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Author Topic: foF2 plots  (Read 505 times)

Offline ChrisSmolinski

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foF2 plots
« on: April 26, 2018, 1228 UTC »
As you may know, the HFU has a propagation page, with lots of nifty plots: https://www.hfunderground.com/propagation/

One set of them are foF2 plots from ionosondes, they can be found here: https://www.hfunderground.com/propagation/#skipzone

There's several of them in various parts of the US (there are also some overseas, not displayed on the page). You can view all of them on my DX ToolBox app, btw, which is available for Windows/Mac: https://www.blackcatsystems.com/software/ham-shortwave-radio-propagation-software.html
and iOS: http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=492113368&mt=8&at=11lb5X

I find these plots to be perhaps the most useful real time gauge of propagation conditions, especially for the 43 meter band. So what's being measured? foF2 is the highest frequency that will be reflected from the ionosphere if directed straight up (which then gets reflected back down to the transmitter site).  Any frequencies higher than this just go off into space(*) (mostly, there can still be some reflection, but signals are usually extremely weak).

If you think this sounds like NVIS (Near vertical incidence skywave) propagation, you're correct! And that's the typical mode most stations in the 43 meter band use, especially in the daytime, but also at night, for non-DX propagation. As the distance between the transmitter and receiver increases, the angle becomes less than 90 degrees (perpendicular) and frequencies higher than foF2 can be propagated.  This can be computed with simple geometry, if you know foF2 and the height of the F2 layer (also displayed on a graph next to the foF2 plot) you can compute the angle and then the skip zone distance around the transmitter site. There's actually a calculator on the page to do just that.  At large distances, propagation on the higher bands (20 and 10 meters for example) becomes possible.

The foF2 plots have lines with three different colors. The blue line is for today, the red line is yesterday, and the green lines are several previous days.  So look at the blue line to see what the current foF2 value is. The other lines are useful to see past trends. Assuming the solar activity has not drastically changed from yesterday, today's plot usually follows it, with some wiggles. An increase in solar flux (due to more sunspots) will cause an increase in foF2.  Note that this is independent of the K and A index (although they of course are affected by solar activity as well). For good propagation, we want high foF2 values, the higher the better.

There are both diurnal and seasonal trends. Right now, the typical pattern is for foF2 to fall at night (which makes sense, UV from the Sun is not reaching the ionosphere above us, so it is de-ioniziong). Then at sunrise, it starts to increase. It reaches a peak around local sunset, currently 0000 UTC here on the east coast. Lately the peaks have been around 7 MHz - which means 43 meters is open for business. Of course you can check the current values to see what is happening on a particular day.

Also consider that what's important is not the foF2 value at your location or the transmitter site, but rather between those two locations, where the signal gets reflected (assuming just one hop). But there's only a few ionosondes to choose from, so selecting the one nearest to your QTH is usually good enough.  Also note how the value varies with latitude. Ionosondes further south report higher values - as the Sun is higher in the sky, causing increased ionization. This is why propagation, especially on the higher bands, is usually best in the tropics, the Sun is highest there, year 'round.

Here's an example plot, from Wallops Island Virginia from last night. I use this site as it is closest to my QTH:
« Last Edit: April 27, 2018, 2012 UTC by ChrisSmolinski »
Chris Smolinski
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Offline Σ

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Re: foF2 plots
« Reply #1 on: April 26, 2018, 1802 UTC »
Nice explanation, Chris. That will be a good resource I can send to newbies when they ask how it all works. :)

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Offline MDK2

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Re: foF2 plots
« Reply #2 on: April 27, 2018, 1945 UTC »
That peak you speak of is local it seems. Boulder shows the peak at about 0200, which makes sense given that the Mountain time zone is two hours behind the Eastern. And Vandenberg CA's peak is around 0300. So it's not a peak relative to UTC, but to sunset at one's QTH.
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Offline ChrisSmolinski

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Re: foF2 plots
« Reply #3 on: April 27, 2018, 2012 UTC »
That peak you speak of is local it seems. Boulder shows the peak at about 0200, which makes sense given that the Mountain time zone is two hours behind the Eastern. And Vandenberg CA's peak is around 0300. So it's not a peak relative to UTC, but to sunset at one's QTH.

Yes, absolutely correct, I'll edit it accordingly. And the peak of course moves throughout the year as sunset times changes.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 270 ft west-south-west beverage / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline KaySeeks

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Re: foF2 plots
« Reply #4 on: April 29, 2018, 2249 UTC »
As the distance between the transmitter and receiver increases, the angle becomes less than 90 degrees (perpendicular) and frequencies higher than foF2 can be propagated.  This can be computed with simple geometry, if you know foF2 and the height of the F2 layer (also displayed on a graph next to the foF2 plot) you can compute the angle and then the skip zone distance around the transmitter site.

An example of the plot that Chris is referring to is at the bottom. I have drawn a red arrow at the relevant height, which is on the black trace. I have highlighted the scale that is used to indicate height. I have also highlighted the indicated height (in km) in textual data, "hmF2".

Here is a map of the digisonde stations around the world: http://www.digisonde.com/digisonde-station-map.html
Here is a location to view plots like I show below: http://ulcar.uml.edu/DIDBase/
Just somebody with a radio and a pair of headphones...

Offline ChrisSmolinski

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Re: foF2 plots
« Reply #5 on: April 30, 2018, 1434 UTC »
Here's a waterfall (netSDR receiver, 43mb folded dipole)  from last night, 0010 UTC 30 April 2018.  You can see five pirates on at the same time:
6880 AM UNID
6905 AM Seven Trees Radio
6925 AM Yeah Man Radio
6935 USB Peace Radio
6950 AM Captain Morgan




And here is a plot of foF2, you can see there was a peak around 6.5 MHz around this time, helping to provide the great conditions (there are other factors as well of course):

« Last Edit: April 30, 2018, 1513 UTC by ChrisSmolinski »
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 270 ft west-south-west beverage / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop