Hello Roelof,
Am 28.07.2013 15:34, schrieb Roelof Bakker:
The loss was flat between about 300 kHz and 1 MHz.
Just like here. Looks like we are quite far away from the cutoff
frequency of the system.
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/
I see it later and may comment then.
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.
Very well!
So now the question is if the band noise still dominates the RX noise in
your configuration, if you are using a floating dipole. You may need to
increase the C of each dipole half to get some more signal level(?).
Then the question is if you can see an improvement, i.e. a better
decoupling from local QRM sources compared to the standard system.
Furthermore, if you would use a more compact transmitter circuit (more
compact, less weight and wind load), it could be possible to reach a
higher distance above the ground and so even more QRM reductiction?
What are your next steps?
Here i'm planning to do some RX tests today, on MF and maybe even LF.
And i started to construct a circuit for a 3.7V battery supply of the
optical TX. Mobile phone accus provide these voltages and should be
quite compact and have a good energy density. And they are cheap! This
should then allow to place the battery and circuit inside one of the
dipole halfs to make the system more symmetric and water proof.
A second fiber optic cable can be used to switch the circuit ON and OFF,
in its position on the roof. So the battery life time is much longer. A
simple LED, driven by the RS232 port and SpecLab could then realise a
timer so the circuit is OFF at daytime and active at night. And the
antenna can always be OFF during transmitting (worth the effort here :-)
). Finally maybe a small solar module, charging the battery during the
daytime? :-)
73, Stefan/DK7FC
73,
Roelof Bakker, pa0rdt
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