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.

What is a Balun and why do I need one?

The article discusses 1:1 or choke type baluns and does not refer to impedance transforming baluns at all.

2 typical 1:1 ferrite core baluns supplied by InnovAntennas

If you do not want to read through the following discussion of baluns and would simply like to buy one to suit the needs of your operation and antennas, follow this link HERE and you will be able to purchase one.

Within this article we will look at the various methods and requirements of a balun in the hope that you will be better placed to make the right choice for your antenna. There are many different types of balun and some incorporate impedance transformation too and it is this area which often leads to confusion.

50 Ohm (Ω) direct feed antennas need a 1:1 balanced to unbalance transformer. However, other antennas need transforming from 12.5Ω to 50Ω up to (and beyond) or 200Ω to 50Ω for example and these types need the impedance to be transformed in addition to the feedpoint balancing effect. InnovAntennas products are designed withtout the need for any impedance matching as matching devices can install loss and noise into the antenna system, this one one reaosn our antennas work so well! You will need a 1:1 balun or choke at the feedpoint of a Yagi to ensure pattern symmetry and best long term results but why is this needed?

Coax Cable - The unbalanced feeder

Coax is a wonderful product. It's invention has allowed the feeding of antennas through a manner of different substances, conditions and places a balanced feed line could not. However, there are issues that need to be over come when feeding a balanced antenna with coax.

It has become commonplace that the Radio Ham or even commercial user use coax to feed antennas due to the convenience of so doing. So much so, commercial modern day transceivers have an unbalanced output designed to feed and receive directly from coax cable. Coax can be feed through walls, under-ground, up the side of our towers or metal poles without any drastic affect on the Antenna or it's tuning - or does it?

The best way to feed most antennas is with a balanced feeder. The reason being, most antennas (including Yagis which are of interest to us here) are balanced antennas. If we feed a balanced antenna with an unbalanced feed line, issues occur at the feed point; namely common mode currents which appear as a result of this balanced/unbalanced miss-match. These currents travel back down the coax and radiate and thus, the coax becomes a part of our antenna system and radiates.

In instances where we are feeding a multi-band vertical or horizontal wire, it may not matter too much to the operator as long as he is radiating a signal from his antenna (despite that there is more likelihood that he will cause interference and have RF in the radio shack due to the coax being apart of the radiating antenna). However, when we are feeding a Yagi, we need to ensure only the antenna itself radiates. Having the coax feeding our Yagi radiating will act to distort the radiation pattern of the antenna. No, this problem will not cause a high SWR. In fact, it may even reduce the SWR seen in the shack giving the ham a false sense of security that things are OK when they are not.

Balanced line feeders

The most common balanced line feeder in Ham Radio today is the 300Ω ribbon used within the very common G5RV antenna. Any twin line can be used as a balanced line feeder; speaker wire, bell wire, even mains flex. They will all have different characteristics but will all provide a balanced feed to an antenna if used that way. The twin feeder will not radiate (or very little anyway) as a result of the phase of the RF in each feed line being 180 degrees out of phase in each leg. This means, one side of the feed line cancels the other out so no radiation occurs.

Disadvantages of a balanced line

Firstly, we will need a balun to use a balanced line feeder or balanced antenna in any case as radios today are not presenting a balanced output. The next point is the effect that any close by objects have on the balanced feed line, walls, buildings in general, towers, all metal objects, ground, everything! We need the feed line to be in as much open space as possible in order to ensure the balance feed line can perform as it should. Beginning to understand why a balanced feed is not in mainstream use for the Ham?

We will not go any further into balanced line theory here as the majority of users will be using coax cable although we will look at the consequences of not having a balun installed below.

The Dipole Centre

The dipole is an important part of the Yagi, anything that goes on (or wrong) here is reflected throughout the rest of the Yagi. Below are 2 images of a dipole (fed element) removed from a Yagi. The cross section at the top representes the dipole itself (with green dots along it) while the down-wire is a representation of the coax or balance line feeding the Yagi. The pink lines indicate current distribution within these 'elements'. Let's first take a look at fig 1.

