[time-nuts] Active LORAN antenna

[Ulrich Bangert]

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 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Schematic.pdf
Type: application/pdf
Size: 17628 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20110826/272a569e/attachment-0002.pdf>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ACAnalysis.pdf
Type: application/pdf
Size: 8329 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20110826/272a569e/attachment-0003.pdf>