Skywave Cancellation For Dummies Explained, Pls?

[Steve Green NEPA]

You know ... for physically handicapped people like me. In my case, I flunked Physics I and Physics II once each.

I've seen that 'cancellation' stuff mentioned here often. I kind of have an idea what it is. WKBW, for example, used to seem to perform their fades at night by dutifully alternating between a 'bass'-sounding warble and a 'treble'-sounding mess before returning to its regularly scheduled sonics. Same principle, right? A magic mix or groundwave and skywave?

If that's so, what exactly is happening? Why do such periods -- WDRC Hartford was another such commotion -- get to sound that way? And from how I get it, the actual AM frequency determines the severity of it all?

I would welcome, if such is possible, as many one-syllable words be utilized, hi. Perhaps others here would appreciate that, too. In my case the tutor(s) are basically dealing with a 15-year old who wasn't told in school why a steel ball-bearing rolling down 'a frictionless' plane was going to help me meet girls.

Actual DX later * did * help.

 

[K6JHU]

Much easier if I could do it with a couple of diagrams.

Since you mention sky wave and ground wave, I suspect you have an idea of what they are. The signal reflects off the ionosphere (sorry about the multi-syllable) at night. But the ionosphere is not like a mirror. More like the ocean with waves on in. So sometimes the signal reflects off a peak and sometimes off a trough (so to speak). That means that the signal will arrive at slightly different times at the radio because the length of the path is slightly different. Meanwhile the ground wave is not affected by the ionosphere. So that arrives at the radio without shifts. The two signals (ground wave and reflected) mix together at the radio sometimes adding and sometimes subtracting. You hear that. If then the ionosphere settles down then normal sonics return.

 

[Ai4i]

That is phase cancellation.
There is also selective fading, which is mostly noticeable when only multiple skywaves are present and cancel each other out.

Also, here is why stations at the top of the band fade more rapidly than stations at the bottom of the band.
You can compare their wavelengths to the sizes of basketballs and golfballs thrown at rough, moving walls.
The basketballs will be reflected at much more predictable and regular angles that the smaller golfballs.

On shortwave, the wider the emitted signal is, the worse the fading will be.
SSB is least affected, AM is worse, and FM STL's are much worse.

 

[Steve Green NEPA]

Thank you there, K6JHU and Ai4i.

In the works is the relaying of your treatises to the NYBoardofEd (specifically signaled to their Physics Department at Brooklyn Tech).

Just one more question of you folks, if I may :

Why is there a 'bass' fade plunge and and then a 'treble' fade return to normalcy ?
Perhaps we were DXing via better radios then -- in the late Sixties. There indeed were better rigs available back then. Can the skywave/groundwave cancellation-incest still affect even the meagre sonics on AM to such a sonic degree?

On the AM dial? Obviously, the answer is yes.

Perhaps I should get an oscilloscope. But I've had two or three of them done on me in the past.

Thanks for your help, fellers!

 

[Ai4i]

Thank you there, K6JHU and Ai4i.
Why is there a 'bass' fade plunge and and then a 'treble' fade return to normalcy?

Perhaps I should not have suggested that phase cancellation and selective fading are not co-phenominal.
As I explain what you are describing, you will understand how intuitive what you are describing really is.

Phase cancellation swings across channels selectively, from the bottom to the top or from the top to the bottom.
Say you are listening to a station on 1 MHz:
The cancellation will begin at either 992 KHz and drift up to 1008 KHz or it will begin at 1008 KHz and drift down to 992 KHz. You will hear a loss of highs when the cancellation is at least 3 KHz off of the carrier, then a loss of midrange when it is at around 800 Hz to 1½ KHz off of the carrier, and then a loss of bass, severe distortion, and another loss of bass as it passes through frequencies near the carrier and the actual carrier frequency, then another loss of midrange, and finally another loss of highs, and then it will pick itself up, brush itself off, and start all over again, as Ginger Rogers explains to Fred Astair here.
When the cancellation is on the carrier frequency, the station will then be coming in as true double sideband, suppressed carrier.

This can be remedied by inserting a phantom carrier into your receiver with either a BFO (beat frequency oscillator),
or by holding another receiver next to it tuned tuned to the first IF frequency,
usually 455 KHz, below the station to which you are tuned, in this case 545 KHz,
or by injecting a dead carrier directly into your receiver on the frequency of the station to which you are tuned.
Receivers with synchronous detection, such as those with switchable kahn-hazletine chips, do this wonderfully.