An introduction to the LFA Yagi

To the novice, the driven element of the LFA Yagi might seem like just another folded dipole. However, this assumption is far from reality. In this piece, we will explore in depth the reasons why the low noise LFA truly stands apart from the crowd.

Beyond its distinctive rectangular loop, surrounded by parasitic elements, the LFA Yagi's true uniqueness stems from the additional control parameters incorporated during the optimisation process. These intricacies significantly bolster the Yagi's performance, enhancing its noise reduction, gain, Front to Back Ratio (F/B), and bandwidth attributes. Following this introduction, we'll delve into a detailed breakdown of its key features.

4 x 10el low noise 50MHz LFA Yagis on 17.7m long booms at KJ9I

Below is a list of key features which shall be elaborated on further within this piece.

• Low noise Yagi – ‘The Urban’ or City Yagi
• Distinctive Close loop driven element
• Optimised performance with direct 50Ω feed
• Extended bandwidth capabilities of the LFA
• Closed Loop Impedance - explained
• Driven element with DC ground
• Lower native impedance
• Self-Balancing Yagi
• Symmetrical in both AZ/EL
• Band Pass Filter properties of the LFA
• Higher power handling
• Phased Loop cancellation properties
• Enhanced Optimisation Dynamics of the LFA
• Best-in-class G/T
• Evolution of Computer Optimisation and the LFA’s place in it
• Real World Translation of the Perfect Antenna Model
• Future-Proofing the X-pol Yagi: Centralised elements & Cable Integration

The Low Noise LFA Yagi – The Optimal Choice for Noise-Laden Urban Areas

Before we look deeper into the specifics, let's set the scene with a tangible demonstration of its real-world performance. A Canadian ham, equipped with a 7-element 50MHz Yagi from a distinguished USA-based brand, chose to transition to a low noise LFA, maintaining both the length and the number of elements. With the new LFA proudly perched on a fresh tower, plans were set in motion to retire the older 7-element American model in favour of an HF multi-bander.

However, prior to the final switch, he was driven by curiosity. Engaging in A/B tests, he connected both antennas to distinct sockets on the same radio. In the comparison video provided below, you'll observe that while the signal strengths might appear similar, the low noise LFA's remarkable reduction in the noise floor transforms barely perceptible signals from the original 7-element antenna into crisp Q5 signals.

.

So, what magic does the low noise LFA Yagi weave to stand out in its performance?

The Distinctive Closed Loop Driven Element of the LFA
The low noise LFA Yagi's signature closed, rectangular loop driven element is a marvel of engineering adaptability. Unlike the static nature of traditional dipoles, the LFA's loop thrives in a three-dimensional space, granting it unparalleled flexibility of adjustment.

Here's where it stands apart: In traditional Yagi optimisation, when the dipole shifts, it either approaches the reflector while distancing from the director or vice versa. However, the LFA's loop, with its unique three-dimensional construct, doesn't abide by this limitation. It can expand or contract along the boom, allowing it to adjust its distance from both the reflector and the director simultaneously. This means it can move closer to both, further from both, or selectively change its distance to only one of them whilst keeping its relationship with the other constant.

Furthermore, the choice of the loop's feed point placement—be it nearer to the reflector or the director—introduces another dimension of optimisation. Each placement brings forth distinct performance characteristics, which are then meticulously refined through subsequent optimisation.

It's this multifaceted adjustability, combined with detailed computer optimisation, that sets the LFA loop apart from any other Yagi, making it a beacon of innovation in Yagi design.

Optimised Performance with a Direct-Fed 50Ω LFA Yagi

Traditional dipole-fed Yagis operating at 50Ω impedance often deliver average performance. Enhancements in performance can be achieved by reducing the feed point impedance. However, this brings about its own challenges. While the lower impedance can bolster performance metrics like forward gain and F/B, it also necessitates the integration of a transformer or matching device to restore the feed impedance to the standard 50Ω. This adaptation leads to inefficiencies, possibly negating the advantages of the lowered impedance. This is where the low noise LFA Yagi, with its unique loop design, truly shines. The LFA offers the enhanced performance characteristics typical of a traditional split-dipole Yagi with lowered impedance, yet it maintains a direct 50Ω feed, eliminating the need for additional matching devices. This combination ensures optimum performance without the associated losses of traditional designs.