Fig1. A dipole with a balanced feed

Within Fig1 we can see a nice, clean and balanced distribution of current through the dipole itself with no radiation within the feed line. This represents a scenario where a perfect balun is placed at the feedpoint or the antenna ia fed with a balanced line feeder.

Fig2. A different story if the feedpoint is not balanced

Fig2 shows the distortion within the dipole if coax is connected and a balun is not installed. This may not impact vertical or mono-pole antennas so much but for a directional antenna such as a Yagi,  this is disasterous! Ensure you have a good balun at the feedpoint of your Yagi to ensure your antenna pattern stays clean, symmetrical and you see the performance you deserve.

The balun - What does it do?

In this instance we are discussing 1:1 baluns or chokes which takes an unbalanced input from your coax line and allows connection to a balanced antenna feedpoint. Have you wondered where the name balun comes from? Balanced to Unbalance, that's it.

----------------------------------------------------------------------------------------------------------------------------------------------------

The 1:1 balun

Fully Symmetrical Coaxial Balun

We need a 1:1 balun at the feed point of our Yagi. This means the balun will connect to a 50Ω unbalanced line and present a balanced 50 Ohm output. There are a number of ways to do this in my opinion. The first is a coaxial 1:1 balun as described by I0QM at this link:  http://www.iw5edi.com/ham-radio/files/I0QM_BALUN.PDF and for which there is a photo below.

This uses 2 pieces of additional coax cable at the feed point of the antenna. A full explanation is given with the document created by I0QM. This is the best method of producing a fully symmetrical balance at the feed point (using coax cable) without the losses seen in a torroid wound equivalent. However, it does have a number of draw backs. The first is it is relatively narrow in bandwidth.  This is course one of the major benefits of G0KSC Yagis (being wide-band) so do we really want to inhibit performance? The next is the additional connections and coax we are introducing which means additional losses in our feed system in addition to another point where weather (water mainly) could gain access to our antenna and feed line and as a result, de-tune our antenna or make it completely defective.

Finally, in addition to being narrow bandwidth, this type of balun can only be used on one band rendering this balun useless for our multi-band Yagis.

A Fully Symmetrical Coaxial Balun

----------------------------------------------------------------------------------------------------------------------------------------------------

Choke balun

The choke balun is a a cheap and easy method of removing common mode currents from the feedline, provided it is manufactured correctly. Basically, this is a coil (a few turns) in the coax feeding the antenna as close to the feed point as possible. This coil acts as an RF choke and prevents the common mode currents returning back down the feed line. However, Current may not be symmetrically distributed through the radiating element using this method. While it does work, it is not an 'ideal' method in all instances.

Firstly, the higher in frequency you need a choke to function, the more difficult it is to achieve a fully functional choke using coax. For example, with many good quality coax cables, the outer sleeve of the coax is too thick to provide sufficient capacitance needed between the turns in order to provide the combination of L+C needed to enable choking. Next (and although varies with coax cable type used) the coiled choke is fairly narrow band so provides performance over a relatively small area so although a common perception is the choke is a good fit for multiband Yagis, it is not.

The above said, this method of feed point choke has a number of benefits listed below:

• The choke balun is easy to make and implement although in highe bands (VHF/UHF) it is advisable that a Vector Network Analyser is used to confirm the antenna is working correctly.
• No additional connections required - for me this is one of the most important benefits, especially at VHF. Use one single piece of coax from the back of the rig, through the coaxial balun and right up to the feed point. This minimises losses and connections and therefore, any potential issues that can happen at a later stage in the life of the antenna and it's feed line
• The best part of this choke balun is it is extremely easy to make! Lets look now at how this is done

See bottom of this page for Choke build instructions

----------------------------------------------------------------------------------------------------------------------------------------------------

Pawsey Stub

19/11/2010  Many Hams have seen inconsistencies with the Pawsey stub, largely believed to be due to the varied velocity factor seen in most coax cables. Steve, G3TXQ contacted me with a very interesting viewpoint and provided some seemingly valid information explaining why only coax of the same type should be used for the Pawsey stub, not a separate, dissimilar wire. I therefore would no longer recommend this style of Pawsey stub but will keep the information here. My antenna balun with a feedpoint choke may give the best, all round performance:http://www.hamradio.me/antennas/coax-velocity-factor-part-2.html#more-1528

