Introduction
Earth-Moon-Earth (EME) communications represent one of the most challenging and rewarding frontiers in amateur radio. For EME enthusiasts, the goal is to optimize every aspect of the station to overcome extreme path loss and achieve the best possible signal-to-noise ratio. Antenna systems play a critical role in this pursuit, and cross-polarized Yagi antennas (XPOLs) are often the weapon of choice for their ability to provide dual-polarization for signal reception and transmission. However, ensuring that the elements of the Yagi antenna are perfectly centered through the boom is a key design factor that cannot be overlooked. Failure to do so introduces pattern distortion, polarization inefficiencies, and measurable degradation in system performance, including detrimental impacts on noise figure and G/T (Gain over Temperature).
This article delves into the technical reasons why centering the elements in cross-polarized Yagis is essential and explores the consequences of off-center element placement. It also provides a deeper understanding of how technological advancements have improved Yagi antenna performance, making them an essential tool for Hams focusing in EME. By incorporating modern simulation tools and paying careful attention to the physical construction of the antenna, operators can push the boundaries of what is achievable in EME communication.
A man that knows - Joe Taylor K1JT commissioned Justin G0KSC and InnovAntennas to design a 'no compromise EME and Radio Astornomy array to install at Princeton University in NJ, USA (W2PU). 2 of the 4 perfectly alligned XPOLs for 432MHz can be seen here.
The Legacy of Yagi-Uda and Early Optimization Techniques
The Yagi antenna, often known simply as the Yagi, was originally invented by Shintaro Uda and popularised by his colleague Hidetsugu Yagi in the 1920s. The Yagi-Uda antenna design quickly became renowned for its simplicity and impressive gain. In the early days, achieving optimal performance with a Yagi antenna was often done using a manual "cut and try" approach. Builders would construct the antenna, cut or move elements, and manually measure performance each time to determine if the changes resulted in an improvement.
This manual optimisation was effective to a point, allowing enthusiasts to develop Yagi antennas with reasonable gain and Front to Back ratio (F/B). However, it lacked the precision necessary to reach the deeper levels of optimisation seen today. The advent of computer simulation revolutionised Yagi design. Programs like NEC (Numerical Electromagnetics Code) allowed designers to simulate countless variations at a rate of 10 or more adjustments per second, honing and refining performance to a level far beyond what could be achieved manually.
As computer technology advanced, so did the ability to model increasingly complex antenna systems. The introduction of user-friendly software such as EZNEC and MMANA-GAL made these sophisticated modelling techniques accessible to amateur radio enthusiasts, allowing more precise and efficient optimisation of their Yagi antennas. These tools enabled users to experiment with different designs in a virtual environment, dramatically reducing the trial-and-error nature of early Yagi optimisation techniques and making the development process more efficient and accurate.
The advancements in modelling software have also allowed designers to consider other critical parameters, such as impedance matching, feedline interactions, and environmental factors like ground effects. This holistic approach to design has led to further refinements in Yagi antennas, pushing the limits of gain and F/B while maintaining simplicity in construction. The combination of precise modelling and practical construction techniques has been key in achieving the high performance demanded by EME enthusiasts.
The analyser at KG6NK - allowing finalisation of the driven element as InnovAntennas does and with the antennas attention to detail being second to none, incredibel return loss results can be seen. -54.12dB @ 144.126MHz on this antenna (shown) and -48dB on the opposing plane.
Cross-Polarization and the Role of Symmetry
Cross-polarized Yagi antennas are typically designed with two sets of elements arranged at right angles to each other. This configuration allows simultaneous horizontal and vertical polarization, which is especially important in EME operations to counteract the unpredictable Faraday rotation of signals as they pass through the ionosphere. Faraday rotation can alter the polarization of signals, making it essential to have antennas capable of receiving both polarisations effectively.
For this dual-polarized design to work optimally:
- Each polarization plane must be independent and unobstructed.
