This site is designed to be viewed on a laptop or desktop. It will not display correctly on a phone.
This is a skywave playground. It is designed to help the user understand how the distance between earth and ionospheric regions might be affected by the angle at which an HF signal is sent from a transmitter and the angle at which it is received.
The site has several tools, and three basic designs:
Single-hop diagram on this page and others.
Great circle maps with refraction point markers Miles Kilometers
In the single-hop design there is the design shown below where you can modify height of the ionosphere and angles for arrival and departure in miles. There is another for kilometers. Click here to see the metric tool.
In the multiple-hop hop design, there is a tool in units of miles. Click here to see the multi-hop tool.
In the great circle maps, there are markers where a signal might refract off the ionosphere or the earth surface.
You will also find that there are several narrative pages that add context and perspective to the tools. One page talks about multi-path propagation. Click here for that. Another page talks about using the tool for exploration. Click here for that one.
The chart that is below, plots the path of an HF radio signal from point A to a refraction area in the ionosphere, and then on to point B. The chart allows you to change three variables: great circle distance, the angle of departure/arrival at point A, and the angle of departure/arrival at point B. With those three values, it will plot a position in the ionosphere where the signal may be refracted. You can also use a left-click-and-hold on a mouse to move the little red dot and all values will update as you move. Move the dot around and watch for changes in the distance traveled in units of miles and wavelengths. The chart will automatically resize as the values change and that is part of the display.
In an HF communication circuit, the transmitter can be at either point A or point B. This is not an actual predictor unless an operator knows the precise angle of departure/arrival, and the precise refraction area in the ionosphere. With the current technology, that angle cannot be precisely determined and specifics about the refraction area are very limited. Therefore, everything is really speculation. Still, the tool allows those who are interested to think about what goes on when their signal departs and another arrives.
There are a few caveats. One is this. By setting great circle distance and angles of departure/arrival at angle (a) of 45 degrees and a receiving angle (b) of 45 degrees, the estimated ionospheric refraction point is above the highest level of the F2 layer. It is unlikely that there will be a refraction from such a high area above earth. Some programs might mark it that as an error. I left it, however, so the user can understand how the geometry works, and why that is not a likely scenario. In the same way, if the user enters a great circle distance that is way too long the results will become unpredictable. Some other entries for distance and angle, will also generate errors, so be cautious about drawing conclusions. Watch for additional explanatory text to be added to this site.
Here is an example. I made a contact on 20 meters with an operator who was exactly 350 great circle miles away. If both of us have a 15 degree angle of transmit and receiving, then the signal will refract in the E region at about half way between us. On another hand, if one has an angle of 35 degrees and the other has an angle of 10 degrees, the signal will also refract in the E region, but at a location closer to the transmitter with the greater angle. So with a lower angle of radiation, from one station, and a 350 mile distance, the other station must have a higher angle in order to receive the signal.
There is also a station that is 175 miles from me. I know it is there because it is listed on the Parks on the Air (POTA) website, but I am unable to hear it, and unable to make contact. If one of the two stations in this configuration has an angle of 35 degrees and the other has an angle of 10 degrees, just as with the station that is 350 miles away, then a refraction point would theoretically be below the D region. We know there is no refraction below the D region. If that is the situation, then it is a possible explanation for the inability to make a contact. Other amateur radio operators report, as I experience, that they are unable to make contact with sttions tht are within a great circle distance of about 350 miles.
Now if we consider an HF connection between Des Moines, Iowa and Berkley, California, it is possible to enter a variety of angles for a and b, and see where the signal might be refracting. A good example is with a setting of 15 degrees for angle (a), and a setting of 7 degrees for angle (b). With a distance of 1,500 miles, the value of h reaches the F region at 220 miles.
Points on the diagram are designated by capital letters, Point A, B, and C. Angles are designated by small letters a, b, and c. The height of point C above the earth's surface is designated with the letter h.
If you set the degrees at point A to 0 and set the degrees at point B to 15, it will look like the line from point A to point C ends at Earth without extending all the way to point A. That is an artifact of the way the program draws the line. Actually, what is happening is that the line becomes a tangent to the Earth circle, with the tangent intersection at point A. Even when the default values of degrees are at 5 for both point A and point B, the circle-drawing function makes it look like the red line doesn't make it all the way to the point, when in fact the signal leaves the point at a slight incline.
If both point A and point B are set to angles 13 or 14 degrees, h is within range of the height of the International Space Station.
One way to describe the geometry of the signal is by suggesting that if we look toward the horizon, and look up by the angle of degrees set for a and b and if we could actually see the ionosphere out there, it would be interesting if we were able to see a spot that has a great circle distance of half the total great circle distance (when both angles are the same) and has a height of h. In the default settings, that is 500 great circle miles and a height of 76.6 miles. Imagine a light bulb at that exact spot. We would be able to see it. Now imagine that the operator at point A turns on the light by sending a signal. Well the operator at point B, cannot see point A, but can see that little light.
Addational commentary on the model may be found here. For a discussion of multipath propagation see the page at this link.
A metric version of the estimator tool may be found here.