4 x 7el 50MHz & 2 x 18el 144MHz low noise LFA Yagis at K1USA

Expansive Bandwidth with the LFA – No Compromises on Performance

The LFA Yagi has made waves in the world of Ham Radio, particularly when it comes to bandwidth capabilities. Traditional beliefs held that native low impedance Yagis would inherently have a reduced bandwidth. Yet, the LFA Yagi, with its unique three-dimensional loop adjustments, has showcased that it's possible to maintain, and even increase, bandwidth without compromising on other performance metrics.

This is notably evident when comparing the LFA Yagi to the benchmark Optimised Wideband Array (OWA) Yagi. The OWA, with its two-dimensional split dipole, has been lauded for its expansive bandwidth. However, the LFA's ability to adjust its loop depth—either uniformly or selectively on one side—and to switch its feed point position between the back or front, gives it an edge in optimisation. This results in a Yagi that not only rivals the bandwidth of the OWA but sometimes even surpasses it, all the while delivering higher levels of performance.

Closed-Loop Impedance Explained

A standalone loop, without being part of a Yagi, typically has an impedance ranging between 200Ω and 300Ω, which varies based on the rectangle's shape. When you introduce parasitic elements, such as reflectors and directors, to flank this rectangle, the feed impedance of the loop decreases. Factors influencing this final impedance include the number, shape, length, and proximity of these additional elements to the rectangle.

This is where intricate optimisation comes into play to attain the remarkable results we see with the low noise LFA Yagi. As computational capabilities advance and speeds increase, the LFA Yagi, too, continues to evolve and improve.

It's essential to clarify that while G0KSC offers free-to-build LFA designs on his website, these represent his initial creations from more than 14 years ago. In contrast, only InnovAntennas provide the latest and most refined antenna designs by G0KSC, including the superior next-generation low noise LFA Yagi.

By meticulously optimising a Yagi antenna with an LFA loop, we've achieved a remarkable feat: bringing the impedance down to an ideal 50Ω without need for any matching device. This optimisation isn't just about hitting the right numbers; it's about harnessing the full potential of the antenna's performance and bandwidth.

In a typical scenario with split dipole Yagis, the initial impedance starts at around 70Ω. But through the process of optimisation, this impedance can drop significantly, often falling between 10Ω and 30Ω. Such a reduction necessitates the use of a matching device, adding complexity and inefficiencies to the system.

The LFA Yagi stands in stark contrast to this. Its unique loop design allows it to maintain a direct 50Ω feed, bypassing the need for additional matching devices. This direct feed not only simplifies the overall design but also ensures optimum performance without the losses often associated with traditional designs.

In the past, there were efforts to optimise Yagis with split dipoles to achieve a final impedance of 200Ω. Some experimented with replacing the split dipole with a traditional compact folded dipole, which naturally transformed the impedance to around 50Ω. While this approach did improve bandwidth over a similar length 50Ω split dipole Yagi, it adversely affected essential performance metrics like Gain and F/B ratio. Consequently, this design method was not widely adopted.

4 x 7el 50MHz low noise LFA WOS Yagis at US0KW

DC Ground – Enhancing Performance and Safety

At the core of the LFA loop system's design is its DC ground connection, astutely positioned opposite the feed point. This strategic design choice is pivotal in optimising the antenna's functionality. The ground connection, situated at a zero current point at the operating frequency, performs a dual function. It helps ensure effective radiation of all RF energy up to this juncture, leaving only voltage which in turn helps prevent common-mode currents travelling back along the feedline. Therefore, this feature also contributes to balancing the feed point, akin to the feed point balancing function of a ¼ wavelength hairpin which is extremely important within a Yagi if gain and F/B are to be maintained as model.

The Self-Balancing Yagi

This approach to grounding goes beyond a simple design choice; it is a vital enhancement that significantly betters the Yagi's symmetrical performance. Such a grounding technique, unique to a loop-fed design, delivers benefits that are unattainable with traditional split-dipole designs. Moreover, the ground connection plays a vital role in safeguarding transceiver equipment from static electrical damage.

