Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

How well can we detect meteors using the Graves transmitter?


Recommended Posts

For the last few weeks I've been trying to get my head around this whole business of meteor detection using the Graves transmitter near Dijon, though specifically trying to detect the ISS has caused me to dig a bit deeper into this. I always find a picture to be more helpful than mere numbers, so I set about constructing a map, which is shown below (courtesy of Google Maps). The Graves transmitter emits in a southerly direction irradiating the southern semicircle at elevations of between 15º and 40º. If we assume for simplicity that the meteor trails occur at 100km altitude, then provided they are within from about 120km to 375km south of the Graves transmitter (assuming I have my sums right :undecided: ) they will be in the radar beam. I've shown this region in purple. The question now is whether they will be detectable.

post-40604-0-54513700-1420108191_thumb.j

If our antenna can detect anything above the horizon, then provided the meteor trail is within 1100km or so, they should be visible to the antenna. I've shown that as R2 on the diagram (from where I live in the south of England). I also wanted to see how the effect of limited visibility near the horizon would affect this, so I re-calculated the range as if I was only able see down to 5º above the horizon; this is shown as R1. Of course, I'm assuming line-of-sight here; whether other radio propagation effects come to play here I don't know.

Now, whether we can "detect" the meteor trails which are within view of the antenna will depend on the strength of the signal, which will depend not only on the intensity of ionisation in the trail, but also on how far away the event is from both the Graves transmitter and our antenna - the closer the better.

So what conclusions have I drawn from this exercise. Practically, we know that meteor trails are detectable, but it would appear that the antenna critically needs to "see" as close to the horizon as possible in order to maximise the chances of detection. That doesn't necessarily mean that the antenna must be horizontal, because its forward sensitivity won't change much over a few degrees, but it does mean that the antenna should be as high as possible and not significantly obstructed on the horizon (and now I'm sure there are many who can prove me wrong here!). Several texts on the subject also suggest aiming the antenna towards Dijon. Now I'm not sure that from England that will be the best direction. I would think that the best chances of detection will be in the zone from West to Southeast of Dijon, being closer to the transmtter and my antenna, and higher in elevation, so when I go back into the loft to put away the Christmas decorations I think I'll swing my antenna to point roughly towards Lyon, about 143º true, or maybe a little further south. I guess this may not be too critical anyway as the 3-element Yagi antenna (or 4-element in my case) isn't desperately directional.

That's my take on the situation, rightly or wrongly. I don't know if it challenges pre-existing theories, but I'd be interested to hear any views.

Sorry to tax the little grey cells so soon in the New Year (and greetings to all) :grin:

Ian

Link to comment
Share on other sites

One thing you might be very surprised at Ian is just how much an antenna being in the loft is affected, the signal is usually attenuated by more than you think.

Having a vertical in the loft here (at 145Mhz and 433Mhz) is somewhat worse than the same vertical outside at ground level, even just walking out of the back door (ground level) is markably better than it being in the loft - received signal strength wise that is.

It will of course depend on what the roof is made of. Ours is the usual tiles, wood frame and felt.

Also, a 3-element yagi has a very broad beam width indeed, so you may notice very little by moving it say from south to south-west.

But yes, ALWAYS mount an antenna as high as possible if horizontal paths are what matter.

Link to comment
Share on other sites

Thanks for the reply Cath, and I agree the loft is second best, but it is a practical and cheaper solution for me, particularly as currently I'm only dipping my toes into the water. It does mean adjustment is easier and it is not built to withstand the rigours of our climate. It's actually a 4-element array, which I hoped would make up a bit for the loss of gain being in the loft. I can easily see the GB3VHF 2m beacon, which I was quite pleased with, though I don't know if that is a particularly challenging target, especially as the centre line passes more or less through where I live near Oxford.

To be honest I'm not expecting much of a change, if any, by moving it, but the purist in me feels it is necessary to try :smile:

Link to comment
Share on other sites

Hi,

Some interesting points.  