- The radiation patterns must remain symmetrical for each polarization.
Perfectly centring the elements through the boom ensures that each plane's radiation pattern remains symmetrical and does not encroach on the other plane. If the elements are offset, the geometry of the antenna changes, leading to unintended coupling between the polarization planes, pattern distortion, and degrading overall system performance. Symmetry is critical to maintaining consistent gain and preventing losses that could reduce the effectiveness of EME communication. Often, antennas with offset elements that are combined into multi-antenna arrays (the most common being a 4-in-a-box configuration, usually supported by an H-frame) carry the same offset on all antennas. However, if the offsets on each boom in the 4-antenna configuration were mirrored, a large amount of the degradation could be avoided.
In EME, the ability to switch between polarisations or combine polarisations dynamically is essential to achieving the best possible signal-to-noise ratio. Ensuring perfect symmetry in element placement allows the antenna to maintain its intended radiation pattern and minimises the risk of cross-polarisation interference. The benefits of this precision become evident when trying to decode weak signals that are already at the edge of detectability.
Effects of Offset Elements on Performance
1. Pattern Distortion
Yagi antennas rely on precise spatial relationships between the driven element, reflectors, and directors to form a highly directional radiation pattern. When elements are not centered:
- Misalignment of phase centres: Off-center elements create phase differences between the intended and actual radiating surfaces, distorting the main lobe of the antenna pattern.
- Unwanted coupling: The offset elements intrude into the opposing polarization plane, leading to coupling between the two sets of elements. This coupling disrupts the clean separation between polarisations and leads to degraded performance.
- Coiled Coaxial Chokes: This is also true for antennas with coiled coaxial choke baluns installed. The large surface area of these coiled baluns presents a significant electromagnetic presence in the opposing plane, which will lead to additional coupling in an extremely sensitive part of the antenna (either side of the dipole) and lead to a compounding degrade of system performance.
Distortion of the radiation pattern reduces forward gain and increases sidelobe levels, making the antenna more susceptible to interference from unwanted directions. In EME operations, maintaining a strong signal-to-noise ratio (SNR) is more critical than simply maximizing gain. When modeling the intended array, careful consideration should be given to both the operating frequency and the surrounding environment, including ambient noise levels, before finalizing the design. If your transmitted signal to the moon is not strong enough, adding power with efficient solid-state amplifiers is relatively straightforward and, these days, low cost. However, reducing a high noise floor is much more challenging. The best approach is to use a Yagi optimized for low noise, with highly suppressed sidelobes and a Front to Rear (F/R) ratio of 30 dB or better relative to the main lobe. These Yagis are particularly sensitive to element offsets, meaning even slight misalignments can cause significant performance degradation compared to traditionally optimized Yagis.
HB9DRI, like those as W2PU at Princeton, uses square fibreglass booms on this 432MHz EME array and perfectly aligne centres for best result
2. Polarization Purity
EME signals are weak and subject to polarization mismatch losses. Cross-polarized antennas minimize these losses by providing clean, independent polarization planes. Off-center elements compromise this purity by introducing:
- Cross-polarization leakage: Misaligned elements contribute to unwanted signals in the orthogonal plane, reducing the antenna's ability to discriminate between polarisations.
- Reduced Faraday rotation adaptability: Impurities in polarization can result in degraded reception of the desired signal, especially when Faraday rotation shifts signal polarization away from the expected plane.
Maintaining polarization purity is essential for minimizing losses in received signal strength. Cross-polarization leakage not only weakens the desired signal but also introduces noise, which can make decoding EME signals more difficult, especially when conditions are already marginal.
Another key consideration in maintaining polarization purity is the role of feedline management. The routing of coaxial cables and the use of baluns can introduce unintended coupling, which further compromises polarization performance. Careful design of the feed system, alongside precise element centering, ensures that the radiation pattern maintains its intended polarization characteristics.
