Tuesday, March 31, 2009

Why good QSOs go bad (propagation)

KB6NU has an interesting propagation question about why good QSO's go to pot.

This evening, just after dark, I called CQ on 40m CW. On the second call, VE3QO, in Ottawa, ON replied to my call. On that first transmission, he was at least 10 dB over S9... Unfortunately, on the next go-around, VE3QO was a lot weaker, and on his third transmission he was nearly unreadable.

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My question is what propagation mechanism is causing this behavior? Is it perhaps the combining of the F1 and F2 layers? If that’s the case, why is the calling station so unusually strong on the first transmission?

OK, I'll bite. I've had similar experiences, not always at sunset. I call CQ -- or answer a CQ -- and signal reports are 59+. After a few minutes, we're both down in the mud. On one level, that's a selection effect. We are more likely to call or be called if the propagation is very good -- we answer the strong stations first. Statistically, you'd expect to start talking to people at a prop. maximum, and things will naturally get worse from there, sometimes dramatically.

My hand-waving "scientific" explanation for both the sunset ("gray-line") and the variability issues comes down to thinking of the ionosphere as a time- and spatially variable propagation medium. We get strong skip when there are (a) strong gradients (acting as mirrors) in ionospheric conductivity (as around sunrise/sunset) and (b) when the gradients have a favorable geometry for a given path. E.g., they tend to lie on ellipsoids that have the two stations as foci, and the ellipsoids are arranged (in Fresnel zones) so that multiple paths arrive in phase.

So it's no surprise that we see a range of signal strengths. Why do we get such a range? We get momentary deep nulls when signals arrive out of phase and cancel. Maybe the question is why do we see the opposite - short periods of very strong propagation? Again, with hand waving, it is reminiscent of the "cusps" you see, for example, on the bottom of a sunlit swimming pool, as the more or less random waves act as lenses that focus light in very bright lines.

4 comments:

KB6NU said...

I don't think the selection effect accounts for this phenomenon because the signals are much stronger than usual. The other idea has merit, though, and I'll have to think about this more.

73, Dan KB6NU

N8SRE said...

If you have deep nulls when multipath signals arrive out of phase, you'll also have the opposite effect -- strong peaks when they're in phase.

SWLs are very familiar with this "selective fading" effect. On AM this causes cyclical fading and sometimes distortion, the latter because the fading is sometimes so wavelength-dependent that the carrier fades and one of the sidebands doesn't, making the signal appear over-modulated at the detector. With a narrow signal like CW you'll only perceive it as a strengthening and weakening of the signal.

I'm not sure if this explains what KB6NU is seeing, though. Selective fading usually has a period of seconds to a few minutes, and what he's describing seems like a longer-term phenomenon.

Martin Ewing said...

N8SRE: Not exactly. A deep null caused by two scattering centers that interfere can attenuate your signal by >30 dB (signal goes almost to zero but only for a very short time), while if they add, they will only double your power, or +3 dB.

What we do seem to see are relatively short bursts of enhancement, maybe 20 dB, like those cusps on the bottom of the pool, but they can last for minutes.

73 AA6E

Martin Ewing said...

Oops. By "relatively short" I mean that the bursts happen for a fairly small fraction of the time on a given path and frequency, perhaps 10% or less.

Since propagation details would be strongly dependent on frequency, it would be interesting to use a swept frequency transmitter and receiver. The pattern over frequency may resemble the pattern over time.