I have to agree with Cath's points about the directional capabilities of a 3 or 4 el Yagi and the fact it is mounted in the loft.  I suspect that the attenuation due to the loft will negate any possible benefits from a slight change in bearing from Dijon.

I have my e el Yagi pointing towards Graves (or as close as I can determine) and it's mounted no more that about 16ft off the ground (where it's easy to get at) and I'm getting good detection rates, including the ISS.

I have wondered if the beam pattern of Graves radar ( detailed here)  has any impact on reception or how detections are displayed on my Spectrum Lad waterfall screen.  However a number of web searches haven't turned up an answer to this one.

I often see detections similar to this one and have started to wonder if the 3.2 second duration of each of the 6 azimuth beam step could account for this.

Hard to be sure from the image, but those lines are 5 sec apart and the size of those 'blocks' are usually between 2-3 sec long.

I need to look at some of the other similar images to check but I'm fairly sure they are a similar length each time and that there are usually 5 or 6 of them (matching the number of azimuth steps)

Meteor20140907195601x.jpg

Al

Link to comment
Share on other sites

Hi Al

I've wondered what the effect of the Graves stepped transmission does to the observed pattern, but I suspect it is quite complicated because I would think that the radiation patterns from each segment overlap to some extent. I've seen on-off patterns in traces, but by no means as consistent as yours. I've included some examples below.

This is an ISS trace, which gives 1.5s on, 3s off, 1.8s on for the last 2 clear detections .

post-40604-0-17859900-1420236763.jpg

Another ISS trace (shown in my recent post "I detected Santa's sleigh") gives 2.1s on, 1.3s off, 3s on.

The following is a meteor trace. 0.9s on, 0.4s off, 0.4s on.

post-40604-0-19820300-1420237057_thumb.j

The following is a meteor trace. 1s on, 0.5s off, 1.1s on, 0.4s off, 1.2s on.

post-40604-0-40030300-1420237216_thumb.j

The last one is 1s on, 0.3s off, 0.1s ?on, 0.2s off, 0.4s on.

post-40604-0-32956300-1420237252_thumb.j

So not very consistent and nothing like the 3.2s step time. Some references put it down to constructive/destructive interference between the reflected radio wave from different parts of the trail, but that wouldn't apply to the reflections from the ISS.  I would also suspect that the timings for the ISS would be affected by the relative speed of the ISS.

Ian

Link to comment
Share on other sites

Another aspect I've been pondering on is how the polar response of the antenna affects what we observe. It would seem that Yagi antennas have a fairly smooth polar response in the plane of the antenna, but have a deeply notched response in the plane perpendicular to the antenna. See here for example (you'll need to scroll down the page to see the plots). From what I deduced in my first post in this thread, we would require a good antenna response over the first five or so degrees from the horizon up, but it would seem that for a horizontally mounted Yagi its angular elevation is likely to be particularly critical, because of this highly variable sensitivity with angle. Clearly, mounting the antenna vertically should essentially overcome this problem, irrespective of any polarization issues. Now I don't know whether this actually makes any practical difference. Indeed, the original article, whilst recommending mounting vertically, went on to say that no significant difference was found between the mounting orientations. Why that should be so I don't know (or perhaps I'm just raising red herrings :embarrassed: )

My antenna is currently mounted horizontally. When I'm next in the loft I'll rotate it vertically (if I have sufficient height available) and see if I can observe any difference.

Ian

Link to comment
Share on other sites

When I did meteorscatter a few years a go nearly everyone had horizontal ants.

I had two stacked 13 elements at 40feet agl but have a lousy takeoff, 600feet of chalk in most directions.

Stacking in theory would bring the main lobe down but I had the ants set to about 5degrees of elevation

to try and get over the chalk.

IMO the most important thing is a clear take off with the ants as high as possible.

This might not be so critical with todays digital processing as it's now possible to receive signals that one

could'nt hear back in the day.

I have been thinking about getting back into this meteor lark, purely receiving, still have a 25element log periodic on

the mast, just needs new cable to the ant and the rotator connected.

Anyway it's always worth experimenting so give it a go.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.