A 432MHz X-pol LFA Yagi showcasing perfectly centred elements and no feed point boxes, essential for accurate software pattern model replication.

Symmetrical in both AZ/EL

An additional advantage of the LFA loop system is its uniform alignment with all parasitic elements. This alignment ensures perfectly symmetrical elevation (EL) lobes, an attribute elusive in traditional folded dipole Yagis. In conventional designs, where the folded dipole sides are positioned above and below the boom, elevation lobes tend to be asymmetrical. This distortion in lobes becomes increasingly pronounced at higher frequencies, and these complexities are compounded within X-pol designs. The LFA Yagi, with its forward-thinking design, overcomes such challenges and ensures a distortion-free 3-dimensional pattern.

Optimised Band Pass Filter (BPF) for Enhanced Noise Control
Building on the unique attributes of the LFA Yagi, it's important to note the sophisticated implementation of a Band Pass Filter (BPF). This feature is intricately linked to the DC ground, which is effectively invisible at the antenna's design frequency. As the frequency deviates from this set point, there's a significant increase in impedance. This deliberate design choice not only results in the BPF effect, adeptly minimising noise outside the designated band but also leads into the broader scope of benefits encompassed in the LFA Yagi's design.

This BPF property extends its benefits beyond transmission, where it plays an important role in reducing the risk of causing interference. It also acts as a vital safeguard during reception, significantly lowering the likelihood of receiving interference from out-of-band sources or high-powered transmitters. This characteristic is particularly important in bands like 50MHz, especially in regions where older, powerful TV transmitters operate adjacent to the ham radio band. The integration of this BPF feature into the LFA Yagi enhances both transmission quality and reception ability, making it an indispensable asset for operators across various environments.

4 x 22el X-pol low noise LFA Yagis for 144MHz at W6TCP. All element perfectly centred mid-boom - the only way to ensure pattern symmetry and avoid pattern distortion and G/T loss

High Power (QRO)? No Problem!

Historically, the cubical quad was designed to facilitate very high-power transmissions, especially at elevated altitudes where the air is less dense. In such conditions, transmitting high power to split dipole Yagi antennas could lead to coronal discharge from the element tips – a sparking effect. This could cause the tips to melt, resulting in the breakdown of the driven element and potential damage to transmitters.

However, the low noise LFA stands distinct. While oriented differently compared to the typical quad beam, the LFA's loop is a full-wave loop, comparable to those used in quad beams. Its closed-loop design eliminates the issue of melting tips, enabling it to handle much higher power levels. In fact, certain commercial LFA Yagi variants can manage powers in the tens of kilowatts at specific frequencies.

Phase Cancellation in the Driven Loop – Minimised Side Lobes

An essential feature, harnessed during the computer optimisation phase, is the flat end sections of the LFA Loop that sit parallel to each other on either side of the boom. A combination of the final rectangle shape of the LFA loop, coupled with the number and proximity of parasitic elements, can result in these loop ends being 180 degrees out of phase. This phase cancellation effect subsequently leads to pronounced Front to Side (F/S) nulls. As a result, any potential unwanted forward-facing side lobes are significantly suppressed, ensuring a more distinct, singular forward lobe. Furthermore, this effect can also substantially enhance the F/B capabilities.

With side lobes either minimised or eradicated, the low noise LFA becomes an optimal choice for ultra-low noise applications or precise point-to-point transmissions. This standout feature is a prime reason the low noise LFA has been chosen for specific defence applications.

Beyond the Basics: The Enhanced Optimisation Dynamics of the Driven Loop

Within the realm of computer optimisation software for Yagi design, standard parameters like element position on the boom and element length are typically manipulated to achieve an optimised Yagi. This software offers precise pattern and performance simulation tools, facilitating a real-world preview of each design iteration. It continually adjusts element sets, both collectively and individually, modifying their positions and lengths. After each adjustment, it evaluates the results, retaining improvements and continually refining.

