Utah Fades: A TouCans Lab Book Entry

A few afternoons ago, I watched the signal from TouCans fade from an s9 into the Utah SDR to nothing in a matter of minutes. There was nothing wrong with the rig. I was literally getting to observe the effect the F2 layer has on propagation on twenty meters.

Schedule

00:02 UTC/16:02 PST: Almost at S9 into Utah

00:32 UTC/16:32 PST: Running at S9 into Utah

00:36 UTC/16:36 PST: Already down to S5

00:37 UTC/16:37 PST: The signal is all but gone

What Happened?

Here's the F2 critical frequency, (fof2), along the path to Utah at each of the above time steps.

The critical frequency is the maximum frequency that will be reflected straight back down to the ground at any point rather than piercing the ionosphere and propagating out into space.

When I say along the path, I mean the value for fof2 reported by GloTEC at that time. GloTEC collects a number of ionospheric data sources and assembles them into a model of what the F2 layer looks like at any location on Earth at a given time. Samples are 10 minutes apart. The samples are also gridded on the Earth. The grid between San Francisco and Utah is shown in the example below. Notice that the path crosses four grid squares. These correspond to the four points on the graphs below.

Here's the data. When the signal was appearing at S9 in Utah, the path to NG7M (also in Utah) showed

Just before the signal disappeared altogether, we had

and ten minutes later when the Utah SDR was no longer picking up Project TouCans, we had

What Does it All Mean?

We might be on to a way to quantitatively determine when the East Coast will start to open up. When the critical frequency in San Francisco, (zero degrees along the great circle arc to Utah), falls below 8,000 kHz, (8 MHz), our 20 meter signal at 14.0574 MHz can break through the 'mirror-shiny' F2 and cast along farther before it is deflected back. Not only that, but from the Dept. of Commerce book, we have a formula to determine how far the radio wave should be able to propagate based on the fof2 numbers along the path.

This is where the height of the F2 layer finally comes into play. The rig was spotted at W1NT starting about a minute after it disappeared from Utah. The height at that time was

In other words, the height of the ionosphere at that time was about 297 km. The calculated launch angle is a little off because the height of the transmitting station is a little off. (The station was about 111 meters higher on the surface of the Earth than the initial assumption of sea level.)

Ten minutes earlier, the F2 height was about 294 km. That might not have supported a single skip. Also keep in mind though that the critical frequency overhead simply might not havvea supported the signal breaking through at all because the F2 layer was at 277 between SF and UT.

Things we need to be able to calcuate:

Lowest F2 height supporting single skip between two stations corrected for altitude
The launch angle corrected for transmit station altitude
The maximum frquency that would reflect between SF and UT based on the secant law and fof2 at the midpoint of the great circle arc between the two.

Things we need to measure:

Does Utah disappear at the same crtical frequency every day?
Does it appear at the same frequency every morning

Is the appearance frequency substantially different?

What are the f2 heights at both of these times, (morning and evening.)

Are they consistent?

Final Note

I hadn't realized how correct the negative angle was until I zoomed out along the path

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