Gentlemen, my friend Frank and I both miss the matching actice antenna for our Standford Research FS700 Loran frequency standards. We also do not have a schematic of it. Nevertheless we have tried to re-engineer the circut from the part's list and the circuit description in the FS700 manual. Our result is shown in the accompanying schematic.pdf.
The generator in the left in conjunction with the 10 k resistor represents the expected high footpoint impedance of the very short (3 m) antenna. The VDD in the right in conjunction with the 100 ohms resistor represents what it believed to be inside the FS700. Not shown here is a 100 k resistor that may be included between the filter output and the 7 k resistor to form a 30 dB divider together with it. Also not shown is a 390 micro henry inductance which's impact is not to be found in the circuit descripion. Also not shown is a small neon bulb for overvoltage protection. The ac analysis shows that the overall filter function is well around 100 kHz. But it seems as if the fet (a 2N5991 in the original) would not have enough gain to counteract the overall damping included in the front end let alone to deliver some additional gain for the receiver's front end. We both feed the receiver currently with dipoles for 80 m ham radio and a resistor that makes the receiver think the active antenna is connected. The receiver has a possible total of 130 dB gain and my receiver says it uses 94 dB gain while Franks receiver says it uses 102 dB gain. The noise margin is also a bit better on my location. However, if one looks at the manual f.e. at page 17 one could get the impression as if a receiver gain of 75 dB may be considered much more normal as our values. Instead of improving the reception everything gets even worse if the active antenna is connected. I should also note that my qth is only 200 km away from SYLT (our German Loran station) while Frank is even 70 km nearer to it. Questions: 1) Anyone an idea what we are possibly doing wrong? 2) Anyone an idea for the 390 micro henry inductor? 3) Anyone own the original schematic? TIA for your help Ulrich Bangert www.ulrich-bangert.de Ortholzer Weg 1 27243 Gross Ippener
Schematic.pdf
Description: Adobe PDF document
ACAnalysis.pdf
Description: Adobe PDF document
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