However, the low noise LFA's driven loop introduces a groundbreaking dimension to this process. Unlike traditional driven elements, the LFA loop's shape and orientation offer an unprecedented level of flexibility in optimisation. Beyond the standard adjustments of moving the entire shape along the boom or altering its width, the loop's depth along the boom can also be modified, unveiling a third dimension of optimisation possibilities.

To illustrate, consider the limitations of adjusting a standard split dipole between the reflector and first director. Moving the dipole closer to the reflector simultaneously distances it from the director, which might present both advantages and disadvantages. In stark contrast, the LFA Loop's design permits simultaneous, independent adjustments to BOTH sides of the loop. It can be configured to approach both the reflector and the director concurrently. Alternatively, the distance between the reflector and the loop might remain constant, while only the spacing between the loop and the director is adjusted. Additionally, the loop can be manipulated so that it distances itself from both the reflector and the director simultaneously. This unparalleled adaptability facilitates enhanced performance in noise reduction, bandwidth, and gain, setting the low noise LFA Yagi apart from other antenna designs.

8 x 18el 144MHz low noise X-pol LFAs at OH2BC - the LFA remains operations in wet weather and even covered in snow - unlike many competative products

Best in Class G/T Performance

G/T (Gain over Temperature) is a mean opinion score created by taking the antenna's forward gain figure and comparing it against the level of noise the antenna receives, measured in degrees Kelvin (K). The lower the figure, the better the performance. This score offers an insight into the antenna's ability to receive weak signals at an antenna elevation angle of 30 degrees.

The Evolution of Yagi Antenna Design and the LFA Yagi's Place in It

The modern age of Yagi design has been shaped significantly by the introduction and evolution of computer modelling software. Early software versions, specifically those reliant on NEC-2, NEC-4.2 and MiniNEC engines, had limitations in their ability to accurately model complex driven element shapes, such as the LFA loop, Quad, and Delta loops and more so, matching devices. This often led to the omission of vital components, like matching devices, from certain Yagi types in the software models. As a result, the performance predicted for these Yagis in software models was overly optimistic since the real-world addition of matching devices introduced losses not accounted for in the software model.

Lionel's, VE7BQH, comprehensive list offers a comparative analysis of Yagis ranging from 50MHz to 430MHz. Despite being populated predominantly with antennas designed in earlier software versions, LFA Yagis from previous generations are notable inclusions. It's testament to their enduring quality that even these earlier LFA models exhibit best-in-class performance when set against other, even newer Yagis on the list.

However, this list doesn't showcase the most recent, improved versions of low noise LFA Yagis. These modern iterations are crafted using sophisticated software packages beyond the scope of Lionel's list, which only accepts antennas designed using the NEC-2 or NEC-4.2 calculation engines. These cutting-edge LFA Yagis employ advanced software that accounts accurately for various real-world influences, such as booms, insulators, and even the effects of the coax cable and baluns.

While the LFA Yagi's unique loop shape eliminates the need for any matching devices, older designs, particularly those modelled in previous software versions, often incorporated such devices post-model, leading to discrepancies between model predictions and real-world performance. The LFA Yagi's design, enabled by the capabilities of modern software, results in a direct 50 Ohm impedance without necessitating external matching devices. As a result, the LFA Yagi can proudly boast its real-world performance figures, as the software model predictions accurately represent the built antenna's capabilities, unimpeded by the addition of post-model matching devices.

Future-Proofing the X-pol Yagi: Centralised elements & Cable Integration

Recent iterations of the low noise LFA Yagi antennas have been meticulously designed for flawless integration into X-pol or Crossed Yagi configurations. Traditional X-pol Yagis often grapple with pattern distortion and diminished G/T figures when transitioning from software models to real-world antennas. This issue is primarily due to the mechanical structure of the dipole or matching device extending beyond the typical two-dimensional plane. Such distortion is exacerbated in designs featuring 'above boom' mounted elements, where each plane of elements intrudes into the EH (Electromagnetic Field) field of the other.

