also,
avoid sharp Right angle bends and Roundabouts, keep straight ahead. If the
bend is too sharp there is a tendency to go straight on and burn a hole in
the fibre cable. Good Coax might be the best bet in the the amateur context.
Apart from the broad band advantage of fibre I don't see the necessity and
extra engineering required, interface connections etc for a simple preamp
connection on MF.
 G3KEV

----- Original Message -----
From: "Ken" <[email protected]>
To: <[email protected]>
Sent: Monday, February 01, 2010 5:56 PM
Subject: Re: LF: 2nd try of sending the pic of my loop preamp with the BF862


> Hi Stefan.
> 150dB/km, is a very high loss for modern fibre, even thhe early 850nm
> multimode fibre was only 1 or 2dB /km and modern single mode fibre at
1300nm
> or 1550nm the attenuation is substantially  less than 1dB/km. What optical
> wavelength is the data sheet quoting this attenuation figure at? If you
are
> using a red LED into a single mode fibre then the attenuation will be
high,
> the launch power will also depend on how the  LED is coupled into the
fibre,
> ideally the LED should be mounted in mating coupler so the so the fibre
may
> be mated up to it, air gaps of fractions of a millimeter will introduce
> attenuation. The same applies when you mount your receive photodiode. In
the
> 1980's and 90,s I spent many years both specifying and commissioning
optical
> line sysyems in the UK. I hope this info will be of use to you.
>
> 73s.
> Ken
> M0KHW
>
>
>
> ----- Original Message -----
> From: "Stefan Schäfer" <[email protected]>
> To: <[email protected]>
> Sent: Sunday, January 31, 2010 10:43 PM
> Subject: AW: LF: 2nd try of sending the pic of my loop preamp with the
BF862
>
>
> Hello Chris,
>
> I am not a communication engineer (but a power electronic engineer ;-) ),
so
> rather an amateur in this field. So my answer is kept simple ;-)
>
> In the datasheet of this cable i just found that attenuation is less than
> 150dB/km, so abt 3dB for my 20m cable. I have tried a cable of 2m (for
> indoor tests) and changed the cable to 20m. The difference (e.g. of DCF39
in
> the Argo monitor) is less than one S unit.
>
> In the moment i use a SFH350V (photo transistor)as the receiver. But
> recently i read in the web that a photodiode has (significant) less noise.
> So i want to try a SFH250V in a next step to reduce the overall noise,
> probably in combination with a second BF862 in the input stage of the RX.
> The SFH250V has even much better switching times. I have already ordered
> some of those. I will try this and send the circuit here...
>
> The cable i use has a PE shielding which is not suitable to exist 10s of
> years in the summer sun. But the distributor told me, it will be no
problem
> for some years. And since LF is a living project, this is enough for me.
>
> The losses of the cable must be compensated by a gain of an amplifier
within
> the system. This gain is not reached without a noise (e.g. by a OPA) but
> this noise can be much smaller than the noise that you fed into a
sensitive
> RX antenna, i expect. But the main aspect is, that one doesn't have to
worry
> any more if it could be possible that a noise is fed from the PC/RX/house
to
> the preamp by the cable.
>
> 73s, Stefan/DK7FC
>
>
>
> ________________________________
>
> Von: [email protected] im Auftrag von Chris Trask
> Gesendet: So 31.01.2010 14:57
> An: [email protected]; [email protected]
> Betreff: Re: LF: 2nd try of sending the pic of my loop preamp with the
BF862
>
>
>
> >
> > Some OMs informed me, that the picture of the loop preamp is
> > in a strange design. That may be come, since i (have to) use
> > outlook exchange (?)
> >
> > Now i try to do it again. Those who asked for the picture,
> > please give my a short confirmation if the picture is in the
> > correct format now.
> >
> > I do some modifications to the RX in the moment and will send
> > the new and perhaps better circuit soon. But the setup is
> > already working vy fine!
> >
>
>      That is a very interesting approach to minimizing induced noise on
the
> RX cable.  What are the loss characteristics of the optical link,
including
> the diode(s)?
>
>
> Chris Trask
> N7ZWY
> WDX3HLB
> Senior Member IEEE
> http://www.home.earthlink.net/~christrask/
>
>
>
>
>
>