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Date: Sun, 28 Jul 2013 15:34:37 +0200
From: Roelof Bakker <roelof@ndb.demon.nl>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Re: MiniWhip antenna, fiber optic
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Status: O

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