This concern is further compounded when a feed point box is added. Not only does the box itself cause distortion, but it also introduces at least one additional point of connection in the feed line, leading to loss and potential unwanted impedance transformation. In scenarios where connectors are 'open' on one side, this can lead to RF leakage and further loss. InnovAntennas addresses this by minimising connections within the feed line. By removing the feed point box, a direct coaxial line connects the feed point to a low noise Masthead amplifier (LNA), typically used in low noise systems. This approach eliminates unnecessary impedance transformations and ensures a low-loss, direct connection.

The integration of an additional, vertically polarised Yagi in traditional systems to create the ‘X-pol’ can further complicate pattern maintenance due to third-dimensional intrusions. The LFA Yagi, with its innovative design, maintains the perfect alignment and centrality of both horizontal and vertical elements within two planes, in addition to perfect alignment with the feed point, ensures preservation of the antenna's pre-X-pol performance in terms of Gain, Pattern, and G/T.

In sharp contrast, the low noise LFA Yagi's design maintains all elements, feed point, and loop strictly within two planes. This adherence to a streamlined design not only ensures consistent performance but also means that the antenna's noise figure and G/T results are true reflections of theoretical models and empirical measurements. The result is a Yagi antenna that excels in both X-pol and traditional configurations, offering unmatched reliability and performance.

8 x 14el 50MHz low noise LFA WOS Yagis at OH2BC - A monster system with each antenna boom 11m long. We advise on the correct way to install mutli-antenna systems to ensure best results.

Real-World Translation of the Perfect Yagi Model

In the world of Yagi design, the ideal Yagi would appear as close to its software model as possible, suspended in free space without any form of support, and its driven element receiving RF without the need for coaxial cable. However, the reality of construction demands a more pragmatic approach. The challenge is to bridge the gap between the ideal and the tangible without compromising performance.

Element support is non-negotiable in real-world scenarios. A boom is essential for aligning and supporting the elements. Minimalist construction methods, keeping contact with the elements as brief as possible and centred, are paramount. The farther this contact extends along the element's length, the more pronounced its negative influence becomes.

Advancement Through Elimination: Why Removing the Feed Point Box Matters

The intricacies of VHF/UHF Yagi antenna design are notably influenced by real-world implementation choices, with the feed point box playing a pivotal but negative role. Many software models overlook this component, leading to predictions that are not a true representation of what the end-user experiences. Not only does it retain moisture over time, but its presence also distorts the antenna pattern, influences the noise figure, and ultimately degrades the G/T performance. This degradation contrasts sharply with the model's predictions if the feed point box was omitted. The only discernible benefit of the box is aesthetic, making the Yagi appear more polished.

InnovAntennas confronts these challenges head-on. We prioritise the use of RF-friendly materials and design with minimal supporting structures. We avoid feed point boxes entirely, sidestepping their associated problems. Instead, we employ a direct connection using small, durable user-serviceable connections. This ensures longevity. Additionally, it offers users easy maintenance without the need for proprietary parts from InnovAntennas. Our Yagis strike a harmonious balance between the perfection of a software model and the practicalities of real-world construction, delivering industry-leading performance by setting new standards in real-world model replication.

For those who aim for the apex of low-noise performance, with a steadfast commitment to design integrity and longevity, the low noise LFA Yagi stands unmatched.  we've completely eliminated the traditional feed point box, commonly susceptible to moisture retention and the ensuing performance deterioration. This deliberate engineering choice ensures a moisture-free, low-loss, low noise connection - a feature notably absent across the industry and one that ensures our G/T results are closer to predicted noise figures than anyone else. Our approach safeguards against the common pitfalls of water ingress, which can lead to tuning fluctuations and potential transceiver damage from impedance discrepancies. The LFA Yagi offers a universal pillar of dependability and superior performance in any environment, catering to those who prioritise peak performance without compromise.

4 x 20el 144MHz low noise LFA Yagis at FM5CS - Perfectly centred elements and absolute minimal supporting structures around the antenna to ensure modelled pattern replication.

