(meteorobs) FM observing question - east or west transmitter?
Bruce McCurdy
bmccurdy at telusplanet.net
Mon Jun 6 16:38:22 EDT 2005
Al Degutis wrote:
> I've done some basic research (make that last minute research for the
Arietids) on FM radio meteor observing. I understand the concept and
am trying to find some FM radio transmitters located at the proper
distance (approx 1300 km, or between 800 km to 2000 km) when it dawned
on me... does it matter where the transmitter is located in relation
to my location?
I've been doing radio meteor observing here in Edmonton for over three
years now with the Sky Scan Science Awareness Project. FWIW, I'm an observer
first and a radio technician a very distant second, but I have had lots of
interesting results.
My home detector, the Northern Claw Radio Meteor Observatory, has been
running almost continuously during that time. It consists of a three element
Yagi (description at http://www.skyscan.ca/3ElementYagi.htm ) on my back
deck, wired through my basement window to a digital radio and on into the
sound card of my computer, where the signal is monitored by Jim Sky's Radio
Skypipe software (www.radiosky.com) . The Yagi is pointed approximately SSE
(there's no point in aiming north from here!) at about 20° altitude. I tune
to 92.1 which has no local station, but there is one in Calgary just 300 km
south of here, and there are a number of other transmitters at various
distances.
A spike on a data file yields no information about its source, and in
any event the key question is not what the transmission actually says but
simply that it is received at all. To paraphrase Marshall McLuhan: "The
meteor IS the message." (Sorry, couldn't resist. :) However, at times of
high activity I often plug in a set of speakers and listen in. There are
lots of clues -- announcer's accent, name of a city, different time zone,
type of music, change of song in mid-signal -- that suggest not all radio
bursts are from the same transmitter. Occasionally I catch a call signal
that unambiguously confirms the source.
By far the lion's share of call signals that I hear come from CJAY 92, a
Calgary rock station, and the majority of bursts I hear are rock music or
obnoxiously loud commercials which are suggestive of that same source. This
is well within the suggested lower limit of 700 km for forward scatter. No
doubt this isn't the optimum setup for maximum sensitivity, but in my
defence all I can say is: it works. During the Geminids' peak I was
consistently recording over 100 "hits" per hour. (Of course, many of these
are underdense bursts which are much more difficult to pinpoint to their
source.)
At the other extreme, during the 2002 Leonids we recorded a minute-long
overdense burst from Classic Country 92 which I determined to be KTFW in
Fort Worth Texas, some 2700 km from here and well beyond the suggested upper
limit of 2100 km.
While maximum sensitivity is obviously desirable, I think perhaps it's
more important that the detection system is consistent within itself. I
compare my results from the Geminids etc. to a background "sporadic" rate
obtained with the same detector tuned to the same frequency during times of
low shower activity, so I'm comparing apples to apples as much as possible.
A more sensitive detector might show a ratio of 200:20 instead of 100:10,
but what more does that prove?
I have had much less success with daytime showers ... there is lots more
radio noise in the daytime, and I find I get a much higher percentage of
overdense spikes which I suspect are reflections from aircraft rather than
meteors.
As for antenna direction, we offset ours about 30° to the direction of
the primary transmitter in Calgary, as suggested in Joseph Carr's book
RadioScience Observing. Any shower radiant will of course rotate right
through this azimuth. I have not conducted an analysis of how the changing
antenna-radiant angle affects sensitivity, although presumably this could be
done with a large data set. Perhaps there is an expert on this list who
would care to address this aspect of Al's question.
regards, Bruce
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