Propagation question

[MDK2]

I'm curious about what the more experienced of you think about this. Last night, Wolverine Radio came on with a very good signal here in Colorado at 6950. Apparently I was the only one getting it well based on the logs, so he switched frequencies to 4020, after which most midwest and eastern listeners reported much better reception, while it was below the noise floor here. I first wondered if I had been benefiting from some NVIS propagation when he was on the higher frequency, but then it occurred to me that that's supposed to be more likely on lower frequencies. Am I wrong? Was 43m just long last night? Just trying to learn what I can from the experience. Thank you.

[Skipmuck]

Although there are others here much more capable of answering the question, I'll take a crack at it. When Wolverine came on last night around 0200 UTC, the foF2(Maximum Usable Frequency) as reported by the Wallops Island ionosonde was hovering just below 3 Mhz. The hmf2 layer was bouncing around quite a bit, but was in the 300-350 km range (height). There is a formula on this link https://www.hfunderground.com/propagation/ that gives the Skip Zone for these foF2 and hmf2 figures as in the neighborhood of 900 miles. That means Wolverine's signal was skipping over listeners in a 900 mile radius from the xmtr location. It only means that the signal was going further out, so your location in Denver was outside the skip zone while for the rest of us, the signal was greatly attenuated. When Wolvie switched over to 4020 Khz, the figures for f0F2 and hmf2 from Wallops Island plugged into the formula yield a skip zone of roughly 350 miles. Therefore, the east coast and midwest received a much better signal because the signal was no longer bouncing over us. But the signal probably had to do a 2nd bounce to reach your location in Denver. Remember, it is the location of the xmtr and the MUF at that location that defines the Skip Zone. Not the location of the listener. When the band "goes long", it means the transmitted signal is returning to earth at a greater distance from the transmitter. I am using the Wallops Island ionosonde because it seems to be the closest one to my location here in Western Massachusetts. If I used another ionosonde from another location such as Alpena, Michigan or Boulder, Colorado....then the figures change for foF2 and hmf2 and the formula for the Skip Zone change accordingly. Not knowing the location of a given transmitter and therefore choosing the best ionosonde location makes the Skip Zone results a matter of speculation...I hope this makes some sense!

[ChrisSmolinski]

Yes, last night 43 meters "went long" and was no longer open for NVIS. So there was a radius around Wolverine where it could not be heard well, while outside that radius, reception was much better, out to a maximum distance where the signal again became weak. So think of the area that could receive it as a donut shape. Had the transmission occurred much earlier, while NVIS was still possible, the reception area would be circular in shape.

When they switched to 4020 kHz, reception via NVIS was again possible on the east coast. I imagine a weaker signal was received at your location due to a number of possible reasons: more noise on lower frequencies, the station's antenna is lower (as a function of wavelength) at 4 MHz vs 7 MHz, so less of the signal is radiated at low angles favorable for DX, and the ionosphere is probably reflecting less of the signal at 4 MHz vs 7 MHz. I'm sure there additional reasons I've ignored  

I would not consider your reception on 6950 as NVIS, rather just typical low angle DX.

EDIT: Also, what Skipmuck said  

EDIT #2: I'd need to think about it some more but I suspect the condition of the ionosphere at the location(s) where the reflection(s) (OK, refraction) takes place is important, vs just at the transmitter site. For NVIS, this location is indeed right above the transmitter. For DX listeners it would be where the bounce takes place. Or multiple reflections if you are further away. And in that case, you have reflections off the ground as well. And those conditions probably matter. If a ground reflection occurs over a lake or ocean or other conductive surface, I imagine you get a stronger signal, vs over poor conductivity ground. Maybe this explains some of the fading we experience. As the height of the ionosphere changes, the geometry does as well, which could move the location(s) of those secondary ground and ionosphere reflections.

[MDK2]

Thank you for your answers. This gives me a greater understanding which is what I was after.

[James Brownyard]

This is all dependent on what time of the year it is and where we are in the 11 year solar cycle.  Just a few years ago, when there were more sunspots, 43 meters was both short and long well after sunset. Now, as we head towards solar minimum, there is less charging of the ionosphere, which pushes the critical frequency for NVIS lower. Things should improve some as the days get longer, but overall, it will get worse before it gets better.

[Pigmeat]

Last night was a weird night for listening, period. MW stations that are normally rock solid after dark were fluttery at best here and dx was minimal.