Dear VLF,
Full of euphorism i want to tell you about my new VLF RX antenna.
You know my active antenna for VLF which is in use at my grabber:
http://www.iup.uni-heidelberg.de/schaefer_vlf/8,97kHz%20active%20E%20field%20antenna%20by%20DK7FC.jpg
I have had the idea to separate the E field active antenna into a front
end and the rest of the circuit can be left in the shack!
The front end consists out of the first JFET, the 1MOhm and 10 MOhm
resistor, the electrode(wire) and the C=20 nF. The diodes can be left
away. The rest of the circuit is now placed inside the shack.
This has the following advantages:
-The gain and or filters can be adjusted and optimised in the shack!
-There is no need for a (separate) power supply on the top of the antenna
-No need for a (metallic) box on the top of the antenna
-the electronic parts can be shrinked together with the wire(s) by
shrink tubes (waterproof)
-No need for a coupling-decoupling network at the antenna for the power
supply
- If the use of batteries is an advantage, they can easily be
replaced/charged/measured in the shack!
- A single twisted pair cable (e.g. 1/4 of a CAT5 cable) can be used
-When a tiny fishing rod is used to keep the active antenna apart from
the house, the distance can be increased due to optimised lightweight
:-)
I just have used a single twisted pair cable out of a CAT5 cable (now
unshielded) at almost 30m length between the front end and the amp stage
in the shack. This allows me to get more distance to the tram line which
causes strong QRM on VLF on my grabber antenna. Now, this new antenna is
on the other side of the house.
See the difference between the 2 wideband captures: Old system, bottom
capture of
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber.html
and the new system, bottom of
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber2.html
So, at least on VLF this is a great chance to reduce the local noise.
Maybe some others can do the same since a very small twisted pair cable
is almost invisible and cheep (i've used a 10m cable and soldered the
pairs in series). No matter what the maximum distance might be but i can
imagine 100m will be not the problem! I will check to do the same on LF.
Since the length is still << Lambda i do not expect much trouble.
73, Stefan/DK7FC
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