CHAPTER 5:
OBSERVING TECHNIQUE - RADIO

Currently, monitoring radio meteor activity is beyond the scope of NAMN. But to those who wish to investigate it, the following should prove to be a start.

Monitoring meteor activity by radio got its start right after World War II. It was then found out that meteors could reflect radio signals. There are two ways that radio meteors can be detected. One is known as back scatter. This is monitoring with radar equipment and is mostly in the realm of professional work. A simpler method, and one that amateurs can participate in, is forward scatter.

It has been learned that when a meteoroid enters the atmosphere, it produces an ionized column of gas molecules. This ionized gas has the ability to reflect radio signals between a transmitter and a distant receiver. The frequency range that this occurs at is between 40 and 150 MHz. Although the optimum lies between 40 and 70 MHz, the common FM band is frequently used for back scatter work. This FM band is between 88 and 108 MHz.

The technique used to register forward scatter meteors is actually quite simple. A receiver is tuned to a distant radio station that is located below the observer's horizon. The distance between the receiver and transmitter varies from between 200 and 1000 miles. Most often a commercial radio station is used as a transmitter. Aircraft ground beacons are also suitable transmitters. The station may not be heard, usually only background noise is apparent. At the moment a meteor appears at the correct angle, there is a short contact (usually 0.1 to 4 seconds) with that radio station. For larger meteors, it is possible to have longer lasting signals, often up to minutes in duration. This contact gives itself away as a signal increase in a piece of music, voice or noise. Since this technique uses electromagnetic waves, it can be used during the daytime, when there is a bright moon, or during cloudy and rainy weather. Therefore, this method is ideal for continuous periods of observing.

The best FM radio to use is one that is digital and that has a shielded cable connection where the antenna plugs in. The digital type of radio is easier to set on the desired frequency. A Yagi style FM radio antenna is also required (at the time this was written, Radio Shack sold suitable antennas for about $16 and local TV antenna companies sell a more directional FM radio antenna for about $150). Unfortunately, which type of antenna to use depends on trial and error. It would probably be best to try the lower cost antenna first. Assuming that this type is used, hook up the antenna to the radio with a coax cable.

To find a frequency, try them one at a time from 88.0 MHz to 108.0 MHz. Write down all the frequencies where there is no music or talking. All that should be audible is static noise. Be sure to turn the antenna all the way around (360 degrees) to find this static area. Do not direct the antenna very high, because in most places in the United States, the antenna may receive unwanted, continuous reception.

If you're lucky, you'll find several or more frequencies where nothing but static is heard. These are the potential useable frequencies. For those who are unable to find any, there are two options. Move your radio equipment to another location, preferably in a valley and many miles away from the city (30 or more). Another alternative is to buy the more expensive, more directional, and most likely, larger (12 foot long antenna), and try all the frequencies again. If the directional antenna still doesn't work, try another location and/or consider a different frequency band.

On useable frequencies, it is probable someone is transmitting on it at least 200 to 1000 miles away, but at an unknown direction. The only way to determine the direction is to go to the local library and find a book that lists frequency and station locations. If the library doesn't have such a book, it may be possible to find it by looking in the phone books of different cities located within 200 to 1000 miles. Ideally, choose a station that transmits over 30 kilowatts and is located about 300 to 500 miles away.

Next, find your location and the location of the transmitting station on a suitable map. Taking geographical north as being 0 degrees, find what degree angle the transmitting station is in relation to the observing location. This is the transmitting station's azimuth.

Then, with your antenna pole at the middle, use a compass to find true north. Be sure to do this with the pole and other metal out of the way or a false reading will result. Turn the antenna to point toward the transmitting station. Using a protractor, tilt your antenna so that it points up at a 45 degree angle instead of the traditional horizontal angle.

Now, just listen to all the static. When a meteor passes by, it may or may not (depending on the angle among other things), produce a signal. The radio can detect meteoroids down to 8th magnitude at least, with most being very short signals on the order of 1/4 second. They are actually small segments of what's being transmitted. They sound like bumps, thumps and chirps. The longer signals will register as pieces of music or talking. These are usually very sudden, loud and clear, and will begin and end abruptly. Aircraft can interfere when they fly nearby, but they usually produce signals which are gradual before getting real loud.

During observations, it is advisable to use one frequency exclusively with the antenna always pointed in the same direction and at the same elevation. This way, day to day monitoring establishes a reliable pattern that can be compared. On average, expect to hear the least amount of meteor activity around 6 pm local time and the greatest activity around 6 am local time. A typical hourly rate around 6 pm is about 7, while at 6 am it may increase to about 60 or so. Of course, a major meteor shower could change these rates.

One recording method is to purchase a small hand counter and simply press the button for each signal heard, regardless of their duration. If the signal is 1 second or longer, note them separately on paper. Do this for half hour increments during each observing hour.

Interpret signals lasting more than 5 seconds in duration as being most likely caused by a visually bright meteor. It appears that longer signals are produced by brighter meteors (-1 magnitude and up) rather than fainter ones. Also, it's been noted from some observers that signal durations tend to differ from frequency to frequency. Signal durations tend to not be as long at the FM band level as they are with frequencies near 50 MHz. Try to observe visually with a speaker close by. Sometimes, it's possible to get simultaneous events which can be noted both visually and by radio.

Actually, it is possible to listen for meteors any time of the day or night. Generally, the most active 12 hour period is from midnight to noon. Major shower activity usually produce exceptionally high hourly rates both audibly and visually. The radio can quickly become so saturated with radio reflections that it produces continuous radio reception. At this point, it's almost useless to maintain hourly rates. But fortunately, these moments are exceptions to the rule. Observing periods will have the greatest scientific benefit if they are carried out in one hour increments. Periods shorter than this become unreliable.

There are meteor showers that are almost exclusively monitored by radio. These are known as the daytime showers and usually peak shortly after sunrise. The IMO's list of daytime showers include:

H. REPORTING OBSERVATIONS

As mentioned above, the NAMN is not equipped to handle radio observations. Instead, we recommend sending your data to:

It is recommended that observers contact him first so that he can specify the format data should be submitted in. Also, he will need to gather the required information about your location, radio set up, etc.