From: [email protected] (Christian Steyaert) Newsgroups: rec.radio.amateur.space Subject: Meteors by radio: the FAQs Date: Tue, 25 Jul 1995 21:05:20 Organization: V.V.S. Distribution: world NNTP-Posting-Host: vvs.innet.be Keywords: meteors, VHF X-Newsreader: Trumpet for Windows [Version 1.0 Rev A] Hopefully still in time for those who want to observe the Perseids by radio, this year when the Moon hampers the visual observations... METEOR SCATTER - the FAQs Rev. 1, Jul 25, 1995. 0. About these FAQs These FAQs are the result of your various questions and remarks to the monthly RMOB (Radio Meteor Observation Bulletin). Constantly amateur astronomers, radio amateurs and schools looking for new projects, get involved in observing meteors by radio methods, and need the same basic information. 0.1. Revision / posting frequency I am sure there will be many questions and remarks to this first issue. The section with references has to be expanded considerably. And undoubtly, we will hear from existing radio setups for the first time. Rev. 2 can be expected at the end of August. 0.2. Distribution These FAQs are posted on: sci.astro.amateur rec.radio.amateur.space Compuserve's Astronomy Forum, Section 11, Radio Astronomy. Please let me know if you feel that other newsgroup are interesting too or more appropriate. 0.3. Acknowledgement Many thanks to Tom Ashcraft ([email protected]) and other participants to Compuserve's Astronomy Forum. 1. What is meteor scatter (MS) ? Meteor scatter is a form of electromagnetic wave propagation. The ionized trail of meteors (typically appearing between 100 km and 80 km high) acts tempora- rily as a reflector for radio waves. A meteor does NOT generate detectable radio emission itself! In the case of forward scatter, the transmitter and receiver are at different locations. Backscatter can be seen as a special case, whereby the sender and receiver coincide (typically the case of radar). Meteor scatter can be used day and night, allowing daylight meteors streams to be studied. Meteor scatter can also be used for secure communication. Historically, meteor streams were conclusively detected by means of radar at the end of World War II. 2. How to observe MS ? Tune to an "empty" frequency in the VHF band, and wait until a suitable oriented meteor trail reflects a distant broadcasting station. You hear a more or less short (a fraction of a second to tens of seconds) fragment of the remote broadcast. Occasionally, more than one station (together or one after the other) can be heard. Routinely, stations of 300 km to 1000 km away can be observed. The problem however in densely populated areas can be to find a non-locally used frequency. 2.1. Which frequencies to use ? The lower the frequency, the better... but at too low frequencies, the radio- waves bend and follow the curvature of the Earth. A lower practical limit is 40 MHz. More commonly, the VHF is suitable. VHF Band I ranges from 41 to 68 MHz, and is used for TV. The FM band (88 MHz - 108 MHz) is also used frequently. Try as low as possible in the band, and avoid free radio's in the upper band (104-108 MHz). Not to forget are beacons at e.g. 75 MHz. Radio hams listen also to MS in the 2 m (144 MHz) band. The reflections are shorter and weaker than at the lower frequencies. There is little use in trying MS at still higher frequencies. 2.2. Which equipment to use ? A regular FM receiver is sufficient. An all-band receiver is of course better in terms of sensitivity and features such as bandwidth selection. Connect a horizontally polarized Yagi antenna with 4 to 6 elements (not too directive) to your receiver. The antenna can be pointed horizontally, or at a certain angle (the latter avoids direct reception from nearby stations). Keep the antenna feed cable short to minimize losses, or add an antenna amplifier. With a good setup, it should be easy to obtain 10 to 20 meteor reflections per hour when no stream is active (the sporadic background). During streams, this number can get as high as several hundreds. Due to the daily variation in meteor activity, more reflections are received during the morning hours (5h to 6h local time) than in the evening (18h). This can serve as a proof that you are actually observing meteors! 2.3. Always possible to observe ? Unfortunately, other forms of propagation interfere with MS. The worst one is sporadic-E or Es, consisting of conduction clouds in the high atmosphere that make permanent reception of remote stations possible during minutes to tens of minutes. The sporadic-E season is from May to July in large parts of the northern hemisphere. In some regions however, sporadic-E is unknown! There are also tropospheric influences. A temperature inversion can also cause reception over wider ranges than normal. Thunderstorms cause very sharp peaks. On FM, most of these (amplitude modulated) spikes are suppressed. However, DO NOT connect your antenna to your receiver during thunderstorms or when you are absent for longer time. Lightning strikes have ruined several radio shacks. 