This is a very interesting method of balancing the feeder at the feed point of the antenna and I have modified this balun from it's original make-up. All we need to do here is connect the coax to the antenna in the traditional way. However, in addition to the coax, we add a piece of wire to the point of the antenna that the centre core of the coax connects to, run the wire back along the coax and connect this wire to the braid or outer core of the coax at 1/4 wave back along the COAX. As this is a 1/4 wave length of coax, velocity factor needs to be taken into account too. This provides a fully symmetrical balun at the feedpoint of the antenna. In early versions I coiled the Pawsey stub. However, I now recommend keeping this in a straight line and connecting to the boom where the wire connects to the outer braid of the coax. This provides two additional features. The first, it provides a DC ground to the loop or driven element, further reducing potential noise pick up. Second, As it is in a straight line and running along the boom towards the mast, it is taking place that additional feedline would have to and therefore, reducing additional losses.

The G0KSC Pawsey as built by EI2GLB. Feeding the coax directly through the boom helps reduce potential pattern distortion

PA0WRS installed this impedance transforming version of the G0KSC Pawsey stub on his 50Mhz 5el

The 'Pawsey wire' can be clearly seen in the above PA0WRS example. This ensures a fully symmetrical balanced match at the dipole centre. Coax bending around the boom near the feedpoint can distort the radiation pattern. Arranging the coaxial transformer/balun in the above way moves the point at which your feedline has to go around the boom much further along the boom and away from the feedpoint. Any potential issue is drastically reduced the further you move away from the feedpoint so arranging the transformer/balun long the centre of the boom for it's whole length is ideal. Do not be tempted to coil the balun, this servers no purpose other than to reduce the power handling capability!

As discussed above, the point at which the wire connects to the outer braid of the coax is 1/4 wave length along the COAX rather than a 1/4 wave of wire. Therefore, when calculating our length we need to take into account the velocity factor of the coax we are using. The calculation is as follows:

300/Frequency x coax velocity factor x.25 The above is a Pawsey stub for my 70MHz LFA Yagi. The coax I used was Westflex 103 which has a velocity factor of .85 (RG213 is .66) so the calculation was as follows:

300/70.2 x.85 x.25 so my wire length had to be long enough to connect 940 mm back long the coax from the point where the coax was split in two. NOT the end of the centre core of the coax.

The Pawsey stub is an excellent method of producing a true balanced input at the antenna at minimal cost. Again, like the coaxial balun above, this is only good for mono-band antennas due to the relatively small bandwidth.

Please follow this link here for more information on coax cable velocity factors. Don't be put off of an OWL Yagi, if you chose a 12.5Ω version, you can swap the split dipole for a folded dipole and the antenna becones a 50 Ohm antenna and required just a 1:1 balun!

--------------------------------------------------------------------------------------------------------------------------------------------------------

The Ferrite Core Balun

The Ferrite core balun is what I (G0KSC) recommend as the ideal solution for a coax cable fed 50Ω Yagi. The ferrite core balun is a piece of coax which has ferrite cores tightly fitted around the outer sleeve of the coax (normally sealed, InnovAntennas cover the ferrites in glue-filled heatshrink tubing). With this method the choking is formed within a cms or so of the feed point in order there is no exposed coax at all to radiate or de-tune the antenna. Next, the cores used by InnovAntennas have an extremely wide frequency range they function effectively across. For example, the material used for 50MHz, 70MHz and 144MHz has an operational range of 30MHz to 300MHz. This means they are ideally suited for multi-band applications within this bandwidth too. Finally, the ferrite core balun provides another very important function details below.

