Propagation... why do I miss some signals?

[cutty]

Right or wrong, https://en.m.wikipedia.org/wiki/Skip_zone seems to imply that there is no skip zone at ELF.

We are not receiving detecting RF like in a radio wave.

It finally dawned on me why you believe that, Dries. Audio sound waves from a speaker that are detected by our ears actually overlaps a limited range of radio frequencies. (That was difficult for me (and countless others) to comprehend. After all, ~15 kHz can be heard! It "can't" be RF radio waves!)    

Lightning strikes definitely propagate an RF signal that are detected by our BT radio receivers. We are indeed receiving RF radio waves, even though we cannot possibly detect the sound from distant thunderstorms. Think about it: our BT receivers use antennas and probes, not microphones.

There are VLF and LF radio stations whose frequencies are in our VLF or LF band. And some of them continue to be a source of interference to a few stations on our network.

Interestingly, there is a ham band around 137 kHz (2200 meters).

I was wrong when I wrote ELF. The ELF band is either 3 Hz to 30 Hz or 3 Hz to 30 kHz.  

Let's try this approach:  to oversimplify quite a bit.... .and see how much trouble I can get in...

Radio Frequency refers to the "Rate of Oscillation" of an electromagnetic wave. A radio wave consists of a Magnetic Field, and an Electric Field, neither can exist without the other, and they are self propogating and reinforcing. They are produced by and represent  'alternating current' / Voltage.

Lightning does NOT produce a repetitive, 'oscillatiing' signal...it has 'no frequency' ...  it produces an 'impulse' or series of impulses, of energy that is seen at frequenciy 'locations' well up into the megahertz... but they aren't oscillating...the 'amplitude' of extremely short 'field changes' simulates an oscillating signal, but the polarity normally doesn't change, as in AC...  lightning is virtually DC, not AC  It will have little or no Electric "plane component", just a pulse, beyond a "relatively" short distance, is not self propagating, and fades out rapidly with distance, since it's concertated in the 'magnetic field" realm, but this magnetic field can be sensed a very long way because of the original strength..


The  step leader DC Currents can produce instanteous current flow, generating magnetic fields, of shorter duration, and amplitude.. 
The Discharge DC Current produces a very intense magnetic field..  Our loops follow that magnetic surge  which does not "Oscillate".
etc...
BOTH vary in amplitude over time as they occur, which may appear as a 'wave' visually, especially in the case of 'sombrero' signals....

The E probe does NOT detect RF at all... in a manner of speaking, it is a capacitor that lives inside a capacitor.  It mimics the  charge, and change of charge, within that atmosphere / earth capacitor, and as those charged particles build up and decay, the changing potentials manifest as 'voltage' changes. Hence "E Field". Again, those changes can be so rapid, that they can be said to manifest in the 'RF' spectrum, but they are not 'oscillating' and have no relevant magnetic field... in fact, the E probe should be immune to magnetic effects!

We do not detect, with our system, 'radio frequency waves"  ...  we detect impulse' energy magnetic effects and charge state changes which must exist" SOMEWHERE" in the energy specturm... but that does NOT mean they are 'Radio Waves". And yes, any Radio Transmitter or device that produces a magnetic field in our band pass will cause interference... any 'spark gap' can cause interference in the E field.   And a spark discharge may also be detected in the H field loops;.

It's easy to 'mix theories', and term usages.  It's also pretty  much human to have our pet theories, indeed I have mine.

And if you think you can't hear lightning, it' only because of the extremely short duration of the pulse presence. ... you can hear it 'down-mixed' manifesting as 'thunder' since air responds so slowly.... sort of 'lengthing the pulse width"...