VLF Loop Antenna

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My first VLF antenna was a coil, like the picture shown on www.vlf.it This coil was connected to my laptop using the mic input. It worked but I wanted to build me a real loop antenna. Most of the designs for a vlf loop antenna consists of several turns together. Others spaced them a few mm's apart. This seems better to me, don't know why, but it's a feeling...

VLF Loop choc blockTo maintain the spacing between the turns I used some choc-block connectors. From experience I would not do it again, it took one complete day to get all the turns thru it. You have to loosen all screws (24 blocks x 2 sides x 4 legs = 192 screws) and your wire have to fit the small holes. Luckely for me, my daughter was offering some help. She liked it...

VLF Loop CrossThe frame is made out of old legs from a table that otherwise ended up in a dumpster. The diameter of the cross is about 80 cm, 1 complete turn is 224 cm long. For 24 turns you need at least 54 meter of wire. The inner loop, mounted 5 cm away from the main loop, has almost no coupling. So it's useless. As you can see on the pictures, the white turn does the job far more better.

VLF LoopVLF LoopThe VLF Loop antenna is ready.
I connected the pickuploop to my Kenwood R1000 and attached a tuning cap to the main loop. After adding more capacitors I could tune the antenna to the NDB band (Narrow Directional Beacons) As I live just a few kilometers from the airport of Antwerp I received the 2 NDB's very well.
Some info about these 2 NDB's :
ONW on 355kHz, distance to me : 7,8km
WW on 309kHz, distance : 2,6 km
PE1ABR has a page with photographs of some beacons.

Took the VLF loop antenna to a friend of me who has a LC meter. The meter showed an inductance of 512 ÁH (microHenry). Time do to the maths.
The basic formula for calculating the resonant frequency :
Resonant Frequency

The second formula explains how to calculate the capacitor for a given frequency (values are given in Farad, Hertz and Henry). The calculated value for 15 kHz is 219 nF (nanoFarad). I took two small 100 nF caps and soldered them parallel.
The Fr with 200 nF is now 15727 Hz. Connected it to the main loop and the pickup loop to my laptop. I picked up very little.
Connecting the main loop to my laptop did the job. Strange, but it works. I tested the vlf loop antenna in house, lots of noise and 50 Hz harmonics but managed to receive 6 signals without major problems. The names in one of the screenshots are taken from Spectrum Lab. It could be that they are incorrect.
Signals from the vacuum cleaner ? VLF loop Spectrogram 2
VLF Loop Spectrogram 3 VLF Loop Spectrogram 4

I should take the antenna outdoor for some testing. Some day....
Remarks, comments, ... always welcome via the Contact form

73's Joost, ON2BBP