The DF sensor is an original concept and design by Frank Kooiman and thanks to the help of Daniel Verschueren.

This system was developed as a hobby alternative to the existing commercial Boltek lightning detector. The advantages of the lightning radar are the low cost (€40 and up) compared to the Boltek (€350 to €600 depending on the version), the extreme sensitivity of the system, and the possibility of joining the group system via the internet. Where Boltek detectors can detect lightning up to a range of 500km, the LR (lightning radar) has a range of 2000 to 3000km over land and several thousand km over water (e.g. lightning in Florida, south America).
One disadvantage of the LR is that it is not a plug-and-play system and therefore requires some knowledge of electronics and familiarity with a soldering iron. In practice, this is not really a disadvantage since it means that
you learn a lot more about the science of detecting lightning.
The software functions as a single station showing the direction and estimated distance, or connects with other LR stations via the internet to perform a localisation function, displaying the result (specific direction and distance) on a map. In addition these maps can be uploaded to your website.

The electronics consists of an amplifier which boosts and filters the signal from an antenna and passes it to the soundcard of a computer (Line-in).

The lightning strikes are received using a frame antenna set at 10 kHz. At this frequency range the lightning sends impulses over a range of several thousand kilometres. The antenna consists of a frame, around which wire is wound in multiple windings. The antenna measures the magnetic part of a wave and has the advantage that it is less sensitive to interfering electrical fields. With a single antenna, the lightning strike can be detected but the direction cannot be measured. For this reason a second identical antenna is mounted at 90 degrees to the first antenna.

The direction can be calculated from the two signals measured. It is still not possible to say for certain that the lightning strike occurred at one direction, exactly opposite direction could also have been possible (+180 degrees). This is also due to the fact that we do not know if the lightning strike had a positive or negative charge. If you are working with a single station, a third antenna is therefore necessary to detect the charge and therefore the correct direction of the strike. A single station cannot be used to determine the exact position / distance of the strike. This can only be estimated from the strength of the signal, since not all lightning strikes have the same energy. Lightning Radar works in a group of a number of stations and can therefore calculate the correct direction and the position / distance using only 2 antennas.

Now I have in test two ferrite antennas just like the ones used for the TOA system. It's not simple to adapt them to the RDF hardware but some good result is coming :-)

You can find information about Lightning Radar on:
http://www.lightningradar.net
http://lr37.dyndns.org/fr/indexfr.html
 

The TOA sensor is developed thanks to the Egon Wanke Blitzortung.org project.
The lightning location network Blitzortung.org consists of several lightning receiver sites and one central processing server. The sites transmit their data in short time intervals over the Internet to the server. Every data sentence contains the precise time of arrival of the received lightning strike impulse (”sferic”) and the exact geographic position of the site.
With this information from all sites the exact positions of the discharges are computed.
The complete sensor with ferrite antennas is included in a plastic enclosure. The antennas are connected to the VLF pre-amplifier thanks to an RG316 cable. The amplifier is necessary to amplify the lightning signal to be avaluated from the microprocessor. An easy way to realize a VLF pre-amplifier is to use an operational amplifier (op-amp).
After the signal is amplified, it's passed to the evaluation board that make the computations and associates the signal with a time stamp.
The heart of the evaluation board is an Atmel 8-bit AVR micro controller ATmega644 running with a clock frequency of 20 MHz. The board also contains two 10-bit analogto-digital converters AD7813 that operate in high speed mode not powered down between conversions. In this mode of operation the converters are capable to provide a throughput of 400 kSPS.
The pre-amplifier and the evaluation board can be connected by a 1-to-1 shielded cat cable via the RJ45 jacks. This makes it possible to use the same power supply for both boards.
The GPS receiver we need has to provide a one-pulse-per-second (1PPS) output with an accuracy of at least ±1μs, and a serial interface using either RS232 or TTL level. The communication between the GPS receiver and the evaluation board is done with 4800 Baud, 1 stop bit, and no parity. The evaluation board only reads the GPRMC sentence from the GPS output.

The TOA (time of arrival) lightning location technique is based on hyperbolic curve calculations.
The emitted radio signals of a lightning discharge are travelling with the speed of light. These are approximately 300000 kilometres per second. Each received signal is time-stamped at the receiving sites. The time-stamp differences are used for hyperbolic curve generation. The point of intersection of all hyperbolic curves defines the location of the source of the radio signal. The computed position is then be assumed to be the location of the lightning stroke. At least 4 sites are needed to define always a unique intersection of the hyperbolic curves. With more than four receiving sites reporting the same signal there is some redundant information available to improve the accuracy and to verify the performance.