VLF Receivers for SIDs

VLF Receivers for SIDs

This page has information about building VLF Receivers to detect SIDs & Natural VLF Emissions
Two types of receivers are reviewed in some detail, with other types mentioned.

1 VLF Loop Antenna Receivers

2 Mu Metal & Ferrite Cored Antenna Receivers

VLF Loop Antenna Receivers
A simple loop antenna is cheap and easy to construct and can be easily connected to a high gain audio amplifier as a receiver.

A block diagram of a typical system is shown below.



The antenna can be constructed in many ways. One example is shown below where a 1m by 1m loop is wound around wooden pegs supported on a wooden cross frame. The loop can be wound from any wire, but reasonably thick copper wire (about 1mm diameter) will give good results as the coil resistance will be low.

The wire should be insulated with plastic covering, or enameled wire can be used.
In the example shown below 40 turns of wire are employed. The number of turns, the diameter of the wire and the area of the coil determine its Inductance - but also determines the resistance of the coil. This should be kept low (a few ohms is acceptable) and the self capacitance of the coil should always be as low as possible. Thus close winding of the turns should be avoided. If the self capacitance is high, it will lower the loop self resonant frequency - perhaps reducing the output voltage at high frequencies ( say 10 to 24 kHz).



A typical loop antenna for VLF reception
A good example of a loop with widely spaced turns for very low self capacitance is shown below.

This is a very low impedance loop wound with thick wire and would require a coupling transformer to connect it to the amplifier.

Antenna design by Lloyd Butler VK5BR
[http://users.tpg.com.au/users/ldbutler/VLF-LFLoopAerial.htm ]

An example of a closely wound loop antenna with side of 1.5m long, designed for outside use is shown below.
It has a 40dB amplifier which can be tuned to place the antenna resonance at a number of different frequencies, depending on the users choice.
The battery powered amplifier is inside a weather-proof plastic box and is mounted on the antenna framework.









The output from the loop, when suitably amplified can be presented as a spectrum from 10Hz to 24kHz using Spectrum Lab software. See the figure below.


There is usually a deal of man-made interference below a few kHz, particularly mains harmonics, motor noise and earth loop currents from power systems. From about 16kHz to 24kHz there are a number of VLF transmitter signals. These vary with location around the world. Typical signal to noise ratio for these signals is ~ 40 dB. This is more than adequate for use in detecting Sudden Ionospheric Disturbances (SIDs) due to Solar Xray bursts.





A brief introduction to the theory of loop antennas and electromagnetic induction can be found by opening the Power Point presentation { EM Induction.ppt } at the base of this page.

A second presentation gives an introduction to AC circuit theory and resonant loop antennas.
Please open file { AC Circuit Theory.ppt } at the base of this page.

VLF Amplifiers

A suitable audio frequency amplifier for use with a VLF antenna is easy to build. A gain of 40 to 60dB is ideal and a bandwidth of 10Hz to >24kHz is useful.

The 24 kHz figure is set by the bandwidth and sampling rate of the average PC sound card. If you have an advanced card with sampling rates of up to 192,000 samples per second you amplifier needs to have an extended bandwidth up to 96kHz.

For most SID and Natural VLF measurements the 24kHz amplifier / sound card is quite adequate.

There are many designs of suitable amplifiers on the internet - and most use Operational Amplifiers (Op Amps) which are cheap and easily available. Some amplifiers have Field Effect Transistors (FETs) as a first amplifying device if very high input impedance is required. A compact amplifier that can be integrated with a loop antenna - or a Mu metal / Ferrite antenna is shown below.

This design was used in a School STEM project called STEP - Solar Terrestrial Environment Physics.







A low noise version of the amplifier using an OP27 OpAmp is shown below.



The layout of the initial amplifier design is shown below. This was kept as simple as possible so the students with little experience of circuit boards and soldering techniques would be able to construct this amplifier.




The basic circuit for an amplifier based on Op Amps is shown below





Mu-Metal and Ferrite Cored VLF Antennas & Amplifier


Firstly we will describe the Ferrite cored VLF antenna for the school project STEP.
The aim of this project was to construct a compact antenna / receiver suitable for the detection of SIDs that could be simply placed on a bedroom window sill for example and operated 24 hours a day to collect SID events.
In addition the device had to be simple and cheap to build.


The insertion of a high permeability ferrite material made into a rod inside a small multi turn coil, can produce signals as strong as a 1m square wire loop antenna.







A 6 inch long ferrite rod Ferrite rod with coil

In the compact receiver a coil of a few hundred turns of 30SWG enameled copper wire is wound on to a paper bobbin and set on to the ferrite rod. The antenna road is fixed to the circuit board which has a suitable multi-way connector on the other end. See the layout diagram below.



The whole is assembled inside a waterproof plastic tube with suitable ends as shown in the sketch below.











The actual antenna and circuit board are shown assembled as below. The outer plastic cover is ~ 40mm diameter.
The end connector is a 9 Way D Type.




The final VLF receiver for SIDs.




The polar response of the completed receiver was plotted with the equipment mounted on a large protractor as shown below. One of the strong VLF transmitters was used as the signal source. Its amplitude was measured as a function of antenna rotation.

The measurement procedure was almost identical to that for a large loop antenna which is described in the file
{ MEASUREMENT OF VLF LOOP ANTENNA.doc } at the base of this page.

A typical dipolar response diagram is shown below.



Finally a few pictures of students in the Laboratory, building the VLF SID receivers.

Original.