The physics of signal fading & how Rhombic arrays stop the fading
- skylarkcolo
- Jul 21, 2020
- 6 min read
Updated: Feb 21
The core problem: Fading, radio operators call it QSB
Signal fading is a phenomenon that primarily occurs because a receiver does not receive one clean, singular wave. Instead, it encounters multiple copies of the same signal that:
arrive from different directions, creating a complex interference pattern
travel slightly different path lengths, which can vary due to obstacles and environmental factors
arrive at slightly different times and phases, leading to variations in signal strength
When these multiple copies of the signal combine, they can sometimes reinforce each other, leading to stronger signal reception. However, there are also instances where they may cancel each other out, and this cancellation effect is what results in deep fades in the received signal. This phenomenon of fading can be particularly problematic for amateur radio operators, often referred to as "Hams," who rely on consistent signal quality for effective communication.
Why a very large rhombic array helps
A rhombic antenna is designed to spread the receiving elements over a very large physical area (acres) — often extending many wavelengths across. Traveling wave antennas are known for fantastic Improvements in the reduction of signal fading, which is called QSB by amateur radio operators, for both transmit or receive.
This significant size provides several distinct advantages that enhance the antenna's performance in overcoming fading issues:
1. Spatial averaging of the signal
Because the antenna covers such a large area:
Different parts of the antenna “see” different interference patterns, which vary based on local conditions
A deep fade at one specific point in space is not necessarily a deep fade just a few wavelengths away, where conditions may be entirely different
Therefore, while a portion of the array may find itself situated in a null caused by destructive interference, other sections are likely receiving a strong signal. When the antenna system combines these various signals, the effects of fading are effectively averaged out, leading to a more stable overall reception.
Think of it like:
Standing waves on water — one spot may be calm, but just a few feet away, the water can be moving strongly and creating ripples.
2. Sensitivity to many arrival angles
Multipath signals can arrive from slightly different angles due to various factors such as ionospheric refraction or reflection, which can significantly affect signal quality:
A small antenna typically samples essentially one spatial point, meaning it is limited in its ability to capture the full range of incoming signals
In contrast, a huge rhombic antenna can sample many angles simultaneously, allowing it to capture a broader spectrum of signals
The unique geometry and length of the rhombic antenna allow it to:
Favor forward-traveling waves, which are often the most useful for communication
Suppress backward and off-angle reflections that could introduce unwanted noise and interference
This characteristic significantly reduces the chance that a single problematic arrival angle can dominate the signal reception and cause detrimental cancellation effects.
3. Time dispersion becomes less destructive
Because signals can arrive at slightly different times due to the nature of their paths:
On a small antenna, delayed signals can easily cancel out the main signal, leading to poor reception
On a large antenna, those delayed arrivals are not coherent across the entire structure, meaning they do not synchronize in a way that would lead to cancellation
In other words, the phase errors introduced by different arrival times aren’t uniform across the array. When these signals are summed across the entire antenna, they do not align well enough to completely negate the main signal, resulting in a more reliable reception overall.
4. Narrow beamwidth = fewer bad paths
The rhombic’s long length results in a very narrow main lobe, which has several important implications for signal reception:
Fewer multipath components are accepted, which minimizes the potential for interference
There is strong rejection of off-axis reflections, further enhancing the clarity of the received signal
With less multipath interference, there is inherently less fading to begin with, which contributes to a more stable and robust communication link.
This is particularly evident in audio reception, where long traveling wave antennas significantly reduce audio amplitude fading for both transmission and reception. The RF signal rarely maintains a stable phase relationship at both locations simultaneously. This results in random phase and amplitude variations in the signal. The arrival angle and polarization of incoming signals will fluctuate. Typically, this leads to fading; however, with many wavelengths of wire in the air, the likelihood is that while one part experiences a fade, another will not.
Also, emphasize the idea of signal to noise, which is closely linked to the Relative Directivity Factor (RDF). The RDF is defined as the antenna gain in the forward direction compared to the gain in all other directions. This metric is crucial as it evaluates an antenna's capability to focus its reception in a specific direction while minimizing interference from other directions.
