Last Updated on October 22, 2024 by John Berry
Understanding communications path geometry is fundamental to understanding radiowave propagation.
Ray tracing in path geometry
Path geometry drawing embraces ray tracing to describe the path from transmitter to receiver via all the obstacles, clouds of plasma, and reflective layers that might support the communication. There’s a lot the radiowaves encounter along the way. An accurate idea of the path geometry is useful in predicting whether the path will work.
And yet, if ray tracing – the notion of a single ray from one end to the other – is taken literally, misconceptions will abound. Ray tracing is a concept, a model, and no more. The overriding phenomenon is that the signal arriving at the receiver is the vector sum of a plurality of rays, a near-infinite number of rays. There are therefore many geometries in any communication.
Frequency dependence
Communications path geometry is also agnostic when it comes to frequency. A VHF path can go sky-wards, while an HF or LF path can go from point to point. The path loss is of course a function of frequency. And there are a plethora of propagation models we’d use to calculate path loss in each case.
And although ray tracing shows rays as straight lines between points, propagation by refraction through real environments like the Earth’s atmosphere will cause the ray to bend. Ray tracing principles show the ray straight and note the refraction incident at the time. In VHF/UHF/SHF communications where the refractivity of the troposphere changes, we change the Earth’s radius to maintain a straight-line ray.
The essence of ray tracing is the path between transmitting antenna and receiving antenna. In simple VHF communications, the ray might go from TX to RX. But in HF communications, the ray often goes via the ionosphere.
About angles
Communications path geometry is all about angles – typically the angle between the ray from transmitter to receiver in the vertical plane (VRP) with reference to the horizon. When talking about antennas, this angle is often referred to as the launch angle. There may be a big difference between the necessary launch angle to accommodate the geometry of the path, and the available lunch angle from the chosen antenna. Given the former, the latter may stop the path working.
Here are some typical path geometries, noting where launch angles are critical.
| Path type | Geometry | Critical angles |
|---|---|---|
| VHF point to point over an obstructed LoS path | Single ray from TX to RX | Angle of elevation to the first major obstruction |
| Auroral communications | From TX, north to auroral column and back, considering column may be overhead one or both stations | Launch angle of both TX and RX antennas |
| Sporadic E communications | From TX south (or SE, or SW) to refracting patch, then onward south to RX | All antennas point to the horizon with southerly azimuth (from UK) |
| Meteor scatter comms | From TX to meteor trail, then to RX | Launch angle varies from near-horizon to overhead |
| Moonbounce/EME | Both TX and RX antennas point at the moon | None, both antennas always point at the moon |
| Ionospheric communications | From TX via ionospheric region to RX at some distance | Launch angle of both TX and RX antennas |
| HF ground wave | From TX to RX via undulating ground, no ray tracing | Launch angle zero degrees at both TX and RX |
Choosing antennas
In many cases the VRP response of the antennas will make or break the communications. Take this example where the antennas point to the horizon. If both have ±15 degree VRP responses, and the geometry needs a launch angle of 45 degrees at one end, a significant loss will result in that antenna. This antenna loss may render the communications impossible. This is why short, low gain antennas (with broad VRP responses) are best for some geometries and propagation modes.