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Author Topic: What is the ionosphere and how does it affect radio communications?  (Read 6377 times)

Offline Pumblechook

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Does the D-layer never reflect signals?  It absorbs MF (medium wave.. AM band) signals during the day when it is highly ionised.  At night the ionisation disappears (no solar radiation) and signals are reflected by the E-layer.   Seems strange that the D-layer has a large effect only up to around 2 MHz and as far as I know never reflects any signals. 

Maybe some physicists out there can eplain the mysteries of the various layers.  D E and F1 and F2.  Is it to do with density at different heights?  Are there A and B layers?   Why does the level of ionisation effect different frequencies so differently?.  Normally signals don't get reflected about 30 MHz but at periods of high sunspot numbers low-VHF can be received over short wave like distance.. BBC TV from London on 45 MHz (now shut down) for instance being receieved in South Africa  or Australia.  And at times FM band and higher frequencies refelect off the E-layer (unrelated to sunspot max),,Sporadic E.  One theory was that the later was caused by thunder storms. 

I am radio engneer but know little about the physics involved.
« Last Edit: 07/11/2008 19:47:07 by chris »


 

Offline syhprum

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There was a topside ionosphere scanner 'Alouette' operating in the late sixties but its results are only available to members.
It was amusing to listen to as its receiver swept thru the broadcasts bands.
I remember the (unsuccessful) heroic attempts to receive the TV broadcast of the coronation in America.
 

lyner

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The way a wave of a particular frequency behaves depends on the complex refractive index. This has real and imaginary components which govern the amount of bending and the amount of loss. HF is bent less than LF.
The level of ionisation depends on the level of ionising radiations. The situation settles down fairly soon after dawn. The lower layers (D)  have low level ionisation because the radiation has been absorbed mostly by higher layers and only affect the lower frequencies - the higher frequency waves only interact a bit, so they go through. The lower frequencies do interact and, because the air is so dense, energy is lost through collisions. They would be reflected but for the absorbtion which this causes.
Higher frequencies arrive at the upper layers  which will have higher levels of ionisation  (more radiation gets to them) but the loss is less because there are fewer neutral molecules about so they are deflected or even reflected with low loss. The angle at which the wave passes also has an effect. A HF signal at grazing incidence may be reflected but a steep path will let it escape.

When the Sun goes down, the ions in the different layers recombine at different rates. The D layer recombines quickest because of the density and the lower frequencies get through. They can then be reflected by the higher layers but the highest frequencies may well escape completely. Long after sunset, the ionisation level has dropped to a value where only the low frequency HF transmissions and MF/LF are usable.
The D layer allows local MF radio 'Groundwave"  to work well in the day but, once it's gone, the skywave wrecks the reception far away from your local station.
Fo HF comms, the desired transmission distance governs the take-off angle of the beam and you have to choose a suitable frequency, suited to this and the predicted ionisations in each of the Layers.
HF is pretty naff for broadcasting - and the frequencies used are governed by international agreement - but for communications, it can provide a service over enormous distances as long as you can choose your frequency at the time.

VHF, as you say, is not usually affected by the ionosphere but, at low (grazing) angles, even mild ionisation can cause bending. 'Sporadic E' is so sporadic that it tends to be a pain rather than any use.

You don't even need ionisation for UHF waves to propagate between layers of air near the ground at different temperatures - this is called Ducting and can be a right pain in the summer when the South Coast is bombarded with continental interference.

« Last Edit: 07/11/2008 15:37:53 by sophiecentaur »
 

Offline Pumblechook

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Good reply.  I am familar with quite a lot of that professionally and HAM Radio - wise.  I don't quite follow the complex refractive index bit.  I have made contacts via Sporadic E and tropo-ducting..and aurora but never meteor scatter which is quite popular or via the Moon.

Wiki is actually quite good on the subject but rather brief.  Looks like the F layer may reflect medium waves and not the E. 

I like analogies.  I am thinking of a prism which splits light into a rainbox.  It is frequency sensitive..different refraction angles.   I am thinking of a mesh which will reflect lower frequencies better ..wavelength v pitch of the mesh.   

http://en.wikipedia.org/wiki/Ionosphere
 

Offline Pumblechook

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Seems to the crux of the frequency effects....

""If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal.""
 

lyner

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Complex refractive index:
The refractive index of a material tells you about how it affects waves passing through it. The waves cause the electrons to wobble. It takes time to accelerate the electrons and ,once they have moved, they start to re radiate the wave's energy. This, re radiated energy and the unaltered wave energy add together to produce a resulting wave which goes slower than the wave through space. That is well known to happen with light through glass, which slows up. The slowing down effect is due to a phase shift. Electronic circuit elements which cause a phase shift are said to be 'reactive'.
No energy is lost when only this happens. BUT, in all materials, once the electrons are moving they can absorb some of the energy. This is a 'resistive' effect.
Resistance(R) and reactance(X), together, the impedance of a circuit. Impedance is a complex quantity Z=R+iX. This complex quantity is needed to describe the effect of energy lost and phase shift.

Refractive Index also needs to be described as a complex quantity for the same reason. Both speed / phase behaviour and energy loss need to be characterised.

OH yes - and the plasma frequency telly you the maximum frequency that this happens, as you say. But the effect is not 'there or not there' as your post implies.
 

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