Signal-to-noise ratio (S+N/N ratio, or SNR) is a technical aspect that many amateurs overlook, yet if you can't hear them, you can't work them. This is particularly evident in audio reception, where large traveling wave antennas covering extensive areas help reduce audio amplitude fading for both transmission and reception. The RF signal is rarely in a stable phase relationship at both locations simultaneously. This results in the signal having random phase and amplitude variations. The arrival angle and polarization of incoming signals will change, typically causing fading. However, with many wavelengths of wire in the air, the likelihood is that while one experiences a fade, another will not.
The intuition in one sentence
A rhombic antenna is so large that the fading patterns do not align uniformly across its structure; thus, destructive interference occurring at one location is counterbalanced by constructive interference occurring at another location — ultimately resulting in a more stable, fade-resistant signal overall, which is crucial for effective communication in amateur radio operations.
Don't underestimate the performance of the Rhombic or V Beam array, unless you have personally built and used one. Because of their excessive size (area) covering many acres, you see their real advantage of thousands of feet of wire in the air, which creates receive signal diversity, by capturing signals at different times and different angles, vastly eliminating fading QSB, and firing out the transmitted RF in the same way. Traveling wave antennas are very unique and unlike many other antenna in common use.
Final Thoughts on HF Rhombic and Traveling wave Antennas
Rhombic HF and other Traveling wave antennas continue to be an effective solution for long-range communication. Their high gain, broad bandwidth, and straightforward construction appeal to many users who require dependable, concentrated signals across vast distances.
Although they need acres of space and meticulous installation, the advantages typically surpass these obstacles. Whether you are an amateur radio enthusiast seeking remote connections or a broadcaster targeting global audiences, a rhombic antenna can provide robust, clear communication.
THE K0UO Rhombic Antennas features an impressive configuration comprising four exceptionally large arrays, each mounted on a towering 100-foot poles. This significant height not only allows for optimal signal propagation, but also enhances the overall effectiveness of the antenna system in various operational conditions.
To put this remarkable setup into perspective, each of the four rhombic antennas is covering a vast area that is equivalent to five football fields, which translates to approximately 7 acres per antenna. This expansive coverage area is crucial for effective transmission and reception, especially in the context of amateur radio operations where range and clarity are paramount. In addition to the rhombic antennas, There are also three V-Beams, each covering over 3.5 acres each. The need for such extensive acreage is critical for ensuring that signals can be transmitted and received without significant fading and interference.
Furthermore, the antenna system is complemented by a variety of other advanced components, including stacked Log-Periodic Dipole Array (LPDA) Yagi beams that enhance directional capabilities and provide additional versatility in communication. Alongside these, there are miles of Beverage receive antennas, which are specifically designed for low-frequency reception, and are known for their exceptional performance in picking up weak signals from distant sources. Additionally, implemented in this farm is very large steerable HRS curtain array and dipole antennas, which further diversify the antenna capabilities and allow for a more comprehensive range of frequencies to be utilized.
In terms of infrastructure, the setup includes poles and towers that reach heights of up to 195 feet, making them some of the tallest structures used in amateur radio today. This elevation is instrumental in minimizing ground loss and maximizing the effective radiated power of the antennas. Collectively, these installations represent the world's largest ham radio HF wire antenna arrays currently in operation, a testament to the engineering and design that has gone into creating such an extensive and capable system.
K0UO is extremely fortunate to have access to over 1200 acres of land surrounding the antenna site. This vast expanse is not only beneficial for the installation and maintenance of the antennas but also provides an ideal environment for conducting far-field HF antenna measurements. Such measurements are essential for accurately testing and evaluating the performance of these antennas and others in my collection. The ability to perform these tests in an unobstructed area allows for precise assessments of signal strength, clarity, and overall efficiency, ensuring that the antennas operate at their peak potential
Comments