3. G/T (Gain over Temperature) Reduction
G/T is a key figure of merit in EME systems, combining the antenna's gain with the system noise temperature. Pattern distortion and polarization leakage from off-center elements contribute to:
- Lower gain: A distorted main lobe reduces the effective gain of the antenna in the desired direction, directly impacting G/T.
- Higher noise temperature: Unintended coupling between polarization planes and distorted sidelobes increase the system's susceptibility to noise from terrestrial or celestial sources.
It is important to note at this point that comparative lists such as that managed by VE7BQH do not account for element offsets within predicted G/T performance, and thus figures presented on lists such as these could be an overestimation of the actual figures in the real world. A reduction in G/T results in a decrease in overall system performance. In EME communication, where signal levels are extremely low, any reduction in gain or increase in noise can mean the difference between making a successful contact or missing out entirely.
Optimising G/T requires careful attention to every aspect of antenna design, including minimizing noise sources such as resistive losses, poor connections, and thermal noise from feedline components. To achieve the best performance, several aspects should be avoided: offset XPOL elements, the use of coaxial choke baluns within XPOL arrangements, and feed point boxes that enshroud part of the driven element. These factors can all contribute to unwanted coupling and increased noise, thereby degrading overall system performance.
Ensuring precise element placement is key to maintaining a high G/T, which is crucial for successful long-distance EME communication where system noise must be minimised. Precision in design and construction helps maintain symmetry and optimal radiation characteristics, leading to the best possible signal-to-noise ratio and ensuring that every decibel of gain is preserved for effective communication. In addition, any necessary mechanical support or alignment structures that must physically contact the antenna elements should be kept as small as possible and positioned as close to the element’s center as practicable. Larger or off-center contact areas can introduce additional coupling and noise, further eroding system performance.
Practical Considerations for Element Centering
1. Mechanical Precision
Achieving perfect centering requires precise construction techniques:
- Use booms with pre-drilled, accurately spaced holes for element mounting.
- Consider dielectric insulators or clamps that ensure consistent alignment through the boom without introducing mechanical sag.
Mechanical precision is critical not only during the initial construction but also over the life of the antenna. Elements that shift or sag over time can lead to performance degradation. Regular maintenance checks and adjustments are necessary to ensure that the elements remain properly aligned.
Another important factor is the use of high-quality mounting hardware. Stainless steel or non-corrosive fasteners can prevent gradual shifting caused by environmental wear. Additionally, structural supports that mitigate flexing or bending of the boom can help maintain consistent element alignment over time, especially in areas with high wind loads.
2. Material Choices
Avoid materials that may warp or shift over time. Aluminum booms and elements are standard due to their rigidity and low weight, but proper bracing and support are essential to maintain alignment over the antenna's lifespan. Stainless steel hardware can also be used to prevent corrosion, which could lead to changes in element positioning.
In addition to selecting materials that resist physical deformation, it is also important to consider environmental factors such as wind loading, temperature fluctuations, and ice accumulation, which can all impact element alignment. Designing the antenna with robust support structures can mitigate these effects and help maintain consistent performance.
Another consideration is the use of materials that have low thermal expansion properties. Elements and booms that expand or contract significantly with temperature changes can alter the geometry of the antenna, impacting its resonance and radiation pattern. Materials with minimal thermal expansion help maintain performance consistency across varying environmental conditions.
Perfectly centred - on 70cms, perfectly aligning XPOL element is a MUST. Cross booms should also be non-metallic in such instances
3. Modeling and Simulation
Even simple antenna modeling tools like EZNEC, 4nec2, or MMANA-GAL can simulate the impact of element misalignment. Use these tools to:
- Compare the radiation patterns of perfectly centered versus offset designs.
- Analyze the coupling effects and determine the degree of performance degradation.
Simulation tools allow designers to experiment with different configurations and identify potential issues before building the antenna. By using modeling software, operators can visualize how even small deviations in element positioning can impact overall performance and make informed decisions to avoid these pitfalls.
