Dear LF Group,
Yesterday i have tested a new preamp for my RX Loop (44 turns, 85cm diameter,
3dB BW<1kHz) with the BF862
(http://www.datasheetcatalog.org/datasheet/philips/BF862_3.pdf ) that is very
sensitive and especially designed for AM car radios, so for short vertical
antennas like we also use. Its equivalent noise at 100kHz is below 1nV/sqrtHz.
As you know i use a fiber optic cable to transmit the amplified signal to the
RX, to fully avoid coupling (galvanic, inductive or capacitive) of QRM by the
PC and all in the house to the antenna. The loop was mounted abt 2m above
ground, hung up on a tree, for testing. Since the fiber optic cable does not
affect the symmetry, there is no need for a symmetrical preamp and no
interaction with a coax.
The circuit is VERY simple and so i want to present it to you (professional
hand-made!), see picture.
Results were very promising. DCF39, the only signal that was available to that
short testing time was abt 80dB above noise (S9+10dB) and the background noise
@137,7 was now given by my RX, not by the antenna any more.
Length of the fibre optic cable was abt 20m (a fair distance to the house) but
i have also tried 30m without a relevant difference...
If it is interesting for you, i could create such a pretty drawing :-) of the
optic RX (also very simple) that transforms to LF again...
73, Stefan/DK7FC
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