2.4. One step further: recording meteor reflections Meteor reflections can easily be recognized by listening. But a better measure of the shape of the reflection can be obtained in accessing the signal strength and recording it, either on a pen recorder, either feeding it into a computer via an analog/digital (A/D) convertor. Special care has to be taken to shield well the computer, power supplies etc., or the computer signal will be more or less noticed in the receiver. Meteor reflections can broadly be divided in two classes - underdense and overdense - with different reflection profiles. These profiles can be related to the physical characteristics of the meteors. 3. Who else is observing ? During recent years, amateurs in following countries have been recording meteor reflections: Austria, Belgium, Denmark, Germany, Finland, the Netherlands, the UK, the USA. Some of them observe only during the periods of the great streams, others run continuous automated stations. Only a few radars are still used for monitoring streams. One is run by O. Belkovitch ([email protected]), at Kazan University. 4. How to relate radio- to visual observations ? Hearing and seeing a meteor is a unique experience. There is a fairly simple relation between the visual magnitude of a meteor and the duration of the reflection. At 70 MHz, a zero magnitude meteor gives typically rise to a 1 second reflection. 5. How to correct MS counts to standard conditions ? The number of meteor reflections that will be observed from a certain stream is not a simple function of radiant height, as is the case with visual counts (cfr. the zenithal hourly rate). Conversely, deriving the true number of meteors from the observed number of meteors is not a trivial matter, certainly not when more than one transmitter is involved. Certainly, a stream does not necessarily peak when the observed number of meteors is highest! The various parameters, such as antenna characteristics, position of transmitter and receiver, radiant position and other stream characteristics, power of the transmitter etc. can be fed into a simulation program, such as FORWARD (by the undersigned). Although the general activity pattern can be reconstructed, one has to be careful. For visual meteors, the observed number is highest when the radiant is in the zenith (all other remaining the same). This is not so for the number of meteor reflections: the maximum number is observed for a radiant elevation of 45 to 50 degr. When the radiant is too high (say more than 80 degr), very few reflections are received! 6. Where to find meteor stream data ? The International Meteor Organization (IMO), specializing in visual meteor observations, issues a yearly meteor stream calendar, which can be found on several places on the net. Contact otherwise Juergen Rendtel ([email protected]) or Peter Brown ([email protected]). Gary Kronk's home page [http://wums.wustl.edu/~kronk/index.html] gives also a good overview of the various streams. 7. Where to find observational data and reduction software ? Ilkka Yrjola, OH51Y ([email protected]) makes his data available on ftp server FTP.FUNET.FI in directory pub/ham/vhf-work/mssoft42.zip. The software for viewing is available too, as well as stream data. The author edits the monthly RMOB, which gives summaries of recent observations from all over the world. Included are the setup characte- ristics. Just contact me for putting you on the distribution list. 8. References 8.1. Books THE reference is still "Meteor Science and Engineering" by D.W.R. McKinley, McGrawHill 1961. A fantastic book, giving the theory and practical results of the heydays of the radio meteor science. A recent, less astronomical and more communication oriented work is: Meteor Burst Communications (Theory and Practice) Ed. Donald L. Schilling, Wiley Series in Communications, 1993, ISBN 0-471-52212-0. 8.2. Periodicals The regular astronomy magazines, such as Sky & Telescope, Astronomy, or Astronomy Now! run occasionally contributions about forward scatter experiments (due to lack of time: complete list in Rev. 2 of these FAQs). WGN, the Journal of IMO, contain sometimes contributions and letters about radio observations of meteors. ------------------------------------------------------------------------- Chris Steyaert Tel : +32 (14) 31 51 04 V.V.S. Astronomical Association Fax : +32 (14) 22 13 73 Belgium E-mail : [email protected] Kruisven 66 Compuserve: 72650,3513 B 2400 Mol, Belgium 51 degr N, 5 degr E -------------------------------------------------------------------------