Hello Stefan,
I have continued my optic feedline tests with the SFH757V as transmitter
and a SFH250V as receiver.
The latter was lashed up reverse biased to V+ and with a 56K resistor in
series.
From the junction of the resistor and the SFH250V a 10 nF capacitor was
connected to the high impedance input of my standard mini-whip buffer
amplifier.
The signal loss in a 1m long optic link at 500 kHz was about 10 dB; with
a 19m long link this was 20 dB.
The loss was flat between about 300 kHz and 1 MHz.
Photo's of the test setup, optic receiver and optic mini-whip can be
found on my web space:
http://www.ndb.demon.nl/Optic%20mini-whip/
The optic mini-whip is a converted battery fed mini-whip.
The circuit uses a SMD J310 with the SFH757V in the drain and a source
resistor of 220 ohm.
The gate is biased at half V+ via a resistor divider.
Current draw is 24 mA.
The antenna is a copper clad part of the PCB.
The output is the same as my standard mini-whip antenna.
Now the interesting part: on air tests.
The antenna was mounted 5m high and a 19m long optic cable was used.
It did not receive anything at all and at first I thought something was
not working properly.
Double checking everything did not reveal a fault, so the inevitable
conclusion is that an optic link does not work with the mini-whip
design.
This agrees with the remarks sent by Pieter Tjerk de Boer to this list.
The capacitances involved in the circut layout are probably so small
that a proper dipole mode does not exist.
As the box of the optic mini-whip has a loose BNC connector, a 5m long
cable was connected to it.
The shield of this cable was connected to a ground stake at the bottom
of the mast.
There appeared to be a small degree of coupling between the loose BNC
connector and the circuit board ground as I could receive a weak signal
from a semi local broadcast station at 828 kHz.
The next step was to connect the ground lug of the BNC connector to the
circuit board ground.
The broadcaster produced now a signal level of - 54 dBm.
With my standard mini-whip this is - 35 dBm.
The difference of 19 dB agrees well with the loss of the optical link of
20 dB.
73,
Roelof Bakker, pa0rdt
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