diver21 Posted January 29, 2009 Report Share Posted January 29, 2009 Well im doing a course at the moment about comms and me or the instructer cant figure out how HF radio waves can refract fully in the ionosphere. i can understand how when it goes up it gets refracted part of the way to earth (till its parallel to the earth becouse as an increase in height has more electrically dence ions) but as it comes back towards the earth shouldnt it stay up there as a decrease in altitude makes the ions less? any easy way of explaining why this happens? Quote Link to comment Share on other sites More sharing options...
supasi Posted January 29, 2009 Report Share Posted January 29, 2009 This isnt just over my head, this is up in the air :lol: :lol: Quote Link to comment Share on other sites More sharing options...
diver21 Posted January 29, 2009 Author Report Share Posted January 29, 2009 lol, same with anyone else i talk to, thought there might be some ham radio people out there that might be able to help Quote Link to comment Share on other sites More sharing options...
Caryl Posted January 29, 2009 Report Share Posted January 29, 2009 Think of the ionised layer working as a mirror so the signal goes up and is effectively reflected, rather than refracted, so therefore it travels down as long as the ionisation is dense enough. If it's not dense enough then the signal is refracted and will proabably not come back to earth at all. At certain densities it will in fact absorb the signals and will not come back to Earth or pass through. Different layers at different altitudes and different densities all have differing effects depending on the time of day. The angle the signal hits the layer also determines whether it will be reflected back to Earth, refracted slightly, or pass right through. A signal hitting it at a very low angle will reflect back at that same angle so the distance from where it is transmitted to where it is received is much greater than if the signal goes up at a much steeper angle and then comes back at the same angle and therefore is much closer when it comes back to ground. The thinner the ionised layer, the lower that angle must be for that signal to bounce back. The frequency of the signal also has a dramatic effect on where it comes back. Some layers can reflect a vertical signal back down again. Tests are done for this called ionospheric sounding to determine the effects at different frequencies of the ionisation of the ionised layers. Check out ionosond on Google. There are websites you can look at to give you the results of ionospheric sounding as it is done by various organisations. From these you can work out what frequency to use at what time of day to get your HF signal to where you want it to be - with a bit of luck, because the angle of reflection may mean that the signal comes back to Earth either before it gets tyo you or after it gets to you so you may not hear it at all. Also check out MUF (maximum usable frequency) and ALF (alternate lower frequency) Have a look at http://www.ips.gov.au then look under "educational" and there's a huge amount of information on all sorts of stuff to do with propagation, the sun's effect etc. Are you training at Woodbourne? Feel free to contact us if you want to talk to Grant about any of this stuff. He has also run amateur radio classes. (dictated by Grant - Amateur Radio Ham and one who has worked with HF radios for many years) :bounce: Quote Link to comment Share on other sites More sharing options...
Andrew_W Posted January 30, 2009 Report Share Posted January 30, 2009 Is this the big radio we can talk to US coast guards on? Quote Link to comment Share on other sites More sharing options...
Caryl Posted January 30, 2009 Report Share Posted January 30, 2009 Yes, if the ionisation layers are at the right density, the right height and we choose the right frequency it just might work - but then again, it might not. It is also affected by sun spots, of which there are none at present! :lol: Quote Link to comment Share on other sites More sharing options...
diver21 Posted January 30, 2009 Author Report Share Posted January 30, 2009 yea i am training at woodbourne, lol stands out a bit dosent it? but the thing is ive been told that reflection is wrong ant that its like light traveling through diffrent media how it refracts away from the refractive inted when going from less to more dence, then it seems to reverce. Quote Link to comment Share on other sites More sharing options...
Warren Posted January 31, 2009 Report Share Posted January 31, 2009 I'd expect there's a bit of both happening. Even if the radio wave is reflected it's still not going to make it all the way to the US without being absorbed unless it's refracted to a shallower angle as well. This isn't my area of speciality though so what do I know??... Quote Link to comment Share on other sites More sharing options...
sandysme Posted January 31, 2009 Report Share Posted January 31, 2009 http://www.nzscanners.org.nz/ Heaps of radio hams here.... Quote Link to comment Share on other sites More sharing options...
Caryl Posted January 31, 2009 Report Share Posted January 31, 2009 Quite right, the mechanism is in fact refraction and the amount of refraction depends on the density of the ionised layer and the frequency of the signal. The overall effect is similar to reflection but because the refraction varies with frequency, some signals are not refracted as much as others, so do pass right through the layer with only a slight bend in their progress while others have enough refraction to bring them back to Earth again. If you Google "HF propagation refraction index" you will find a lot of information. Quote Link to comment Share on other sites More sharing options...
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