat over the waves, tuning of a receiver for the reception of SSB signals is always difficult. But not with this receiver! In only a few seconds it was tuned to the coast stations and ready. Reception of the weather forecasts and navigational warnings was always very good. Only the short antenna connection was used with a wire antenna of approximately 1 meter in a polyester boat.
(Onno 1986)
The small sailingboat the receiver was designed for.
It was nice to walk a few hours barefoot on the ice where I sailed during the summer!
But... never do that when it is colder than -3C to -4C (26F)!
Index PA2OHH