What is about the minimum wavelength that can bounce off of the ionosphere?
anything from ~10kHz to ~ 50MHz ( 50MHz = 6metre wavelength) will easily reflect off the Ionosphere using either the D, E or F layers
frequencies from ~ 100MHz (3 metres) and up will tend to easily penetrate the Ionosphere. Those higher frequencies, right up into the microwave bands, at least up to 24 GHz (1.2cm) can use tropospheric ducting and other methods of long haul propagation
so as a generalisation, approx 80MHz (3.75 metre wavelength) would be the cutoff freq for Ionospheric reflection.
I have seen/used sporadic E layer propagation up to 150MHz, but relatively rare
Thanks, Dave! Great answer! Do you know of any handheld receivers that could pick up 150MHz? Or would you need something w/ a larger antenna?
There are a number of scanning receivers that cover 150MHz and a wide range of freq's either side.
for example the AOR range of receivers ... the AR 8200 MK3, that I have covers, 500kHz to 2050MHz pretty much contineously. Its a very versatile receiver and altho comes with a "rubber duckie" antenna, the receiver can be connected to a decent external antenna.
Just a comment about reflection off the ionosphere. the lower VLF and HF frequencies say 100 kHz to 30MHz the angle of reflection can be quite sharp but as the frequency increases from ~ 30MHz up to ~ 70 MHz the angle changes substantially, see my drawings below....
NOTE as said in image ... NOT to scale, just a basic pic to give you an idea and something to base further info searching on
the lowest layer is the D layer good for 10kHz to 1MHz (not shown in image)
Then the E layer, then the F1 and F2 layers being the highest up
I wont go into individual descriptions here, there's plenty of info on the www
depending on the time of day and also year the F1 and F2 layers will split into the 2 layers and then recombine into a single F layer
You can see the really low angle of propagation of the radio signal off the E layer
On all layers you can get single hop of signals as I have shown, and you can also get multi-hop of signals too. I have had multi-hop propagation on the 50 MHz (6 metre) ham radio band on a number of times.
As you dig into some studies on the subject, you will find a lot of discussion on what is theorised to be happening in the ionosphere when a radio signal encounters it.
eg.... does the signal reflect directly off the layer at the point of contact. Or is there a degree of refraction that causes the signal to follow the layer for a distance before coming back out and down to the ground.
There's a whole mass of physics way over my head, describing the maths of these interactions.
There is one form of ionospheric propagation that I havent commented on yet.
That is the reflection of a radio signal off an auroral curtain. I have done that on the 50 MHz and the 144 MHz frequencies. Because the aurora has a lot of movement in it, the audio signals are readable but quite distorted and the distortion is worse as the freq increases.
This is also referred to auroral backscatter ... now the drawing below (again, not to scale) that the signal from station B is going out to the auroral curtain and then being reflected back over the top and on to station A. This was what it was like for me when I lived in southern New Zealand. I was in the station B position in Dunedin city and my signals were being backscattered back over the top of me to station A in Christchurch city.
and of course for us in the southern hemisphere the aurora was south of us. It is reversed for those in the northern hemisphere
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