Re: LF: new VLF antenna at DK7FC

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Hi Stefan,

I am unclear about the connection back to the shack. Are you using three wires (in the CAT5 cable) then to feed power, ground and signal back to the shack from the FET? No isolation transformers etc?

Do you use this also on 136kHz and 500kHz with different filters?

73s
Roger G3XBM

2010/11/24 Stefan Schäfer <Stefan.Schaefer@iup.uni-heidelberg.de>

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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