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Re: (meteorobs) Radio Meteors (long)
Hello folks,
I considered answering Conrad's question privately, but because his
question is so common I thought I might post to the entire list with my
answer. If this becomes a bit lengthy, press ye olde DELETE key. In
addition to the below answer, the AMS WWW site
(http://www.serve.com/meteors/) contains additional information and several
links to related sites on radiometeor scatter.
While the standard AM band (560 - 1600 kHz) cannot be used to monitor for
meteor echoes, the commericial FM band (88 - 108 MHz) can be used,
provided that a site is free from band congestion and local RF
interference. This band is the one most commonly used by beginners due to
the relative ease with which meteor echoes can be received. Experience has
shown that such as setup will generally only work in the more remote, or
rural parts of the country, but there can be exceptions.
Below is an excerpt from AMS Bulletin No. 203 (revised), "The AMS
Radiometeor Project." This is a 50 page illustrated "how-too" manual
published by our organization.
begin excerpt>>>
4.2.7 FM Commercial Band (88-108 MHz)
While these frequencies are not as efficiently scattered as
are lower ones, the FM band contains a proliferation of potential
transmitters for use in this project. But this wealth of stations
leads to considerable band congestion, and finding a suitably open
frequency slot may be very difficult, especially in the more
populated areas of the country. Channels in the FM commercial band
are assigned in 200 kHz increments from 88 to 108 MHz, using the
odd numbers, such that stations will be located at the x.1, x.3,
x.5, x.7, and x.9 fractions of the MHz frequency. For Radiometeor
work, the selected frequency should be as low in the band as
possible. The participant should look for three "clear" channels
in a row, with the middle clear frequency used to monitor
radiometeor reflections. Channels adjacent to local FM stations
are not suitable due to "bleed over" from the nearby occupied
channel.
With the high density of FM stations on the band, it can
generally be assumed that any clear channel will have multiple
stations available for meteor scatter, in a variety of directions.
A couple of things should still be kept in mind, however. The
desired distance to useful transmitters is to again be less than
1500 km (about 950 miles) but more than about 300 km (about 200
miles). The operating schedules for target transmitters should
also be considered. For example, most of the transmitters in the
lower end of this band are NPR stations, which often sign off late
at night just as meteor activity is reaching its height. Searches
for suitable FM stations are much easier if an "FM Atlas" is used.
Another advantage of this band is the great availability and
variety of relatively inexpensive equipment which can be obtained.
Survey equipment for this band should consist of an outside
mounted, steerable (manually or electric) log-periodic VHF antenna;
[Note: Another popular antenna is a 10-element Yagi designed for the FM
band and sold by Radio Shack for about $20.]
a store bought FM pre-amplifier, usually about +20 dB; and a good
quality FM receiver with digital display. Using this equipment
alone, meteor bursts from overdense meteor trails can be monitored.
For purposes of the survey, it is preferable to listen to the
demodulated intelligence (music, voice, etc.) from the signal in
order to aid in identifying the distant stations. For actual
receiver system establishment, it is preferred that participants
utilize a high quality receiver capable of operating in the CW or
SSB mode on the desired frequency. Alternatively, it may be
possible to utilize a high quality, digitally controlled standard
FM receiver if the AGC or Power Level meter circuit voltages are
tapped for input to the data collection computer. Again,
experimentation will need to be done to determine the best
equipment setup.
The FM band also has the advantage that it may be possible to
select several suitable stations for meteor scatter in various
directions, and optimize the radiant-to-transmitter aspect for a
particular major meteor shower. As a general thumb-rule, shower
detection by a forward-scatter system is optimized when the shower
radiant passes through an azimuth which is perpendicular to the
transmitter-receiver baseline azimuth. As an example, the Geminid
shower radiant, in the northern hemisphere will, roughly speaking,
rise in the east, pass directly overhead, and set in the west. For
optimal detection of this shower, the transmitter-receiver baseline
for a system should be perpendicular to this, or have a roughly
north-south azimuth. In addition, radiant altitude also plays an
important role. Generally speaking, forward-scatter systems
function best at radiant altitudes of 30 to 60 degrees, with peak
detection at about 45 degrees altitude. Above or below this
radiant altitude range, shower detection drops off rapidly. Thus,
the Geminid shower radiant mentioned above would generate two
distinct system shower maxima as it moved across the sky, rather
than a single peak. Optimum directions for transmitter-receiver
baseline azimuth are list in Appendix II for the major meteor
showers.
It is important to remember that because of the high
sensitivity of forward-scatter systems to radiant directivity (both
in altitude and azimuth), meteor showers detected in this way will
create detected shower maxima which do not correspond to the actual
shower maximum. For showers which occur over a period of several
days, the radio shower observer will notice a system shower maximum
occurring at roughly the same time(s) each day, with peak detected
activity occurring within the same 24 hour period as the actual
shower maximum. This gives a single forward-scatter system an
accuracy of only +/- 12 hours with regard to detecting true shower
maximums. To detect true shower maximums using radiometeor data,
data from several systems must be analyzed simultaneously.
end of excerpt>>>
The geometry of forward-scatter is discussed in greater detail later in the
manual, but the above serves to introduce the basic ideas -- especially
with regard to showers. I hope this has proven helpful.
Take care,
Jim
James Richardson
Graceville, Florida
richardson@digitalexp.com
Operations Manager / Radiometeor Project Coordinator
American Meteor Society (AMS)
http://www.serve.com/meteors/
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