The Very Low Frequency Receiver is a Software Defined Radio (SDR), which makes it possible to receive radio frequencies between 0 Hz – 96 kHz. There is merely a need for a sound card with a line-in input and a “Magnetic Loop” antenna, build from a 0.5 m square frame with 40 windings of enameled copper wire.
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In a second step a simple to build pre-amplifier enhances the systems sensitivity.
Educational school and university projects are eligible to get the software for free.
- Windows 7 – 10 / 32 & 64 Bit
- Mac OSX Lion 10.7.3 – Sierra 10.12.x
Frequency Range: 0 Hz – 96 kHz (depends on the sound card used)
Best Frequency Resolution: 90 mHz
Selectable Spectrum size of up to 2^19 samples
Audio Monitoring through configurable Band-Pass Filter including gain
Highly Efficient Hum-Filter for 60 Hz and 50 Hz power line frequencies
Display-Bandwidth selectable down to a minimum of 94 Hz
Spectrum, Averaged Spectrum and Spectrogram
Longest integration time for the averaged Spectrum is 99 minutes
Different Spectrogram colors and speeds
VLF-Transmitter marker for known stations
VLF-Transmitter signal strength tracking for solar flare observation
Automatic periodic saving of the Spectrum & Spectrogram as picture
The VLF Receiver can be fully tested before purchase. Only it stops after 2 minutes and needs a klick on Start.
The activation allows 3 parallel installations. In fact, it is possible to use the Windows and MacOS X Version in parallel by just using one activation key.
WHAT IS VLF-RECEPTION AND WHY SHOULD I CARE?
The Reception of VLF-Signals can be basically split up into three categories:
- Lightning Strikes (Spherics)
- Distant strong lighting creating radio waves that are subject to dispersion (Whistlers).
- Schumann Resonances within the Ionosphere
- Effects of solar flares on the reception strength of distant VLF-Transmitters
The power network and its connected devices radiate strong electromagnetic waves not only at the power line frequency (e.g. 60 Hz) but also on its harmonics up to 4 kHz. Just connect a short wire to one of the channels of the line-in to see the effect.
Most of todays VLF Transmitters are used to do submarine communication because VLF transmissions penetrate water to a depth of 30m.
Some transmitters are used for ionosphere science.
That screenshot of the VLF Receiver identifies a couple of known Transmitters, mainly used for submarine communication.
This measurement shows the electromagnetic radiation caused by a tram driving by our office. The RF emissions are quite strong and broadband.
The VLF Receiver can be launched 2x times to handle two different antenna setups in a 90° angle to cover all directions.
VLF „Magnetic Loop“ Antenna
The antenna used by us has a side length of 0.5 m and uses 100 windings of enameled copper wire with a diameter of 0.6 mm. 25 to 40 windings are definitely enough to build such an antenna. It would be better to increase the surrounded area then the amount of windings.
We are going to provide a plan to build a pre-amplifier to increase the reception sensitivity soon.
We assembled a simple amplifier to enhance the VLF-Receiver sensitivity.
The used operational amplifier OP27 is an easy to get component. This OP has extreme low noise figures and is therefore perfect for our use case.
We are using a symmetrical power supply to avoid any problems with a virtual ground. Two 9 V batteries are perfect for longer operation.
The operational amplifier is used in its inverting mode. Due to the negative feedback the OP keeps pin 2 at ground potential. That provides a low impedance Magnetic Loop Antenna adaptation.
The resistor R1 reduces the amplification for lower frequencies to mitigate the impact of power line hum in urban areas, having more headroom for the analog to digital converter of the sound card.
If the 50/60 Hz power line hum is not an issue at your location then R1 can be omitted.
To find the right resistance value for R1 the impedance of the antenna needs to be factored in. Our Magnetic Loop already has an impedance of 10 Ohm, therefore we have chosen 36 Ohm.
The DC voltage gain is calculated by R2 / (R-Antenna + R1). Having R-Antenna + R1 = 46 Ohm and R2 = 100 kOhm the DC gain will be around 2174.
For higher frequencies the antenna impedance increases and the associated gain decreases. This is a wished behavior because the magnetic loop antenna becomes more sensitive for higher frequencies.
The pre-amplifier can be built on a breadboard, a perfboard or a printed circuit board. The PCB template can be downloaded here.
THE VLF PROJECT
The VLF Receiver is the start of a long-term project to make it as easy as possible to access the complex topic of radio frequency transmission and reception.
Educational school and university projects are eligible to get the software for free. It is our goal to inspire and to arouse curiosity for technical questions.
If you want to use the VLF Receiver for your educational endeavor then just send us an email to firstname.lastname@example.org with a short description of your project to receive a 100% voucher.
We are going to start a first VLF-Receiver school project to have a prototype educational implementation. Furthermore, that project will produce supporting documents that can be used as guideline in future VLF-Receiver projects.