STUDYING METEORS BY RADIO

Donald Sieber

It is well known that when meteors enter the upper atmosphere of the Earth, the gas through which they pass is ionized as the meteor loses some of its kinetic energy. The resulting ionization trail becomes a good reflector for a certain range of radio frequencies. Both amateur and professional scientists detect and count meteors based on these reflected signals. From this data they can determine the times and dates of old and new meteor showers.

When I first tried listening for radio signals reflected from meteor trails, I was not very successful in verifying that meteors caused the signals I detected. Instead, I was hearing a multitude of noises that I couldn't identify. I tried the frequency bands that others used, like the FM radio band (88 to 108 MHz). I used radio receivers of all types, including television and radio FM receivers, all without success. I finally decided that in order to get into the meteor receiving business, it would be necessary to upgrade my equipment. This required considerable planning, but it was well worth the money and effort.

A Receiving System for Detecting Meteor Trails

A good communications receiver was the first order of business. I selected the ICOM IC-PCR1000, which I purchased online for around $400 US. (Try www.ebay.com.) The PCR1000 receiver is nothing more than a little black box of circuitry about the size of a paperback book. The rest of the receiver is displayed on a computer equipped with Windows 95 or higher and a serial port connection. The computer link is the way the receiver is controlled. I found a great little freeware program called TalkPCR 2.4F2, which can be downloaded via the web. It runs on any windows version and has the added advantage of setting the receiver specs via the serial port. You may then shut down the program and use the computer for other things as long as you don't access that serial port while the receiver is working.

I tuned the receiver all over the FM band to no avail. I then pointed a fringe TV antenna toward Salt Lake City Utah (from Boise, Idaho) in an attempt to pick up TV channel 5 from that city. Normally it is too distant for direct reception, but, if a meteor caused a reflective path back down to my antenna, I should hear channel 5 for the duration of the reflective layer (seconds to minutes). Again, nothing.

I then set the frequency of reception of the ICOM to 77.2495 MHz and the bandwidth to as narrow as I could. The receiver was set for USB (upper side band) reception and all of a sudden, there they were! Signals of around 500-hz audio that could only be from a sporadic event, like a meteor. Boing-boing!

I had tuned the ICOM receiver 500 hz below the picture carrier transmitted by the Salt Lake station and then listened for the beat audio frequency between the upper side band and my receiver's local oscillator. When a meteor struck the upper atmosphere, the TV signal, which normally would have left Earth, was reflected back toward
my antenna.

More Success

Since Boise has some pretty noisy atmospherics and lower frequency meteor strikes are stronger, I tried 500 hz below the picture carrier of channel 3 at 61.2495 MHz. Since there is no channel 3 broadcast in Boise, but there are in many other cities in Western States, I began picking up meteor trails from all around. Since not all the stations were precisely on channel 3, I could hear what is best described as wind chimes--slightly different pitches depending on which city's signal was being received. Sometimes I would hear one station activate and then another a second later. This indicated that the meteor trail was long enough to pass over another nearby TV station. The meteors were playing a tune for me.

I had read that meteors were the most energetic in the morning, because that's when our part of the Earth is moving with our motion around the Sun. Meteor velocities, which are already high, now have this added velocity. During meteor showers, the meteors appear to come from a particular part of space called the radiant. These meteors are remnants of an Earth passing asteroid or comet. If your hourly meteor count is greater in the morning than the evening, then you are probably receiving meteor signals.

Improving the Receiving System

There are several things you can do to make the counting process a little easier. I use an audio filter purchased from Ramsey kits for around $35. The filter selects only a narrow range of frequencies (250 to 750 hz) and cuts out almost all atmospheric noise. You can, in effect, select which TV stations you want to receive. Sometimes weather fronts can make a reflective layer that persists for hours. Sporadic E layers from solar storms can also cause continuous reflections. The audio filter can tune these signals out. I also use filters to tune remote multiplexed VHF seismic stations developed by a local university. So the filters have a number of uses.

Now that you have a meteor signal coming from the speaker of your receiver or audio filter, you will want to save the count for yourself or a meteor organization. One PC program that is useful for this purpose is called SKYPIPE. It is a freeware download that makes a nice graph of time vs. amplitude by way of your computer's sound card. You can count meteors over any time interval you want SKYPIPE to run. I'm a bit too lazy to count meteors this way, so I spent more money for an 8-channel analog-to-digital converter card (PCL-711S from PC Multilab). Using a QBASIC program, the signals can be automatically counted and their strength and duration recorded.

Conclusion

This article describes my system and how I got there. I hope you will find it useful.