Advanced simulation tools, such as Ansys HFSS, can also provide insight into more complex interactions, including the impact of feedlines, matching networks, and environmental factors. These tools provide a more comprehensive understanding of the antenna's real-world performance and help refine designs for maximum efficiency and gain.
Generally, good quality low noise XPOL Yagis will have provisions for the ham to fine-tune the driven element. This helps to remove increased return loss caused by stacking and phasing arrangements.
4. Testing and Measurement
Validate your antenna's performance after construction:
- Use an antenna analyzer or VNA to check SWR and impedance matching for both polarization planes.
- Measure radiation patterns in a controlled environment to confirm symmetry.
Testing is an ongoing process. After initial construction, it is advisable to perform periodic measurements to ensure that the antenna continues to perform as expected. Environmental factors, wear and tear, and accidental damage can all impact antenna performance, making regular testing a key component of maintaining optimal system performance.
Additionally, on-the-air testing with known stations can provide practical validation of the antenna's performance. Comparing received signal reports and analyzing the antenna's ability to decode weak signals can offer valuable insights into how well the theoretical performance translates into real-world conditions.
Quantifying the Impact: Real-World Examples
Let us consider a dual-polarized Yagi operating on the 144 MHz band:
The VE7BQH G/T Table
Another valuable resource for comparing Yagi antennas is the VE7BQH G/T Table, compiled by Lionel Edwards. Lionel's list contains an extensive range of Yagi antennas that are compared in terms of G/T (Gain over Temperature). However, it is important to understand the limitations of this list and the models upon which it is based.
The software models used for the G/T comparisons in Lionel's list do not take into account the physical impacts of matching devices, coax cable exits, or routing, and especially not the offsets that can be introduced in real-world implementations where above-boom mounted elements are applied within XPOL Yagis. Consequently, users of the VE7BQH G/T Table could be misled into believing that certain antennas perform better than others purely based on single-plane Yagi G/T values (even though presented as XPOL within the list), without considering these additional real-world factors that can significantly affect antenna performance with two off-set sets of elements are applied to the same boom.
Note there are no 3D analysed models in Lionel’s list. This is because only models based on the simple 'wire-only' analysis using NEC are presented in the VE7BQH G/T Table. None of InnovAntennas' very latest models that had been developed within Ansys HFSS, which simulates the complete antenna structure and all its components, boom, insulators, coax, balun, etc., can be presented in this list. Therefore, it is essential to recognise that the VE7BQH G/T Table, while valuable, does not provide a complete representation of antenna performance in practical environments.
A 144MHz (2m) system at W6TCP with perfectly aligned elements, curved balun exists and cables to the rear of the booms - the ideal configuration to ensure best-in-class results
Quantifying the Impact: Frequency-Specific Analysis
Off-set ‘Above Boom’ mounted elements within XPOL Yagis
Key Parameters Across Frequencies:
- Wavelength relationship to offset
- Cross-polarization discrimination
- Gain reduction impact
- G/T degradation effects
To understand the practical implications of element offset, let's examine how performance degradation scales across different amateur radio bands commonly used for EME operations. The impact of physical offset is directly related to the wavelength at the operating frequency, making it crucial to consider these effects for specific bands:
144 MHz (2m band) Example:
- Wavelength: 2.08m
- Perfectly Centered Design:
- Clean main lobes with -25 dB cross-polarization discrimination
- Gain: 16 dBi
- G/T: -2 dB
- With 25mm offset (0.012λ):
- Cross-polarization discrimination reduces to approximately -20 dB
- Gain reduction of approximately 0.5 dB
- G/T degradation to around -2.7 to -3 dB
While these effects might seem marginally acceptable at 144 MHz, it's important to emphasize that any degradation in performance is completely unnecessary and should be avoided. Through-boom center mounted elements eliminate these issues entirely, maintaining optimal performance without compromise.
