Ed - KI6PSP: Lightning

I have discovered that lightning causes interference to AM radio receivers from a great distance. This is due to the way the AM recievers operate, Amplitude Modulation, and the spark of lightning causes a wide band pulse of energy around 500Khz. This means that while I am out chasing storms, I can listen to the AM radio around 530Khz, and determine the relative activity of the storms around me.

The interesting part of this, I can use the radio to listen to a storm even though it is not producing Cloud-To-Ground (CG) lightning. The radio can also recieve the Cloud-To-Cloud static discharge as well. The only problem is the radio and antenna of the car is Omni-Directional (I receive the signal from all directions), so I cannot use the radio to determine where the stronger storms are around me.

I started looking online for Lightning Detectors, and what commercial products I found were either expensive, not informative. There are some portable lightning detectors on the market that will estimate the time a storm will arrive, based on the same AM Pulses. Other detectors are dependent on a computer with expensive software, and it uses other similar stations to triangulate the location of a lightning strike.

I started thinking about Doppler Shift Radio Direction Finding, and even asked a local Elmer if I could use the same technology tuned for the 144MHz band on the 500KHz lightning pulses. This was ruled out due to the need to adjust the antenna size and spacing to achieve what I wanted to do, making this a non portable project.

Directional Lightning Detector
below my mobile HAM Radio
APRS and NOAA WX

The Elmer did suggest an alternative technique for radio direction finding, and that was to use the signal strength. This is what most people do when they first go Fox Hunting (Radio Direction Finding), they put a handheld radio against their body, and while rotating, they observe the signal strength meter. Using the body as an attenuator, you find the dip in signal, and the signal is coming from directly behind you.

I came to the conclusion that I could build four Lightning Detectors, and build a four element antenna so I could determine direction of lightning activity. I found some diagrams online for Lightning Detectors, and placed a parts order with Mouser.

I started building the recievers into a box, and used a 5v regulator for each receiver. I had some issues with two of the receivers, and that required some troubleshooting and replacing a few blown components. I also noticed some interactions between the receivers while testing, so I added a diode to the ground of each reciever and that seemed to isolate the recievers from each other.

Built receiver box

I salvaged an IDE connector from an old motherboard and used a computer case wiring harness to enable me to disconnect the front panel lights to work on the unit if necessary. While I was building the units, I added the Red LEDs to each receiver for testing purposes, but I didn't bother to remove them once complete.

Bearing indication of lightning

Testing each receiver is easy, simply take a multimeter on conductivity test, and when you test between the GND and the antenna lead, the circuit should activate the LED for a moment. The lightning simulator from TechLib is also useful, but a similar can also be done with a BBQ sparker or an Aim-N-Flame. A small spark will cause a similar burst of interference as a bolt of lightning. Because of the small size, the sparker test stops working when more than a foot away from the antenna.

Next, I started building a simple antenna array for the recievers. I used a tupperware container, and wrapped cardboard with aluminum foil. I double stick taped the foil so it would stay on the cardboard. I created a small channel under the bottom square, so I would not damage the RG-174/U Coax. I placed two hard drive megnets inside the case, under the base square of the antenna, so I could Mag Mount the antenna to the top of my car.

Ground wire, and magnets on bottom

Before securing the base of the antenna to the case, I poked a hole in the center of the case, and ran a ground wire. This would allow the antenna to use part of the car as a ground plane. This might not be the most efficient design, but I am not transmitting with this unit, only recieving.

Antenna Elements

Next, I started building the ellements. I took the ground from each coax, and soldered them together in the center. I added the cross to isolate each antenna. This design would give me an eight way direction of the storm, in theory. I also added a ground wite to each of the crosses, and soldered it down onto the base. I checked that I had a good ground with a multimeter. Next, I took a few feet of wire for the element and attatched it to the cross with more stick tape.

Here is the completed antenna. Now if I can just get people to accept it, instead of yelling at me, while driving 70 MPH, that I "have something on the roof". I think a gray can of spray paint should do the trick.

Now if I can just get a few thunderstorms in the area.

Ed KI6PSP http://KI6PSP.blogspot.com

This is not directly Ham Radio Related, but Radio theory does apply. Some of my hobbies include Photography and Storm Chasing. I chase storms from a safe distance, and capture lightning as seen in this picture. While chasing storms, I use the AM radio in my car, tuned to the lower part of the band where it is quiet. I can hear the interference generated by the lightning strikes through the AM radio as it is susceptible to the wide band low frequency pulse from the lightning. This method helps to give me an indication of how active the storms are. The Pops I hear on the AM radio are generated by both Cloud-To-Cloud and Cloud-To-Ground strikes, so this can be useful in determining if charges are still building within the storms nearby. One problem with the AM radio, it hears the lightning strikes from a good distance away, but it doesn't give me any direction information.
	With this in mind, I began researching a lightning detector that would react to the same low frequency pulse. I found a few diagrams online that would listen to the AM burst around 300KHz generated by the lighting. I ordered a bunch of parts, and spares, to build a Directional Lightning Detector. After I built the Directional unit, I used the spare parts to make a Portable Lightning Detector. When a pulse is detected, the red LED will illuminate for about a half second. I added a low voltage vibration motor to this unit so I could have it on my side, and be alerted to lightning in the area. This type of vibration motor can be found in old pagers and cell phones. I plan on adding a 75db 3v buzzer to this design, so I can switch between an audible or a silent alert.
Here is the inside of my Portable Lightning Detector. The unit operates from 2 AA batteries. The White switch is for power, and the silver switch is for which alert, audible or vibrate. I was going to use the small speaker in this picture, but I am opting for the buzzer instead.
	I used a 2m/440 Amateur Radio Antenna for this setup, onto the SMA jack I mounted onto the case. Sensitivity can be improved with a longer antenna if needed. This size of antenna should make this unit sensitive for what I need, while keeping the length to a manageable size. Current draw while on standby is about 5mA. When the vibrate motor starts, the draw spikes to about 50mA. Based on 2500ma AA batteries, I estimate a runtime of about 300 hours or 12.5 days of standby time.