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From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <CAKvcm2dpuftdtK6OdQv8fOfiw+4yQrPBQBHJ-FN2gDbT4P_oMQ@mail.gmail.com> <51D498BD.3050007@iup.uni-heidelberg.de> <op.wzq2tgvnyzqh0k@pc-roelof> <005b01ce798a$b0476600$6d01a8c0@DELL4> <op.wzrfpjyyyzqh0k@pc-roelof> <20130706131640.GG5246@cs.utwente.nl> <7D6B8CBB38964C9C82487F5D0F3648FC@FMVXD1232> <20130707203537.GA30691@cs.utwente.nl> <51DA00C8.6000600@iup.uni-heidelberg.de> <51DA0212.8020808@iup.uni-heidelberg.de> <51DA03CE.6020508@iup.uni-heidelberg.de> <51DA1680.6030602@iup.uni-heidelberg.de> <51DB6CB3.2080708@iup.uni-heidelberg.de> <001601ce7c95$f235b360$6401a8c0@JAYDELL> <51DBFF7D.9050807@iup.uni-heidelberg.de> <51DC58E9.5030005@iup.uni-heidelberg.de> <51DF1D06.8020208@iup.uni-heidelberg.de> <51DF43E7.8080300@iup.uni-heidelberg.de> <op.wz4k19tayzqh0k@pc-roelof> <51E125D4.3050804@virginbroadband.com.au> <op.wz5pxy1kyzqh0k@pc-roelof> <51F1A6BB.6070805@iup.uni-heidelberg.de> <51F31086.4080107@iup.uni-heidelberg.de> <1cc492ac82f42955f49b1302d2726a01@xs4all.nl>
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Date: Sun, 28 Jul 2013 16:19:35 +0100
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Subject: Re: LF: Re: MiniWhip antenna, fiber optic
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Status: O

This must be  the  same as  Birds  sitting  on HV lines  ? no  earth   ref 
point  ?

using  some  form of small  loop and the  optic  link  would give good 
results , being  totally  isolated  from  earth, but, the  transmission 
losses  are  high ?

G.

--------------------------------------------------
From: "Roelof Bakker" <roelof@ndb.demon.nl>
Sent: Sunday, July 28, 2013 2:34 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: Re: MiniWhip antenna, fiber optic

> 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
>
>
>