(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 




More information about the Meteorobs mailing list