The LFA Yagi's Feed Point: Uncovered

For enthusiasts seeking unrivalled low-noise performance, the low noise LFA Yagi from InnovAntennas stands as a paragon of excellence. Our innovative design approach centres on more than just avoiding moisture; it vitally focuses on preventing the detrimental shrouding of the dipole, the Yagi's most vital component. Conventional feed point boxes, commonly found in traditional designs, cover a significant part of the dipole's centre, inadvertently inducing noise and degrading both the noise figure and the G/T performance. Discarding this standard approach, the LFA Yagi delivers an unobstructed, low noise experience, ensuring that the antenna's real-world performance closely mirrors the optimised theoretical models. This dedication to design integrity propels the LFA Yagi beyond conventional limitations, offering an experience unparalleled for the discerning operator.

Conclusion

In the realm of high-performance Yagi antennas, the low noise LFA Yagi stands as an unparalleled masterpiece of engineering ingenuity, meticulously crafted to surpass all conventional limitations and redefine the pinnacle of Yagi design. Its transformative features, encompassing a unique closed-loop driven element, three-dimensional optimisation, direct 50Ω feed, and an innovative DC ground connection, have propelled the LFA Yagi to the forefront of Yagi technology.

The LFA Yagi's closed-loop driven element, a hallmark of its revolutionary design, shatters the constraints imposed by traditional dipoles, offering unprecedented flexibility and optimisation capabilities. This unique loop, unlike its linear counterparts, boasts a three-dimensional design, enabling intricate adjustments along its length, width, and depth. This unparalleled manoeuvrability allows for meticulous fine-tuning of the antenna's performance, tailoring it to specific frequencies and environments with unparalleled precision.

The LFA Yagi's three-dimensional optimization goes beyond the physical realm, extending into the heart of its design process. Unlike traditional Yagis, which rely on two-dimensional optimisation, the LFA Yagi harnesses the power of sophisticated computer software to optimize the antenna's performance in a three-dimensional space. This comprehensive approach ensures that every aspect of the antenna, from its driven element to its parasitic elements, is perfectly aligned and optimized to deliver maximum performance.

The LFA Yagi's 50Ω feed eliminates the need for matching devices, a significant benefit that simplifies the antenna installation and minimizes losses. This innovative feature is a direct result of the closed-loop driven element, which naturally brings the impedance back to 50Ω.

The LFA Yagi's DC ground connection, a testament to its innovative spirit, further enhances the antenna's performance and safety. Unlike traditional Yagis, which typically rely on a feed point box, the LFA Yagi employs a direct DC ground connection, ensuring that the antenna's ground plane remains undisturbed, minimizing noise and maximising performance.

The LFA Yagi's symmetrical elevation lobes and Band Pass Filter (BPF) property ensure optimal signal reception in a wide range of environments. The symmetrical elevation lobes, a direct result of the three-dimensional optimization process, ensure that the antenna's pattern remains consistent regardless of the polarization, providing a consistent performance across all axes. The BPF property, a consequence of the antenna's DC grounding, further enhances signal reception by minimizing interference from out-of-band signals.

In addition to its unparalleled performance, the LFA Yagi boasts exceptional versatility. It can be seamlessly integrated into a wide range of applications, from traditional Yagi configurations to X-pol arrays, making it the ideal choice for any ham radio operator seeking unmatched performance and adaptability.

The low noise LFA Yagi, a culmination of groundbreaking engineering innovations, stands as a testament to the power of human ingenuity. It is the epitome of high-performance Yagi design, surpassing all conventional limitations and redefining the standard of Yagi performance for years to come. For those seeking the ultimate in Yagi performance and reliability, the low noise LFA Yagi is the only choice.

Choose the path of InnovAntennas, where the journey transcends the ordinary, and where additional losses are obsolete and optimal performance is the norm.

Ready to elevate your experience with a low noise LFA Yagi but can't find the perfect fit? We're here to help. Contact us with your specific requirements, and let's work together to craft a bespoke Yagi antenna that sets a new standard in design and performance, tailored just for you. This email address is being protected from spambots. You need JavaScript enabled to view it. 

6 x 7el 50MHz low noise LFA Yagis at W7EW vertically stacked on a 200' fully rotatable tower.

4 x 8el 50MHz low noise LFA Yagis at N0TB being redied for EME operation.