Filtering stray signals picked up on the outer sleeve of the coax

Often in urban environments, many modern-day electronic devices generation noise with is picked up by the receiver. However, it is often the case that these signals are along the route of the coax form the radio to the antenna and are picked up along outer sleeve of the coax, travels up towards the antenna and enters the reciever chain this way. The ferrite core balun not only prevent common mode currentls from travelling form the antenna back down the coax, it also prevents these unwanted noises picked out on the outer sleeve of the coax from entering the reciever chan which results in a lower noise floor on your receiver.

A selection of our commerical ferrite core baluns can be found HERE and custom versions available on request.

A Ferrite Core balun customer-fitted to a 4el 50MHz LFA Yagi

 

Creating a Choke Balun

Note: although called a choke balun, this DOES NOT match the unbalanced feedline to the balanced antenna. It simply stops (or helps reduce) common mode currents flowing on the coax as a result of the miss-match at the feedpoint. This will also result in an imbalance in the radiation of the pattern too AND reduce performance. Ultimately when feeding a Beam of any kind, I real 1:1 balun should be used such as the coaxial and Pawsey mentioned above.

I always make mine when the antenna it is intended to be installed upon is complete and ready to be installed on the mast. The reason for this is I am able to accurately install the balun in place by measuring the coax out along the boom and in most case, keep with one length of coax with no joins from the antenna to the radio.

First, we should prepare the coax for connection to the antenna feed point. Assuming we are going to install one single piece of coax from the antenna, through the balun and to the transceiver, cut the coax at one end and prepare it in order that you can see around 5 to 10mm of inner core with the same amount of braid. Keep in mind that the antenna dipole starts at the point that the inner and outer core of the coax split and therefore, this 5-10mm of each should finish where the coax is whole again (see picture).

Slide one ring end over each piece of the coax showing and test the connection on the antenna. the last thing you want to do is solder the rings to the coax and they are not long enough! Once you know where they need to be, solder the joints and ensure there is sufficient heat to allow the solder to flow deep in the joint. This will ensure a good connection and limit the chance of any water ingress.

Next we need to ensure our joint is sealed well reducing the chance of water being able to seep into the coax. One way is to install an insulator box at the feed point. However, from my experiments, it is much more difficult to keep the elements aligned this way. Generally, the dipole ends up a little higher than the rest of the Yagi elements which does have an impact on the pattern. Furthermore, if water were to gain access to this isolation box it could sit inside the box and cause the same issues as it would if it gained access to the coax.

I purchase good quality self amalgamating and simply wrap the feed point of the coax directly. Make sure to use plenty and ensure that you have covered every possible access hole to water. Once this is complete, you are ready to connect the rings to the antenna feed point.

Having connected the coax, decide where you want to place the balun (example in the picture at the top of this page) and mark this point on the coax with a ring of low-tack tape, perhaps insulation tape. Now take off the coax once again for a moment.

Winding the balun

Now we need a few extra tools in order to create our balun. Do you have a spray-can somewhere in the house? This could be furniture polish or a can of WD40. Any standard size household spray can will do. next, we need 4 to 6 strong cable ties that are long enough to go around 4 to 5 lengths of the coax we are using. Place each one of these cable ties face up on the side of the spray-can and place the low-tack tape over one side of the cable tie to hold it in place. now wrap the tape a 1/4 of the way around the can and place another cable tie. Do this until we have 4 to 6 of these cable ties in a ring around the can. Then place another line of tape around the over end of the can to fully secure the cable ties onto the can.

Now locate the mark you made earlier on your feed coax. Hold this point on one end of the can so as it can not move, making sure that the tape holding the cable ties on the can is outside of the point where you are holding the coax.

Now roll the can until you have 4 to 5 turns (50/70Mhz 2 is OK for 144MHz and 1 tight turn on 430Mhz. This should be tested with clip on ferrite chokes after the balun. If the SWR changes, you have too many or too few turns. Change and try again) of coax on the can. At this point, we need to remove the tape holding the cable ties in place on the can. the rolled coax will not be holding them to the can so they should not move. Loop back the cable ties and slot the ends together and tighten as appropriate.

You are now done with you balun! You may now remove the can and install the coax/balun onto your boom, you are ready to go!

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.

Justin G0KSC - InnovAntennas