The Myth of the 5° Angle of Radiation
Are you an HF radio operator looking to work DX? Perhaps you live in a busy built-up area and are surrounded by buildings. You may be wondering if it's possible to achieve long-distance communication with a small garden and limited space. In the past, we were told to aim for a 5° angle of radiation for optimal DX performance. But where did this 5° angle come from? It's not mentioned in the classic textbooks of a few years ago. So, let's explore the reality of HF propagation and discover some hope for those facing challenges in their operating environments.
Understanding HF Propagation
Shortwave signals are propagated by leaving our antenna at a certain angle and hitting the F layer of the ionosphere. The F layer changes in height throughout the day and night, reflecting the radio signals back to Earth at distant points. This is how radio signals on the shortwave bands travel over great distances.
However, if the Earth was flat, we could simply lower the angle of radiation on our antenna to reach thousands of miles. But since the Earth is round, there is a limit to how far a signal can travel before it comes back to Earth. As a rule of thumb, the maximum distance for a single hop is about 4,000 km (or approximately 2,500 miles). This means there must be another mechanism that allows us to communicate with stations on the other side of the world.
The Reality of Low Angle Radiation
Many believe that achieving a 5° angle of radiation is necessary for successful DX communication. However, this notion is a misconception. Let's take a closer look at the evidence.
First, vertical antennas cannot achieve ultra-low angle radiation. The Brewster effect, caused by typical ground conditions, cancels out any signals below 10°. So, in practical terms, 5° radiation is unrealistic and not achievable with vertical antennas.
Horizontal antennas, such as dipoles or yagis, would need to be at heights of 100 ft or more to reach a 10° angle of radiation on the 20-meter band. Such heights are not easily achievable for most stations in the UK. Therefore, 5° radiation is virtually impossible to achieve with horizontal antennas as well.
The Role of Multi-Hop Propagation
So, if low angle radiation is not necessary for DX communication, how can we work stations on the other side of the world? The answer lies in multi-hop propagation. Instead of a single hop, signals can travel through multiple layers of the ionosphere, allowing communication over longer distances. The optimal angle for multi-hop propagation is around 15-25° or even higher.
For example, if you want to communicate from the UK to the United States or Canada, a multi-hop path is required. The signal is reflected by the F layer, hits seawater (which acts as a reflector), and then bounces back up to the F layer again before reaching the destination. With multi-hop propagation, you don't need ultra-low angle radiation. Angles of 15-25° can still provide strong signals for DX communication.
The Importance of Height and Angle for DX Communication
While low angle radiation can be beneficial in some cases, it's not always the best height for DX communication. A higher angle of radiation, around 20-30°, can still work well, especially when utilizing multi-hop propagation. This angle happens to be the sweet spot for a typical half-wave antenna, which is a popular choice for many operators.
For example, with a half-size G5RV dipole at a quarter wavelength above the ground (approximately 32 ft), high angle radiation is predominant. Yet, it can still allow successful communication with stations in the USA and the Caribbean on a regular basis, thanks to multi-hop propagation.
Understanding HF radio propagation is crucial for successful DX communication. While the idea of a 5° angle of radiation may sound appealing, it is not realistic or achievable in practical scenarios. Instead, focus on optimizing your antenna height and angle for multi-hop propagation. With a higher angle of radiation and the help of nature's phenomena, such as the F layer reflection, it is possible to work DX even with modest antennas and limited space.
So, don't be discouraged by the misconception of the 5° angle of radiation. Experiment with different antenna heights and angles, and enjoy the wonders of shortwave communication. With the right setup and understanding, you can connect with stations around the world and experience the thrill of DX communication.
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