432 MHz (70cm band) Example:
- Wavelength: 0.694m
- Perfectly Centered Design:
- Clean main lobes with -25 dB cross-polarization discrimination
- Gain: 16 dBi
- G/T: -2 dB
- With 25mm offset (0.036λ):
- Cross-polarization discrimination reduces to approximately -15 dB
- Gain reduction of 1-1.5 dB
- G/T degradation to around -3.5 to -4 dB
For 432 MHz operations, offset elements should be actively avoided. The performance degradation at this frequency effectively reduces your system to the equivalent of using a much smaller array - negating the investment in larger antennas and potentially compromising months or years of station optimization. These losses can be completely eliminated by utilizing centrally mounted through-boom elements, making this the only acceptable approach for serious EME operations.
1296 MHz (23cm band) Example:
- Wavelength: 0.231m
- Perfectly Centered Design:
- Clean main lobes with -25 dB cross-polarization discrimination
- Gain: 16 dBi
- G/T: -2 dB
- With 25mm offset (0.108λ):
- Cross-polarization discrimination reduces to approximately -12 dB
- Gain reduction of 2-2.5 dB
- G/T degradation to around -4 to -4.5 dB
2304 MHz (13cm band) Example:
- Wavelength: 0.130m
- Perfectly Centered Design:
- Clean main lobes with -25 dB cross-polarization discrimination
- Gain: 16 dBi
- G/T: -2 dB
- With 25mm offset (0.192λ):
- Cross-polarization discrimination reduces to approximately -10 dB
- Gain reduction of 3-4 dB
- G/T degradation to around -5 to -6 dB
These examples clearly illustrate how the same physical offset (25mm) has increasingly severe effects at higher frequencies. For instance, while a 25mm offset might be marginally acceptable for 144 MHz EME operations, it would seriously compromise performance at 1296 MHz or 2304 MHz.
PJ2MM -144 MHz (2m) system elements are perfectly centered, allowing fully capitalized performance rather than settling for performance as if the array were smaller.
Practical Mounting Considerations:
When mounting elements on a typical 30mm square boom with insulators standing the elements 10mm above the boom, several factors become critical:
- For 144 MHz:
- Element offset effects are less severe
- Primary focus should be on mechanical stability
- Careful boom-to-mast mounting to prevent twisting
- For 432 MHz and above:
- Precision mounting becomes increasingly critical
- Consider using precision-machined, small footprint mounting methods
- Implementation of anti-twist boom supports
- Regular inspection and maintenance of mounting hardware
The reduction in gain and increase in noise temperature translate to weaker signal reception, potentially losing critical decibels that could make the difference in decoding an EME signal. At higher frequencies, where the wavelength-relative offset is larger, these effects become particularly pronounced. For serious EME operators, especially those working on bands above 432 MHz, the precision of element mounting cannot be overstated.
Conclusion
For EME enthusiasts, maximising system performance is non-negotiable. Cross-polarized Yagi antennas provide the dual-polarization needed to overcome the challenges of Faraday rotation, but their effectiveness hinges on precise mechanical and electrical design. Ensuring that elements are perfectly centred through the boom is fundamental to maintaining symmetrical radiation patterns, polarization purity, and optimal G/T.
By adhering to best practices in antenna construction and leveraging modern modelling tools, operators can avoid the pitfalls of offset elements and achieve the performance needed to succeed in the demanding world of EME communication. Remember, in EME, every decibel counts—perfection in design and execution is not just desirable; it is essential.
Additionally, by staying informed about new developments in antenna modelling and construction techniques, operators can continue to push the boundaries of what is possible in EME communication. The pursuit of excellence in antenna design is an ongoing journey, and with careful attention to detail, operators can achieve truly remarkable communication capabilities, continuously advancing the art and science of EME operations.
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Justin G0KSC - InnovAntennas
