Sky Waves
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Sounds like Tech Log territory to me !!
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PPRuNe Radar
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PPRuNe Radar
ATC Forum Moderator
[email protected]
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Cron,
Sporadic E propagation results in extreme long-distance communications on 50 and 144Mhz amateur bands, often for short periods of time only (5-10 minutes is common) and of an unpredictable nature.
The precise mechanism involved within the ionosphere is still poorly understood. However, I wouldn't call it a form of sub-refraction.
I found quite a number of websites discussing it using Yahoo! as a search engine. Most were associated with amateur radio clubs or organisations and a bit of web surfing might repay your curiosity amply. My propagation theory is a bit dated now so I can't add much more than this. Hope it helps.
Sporadic E propagation results in extreme long-distance communications on 50 and 144Mhz amateur bands, often for short periods of time only (5-10 minutes is common) and of an unpredictable nature.
The precise mechanism involved within the ionosphere is still poorly understood. However, I wouldn't call it a form of sub-refraction.
I found quite a number of websites discussing it using Yahoo! as a search engine. Most were associated with amateur radio clubs or organisations and a bit of web surfing might repay your curiosity amply. My propagation theory is a bit dated now so I can't add much more than this. Hope it helps.
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I've had trouble with this one too. Super refraction is atmospheric ducting or anaprop. Atmospheric ducting increases the range of VHF signals under normal conditions by about 15% on the line of sight range and on occasions much further, sometimes over 1000nm. I wonder if sub refraction is the opposite, less refraction than normal? I tried searching the web & came up blank.
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I thought I knew a bit about radio propagation, but I'd never heard about sub-refraction. A bit of digging (www.google.com is the world's finest search engine) revealed the Radar Meteorology Glossary at http://www-paoc.mit.edu/Radar_Lab/Glossary.html, which says that sub-refraction is "A condition of atmospheric refraction when radar waves are bent less than normal. Sub-refraction occurs when dN/dh > -39.2 N-units/km.". Less than helpful, huh?
However, a few hits later I found a rather interesting document -- http://www.msc-smc.ec.gc.ca/armp/kin.../profbird.html -- about weather radar and birds, which included the following: "Probably what happened (and what typically happens) is the radar beam experienced super-refraction, or a bending of the beam back down towards the ground. The opposite is called sub-refraction.
The radar beam does not emanate from the radar dish in a straight line. It bends somewhat down toward the earth, but the curvature of the earth is greater than the curvature of the radar beam. Exactly how the radar beam bends is determined by the index of refraction.
The index of refraction of the radar beam in the atmosphere is affected by inversions of temperature and moisture."
Which makes sense. It's also got nothing to do with sporadic E (about which I do know a bit), which is occasionally enhanced reflectivity of the E layer of the ionosphere. As has been said above, this has the effect of causing much higher frequencies than normal to carry over the horizon -- although rarely at radar frequencies. Sporadic tends to happen at VHF and below, which makes it fun for radio hams but a bit of a nuisance for everyone else.
Although 'reflection' from the ionosphere is in fact a refraction phenomenon -- as is all reflection of EM waves, including light from mirrors -- from the above I don't think it has anything to do with sub-refraction. There may occasionally be correlations between weather conditions that cause sub-refraction because of moisture, etc, and sporadic E, but I'd doubt it.
Hope this helps. I learned something new -- viva Pprune! The web site about birds and weather radar is entertaining in its own right, with happy talk about insects showing up at 2GHz and 'bird explosions' caused by enormous flocks of cow birds taking off at the same time and heading in all directions.
R
[This message has been edited by Self Loading Freight (edited 03 November 2000).]
However, a few hits later I found a rather interesting document -- http://www.msc-smc.ec.gc.ca/armp/kin.../profbird.html -- about weather radar and birds, which included the following: "Probably what happened (and what typically happens) is the radar beam experienced super-refraction, or a bending of the beam back down towards the ground. The opposite is called sub-refraction.
The radar beam does not emanate from the radar dish in a straight line. It bends somewhat down toward the earth, but the curvature of the earth is greater than the curvature of the radar beam. Exactly how the radar beam bends is determined by the index of refraction.
The index of refraction of the radar beam in the atmosphere is affected by inversions of temperature and moisture."
Which makes sense. It's also got nothing to do with sporadic E (about which I do know a bit), which is occasionally enhanced reflectivity of the E layer of the ionosphere. As has been said above, this has the effect of causing much higher frequencies than normal to carry over the horizon -- although rarely at radar frequencies. Sporadic tends to happen at VHF and below, which makes it fun for radio hams but a bit of a nuisance for everyone else.
Although 'reflection' from the ionosphere is in fact a refraction phenomenon -- as is all reflection of EM waves, including light from mirrors -- from the above I don't think it has anything to do with sub-refraction. There may occasionally be correlations between weather conditions that cause sub-refraction because of moisture, etc, and sporadic E, but I'd doubt it.
Hope this helps. I learned something new -- viva Pprune! The web site about birds and weather radar is entertaining in its own right, with happy talk about insects showing up at 2GHz and 'bird explosions' caused by enormous flocks of cow birds taking off at the same time and heading in all directions.
R
[This message has been edited by Self Loading Freight (edited 03 November 2000).]
