ARLA/CLUSTER: The 'Loop Fed Array' the original DUBUS article can be downloaded.

João Gonçalves Costa joao.a.costa ctt.pt
Quinta-Feira, 28 de Maio de 2009 - 14:45:30 WEST


The 'Loop Fed Array' Yagi Antenna Feed System

Justin Johnson G0KSC makes public details of the LFA ‘Loop Feed Array’.

The LFA Yagi is not fed with a dipole as traditional Yagi antennas would be. Instead, it is fed with a rectangular loop which is laid flat on the boom of the Yagi in place of the dipole driven element.

The LFA is not ‘The Holy Grail’ of directional antennas and will not give any additional forward gain for any given boom length. However, it has a new and unique combination of properties which can be tailored to provide improvements in many areas over a traditional dipole-fed Yagi.

LFA Yagi Advantages

The advantages of the LFA concept are as follows:

 *   Superior front to back ratio
 *   Exceptionally clean patterns
 *   Ultra-wideband gain and matching characteristics
 *   A new definition of the ‘ideal’ boom length
 *   Flexible feed point arrangement
 *   Direct 50Ω feed
 *   Optimization of loop dimensions.

[http://www.southgatearc.org/news/may2009/images/DSC_0796.JPG]
A 5 element 50MHz Yagi in use on my tower


Superior Front to Back Ratio

Quad beams are renowned for their readily achievable clean patterns, with superior front to back properties over Yagis (when comparing like numbers of elements in small arrays).
The LFA Yagi seems to have picked up this desirable characteristic. In models created so far, front to back figures of between 30­-40dB have been readily achieved in Yagis of 7 and 8 elements without distorting the forward pattern or indeed creating any unwanted sidelobes. In addition, 3 and 4 element patterns look very similar in shape and performance to quad alternatives. This is one of the benefits of the LFA, as traditionally fed OWA antennas tend not to model well unless at least 4 elements are used.

Exceptionally Clean Patterns

Another very nice ‘quad-like’ characteristic of the LFA Yagi is side lobe suppression. This is of particular importance when short boom, multi-antenna arrays are being used for EME work. It can be quite a challenge to eliminate rearward lobes in the elevation plane which will lead to noise being picked up from residential locations (and perhaps causing interference in these locations too). The LFA Yagi provides the best benefits of both small quad and Yagi antennas in one design. The LFA concept also offers more design variables that can be used to tailor the performance to suit individual requirements. Although it may require much more antenna modelling, there is also a good possibility that the design objectives can be surpassed rather than merely achieved.

Ultra Wideband Characteristics

In modelling of both 10 and 11 element LFA Yagis for 2m I have achieved over 15dBi forward gain along with more than 34dB front to back ratio, with no significant sidelobes or unwanted forward lobes and a VSWR of less than 1.1. This is exceptional performance indeed, but even more exceptional considering how well this performance is maintained across a very wide bandwidth between 144MHz and 145MHz. This Yagi provides the nucleus of a very effective array for the serious DXer and EME enthusiast. Figure 4 shows similar wideband VSWR performance for the 50MHz LFA Yagi.

A New Definition of the ‘Ideal’ Boom Length

As any Yagi experimenter will know, for any given frequency and number of elements there is an ‘ideal’ boom length that provides the best balance between forward gain and front to back ratio. Shorter boom lengths will generally provide higher F/B but at the expense of forward gain (though if the boom is made even shorter the F/B will begin to drop as well). With longer booms, forward gain increases but F/B ratio drops away or becomes much harder to achieve. An interesting characteristic of the LFA is that the ‘ideal’ boom length is greater than for a conventionally fed Yagi with a similar director structure, and this tends to give both more gain and improved F/B ratio. This is in part due to the size of the loop and the extra space it takes up on the boom – behind and in front of the loop, the spacings of the reflector and first director remain similar to those of traditional Yagis. When the loop is made more square in shape, so that it occupies even more space along the boom, both the forward gain and the F/B capabilities increase together. Using the 50MHz 5 element Yagi as an example, the loop length (along the boom, the X-axis dimension in the models) is around 45cm and therefore this extra length must be added to the boom. I have achieved 11.3dBi forward gain and over 20dB front to back from a boom length of a little over 5 metres, which thus becomes the new ‘ideal’ boom length for such an antenna.

Flexible Feed Point Arrangement

Another powerful aspect of the LFA is the flexibility of the feed arrangement and how it can alter performance of the Yagi. From the limited experiments made so far, it has been established that if the loop is fed at the rear centre (nearest to the reflector, as shown in Figure 7 ), better wideband performance can be achieved than by feeding at the front centre. Although the feed point cannot simply be switched from one position to the other without re-optimization, a front feed can provide a slight increase in forward gain and F/B; the VSWR bandwidth is narrower than for rear feed but still better than seen in comparable traditional Yagis. As stated above, the length of the loop along the boom can also be varied to change the LFA’s operational characteristics. More experimentation is required to establish which is the best way to feed this antenna.

Direct 50 Ω Feed

As with all of my designs for the amateur builder, there is no need for any matching unit, coaxial stubs or any other tuning arrangements at the feed point. A simple 1:1 balun or coaxial choke is enough. Fine tuning can be achieved by moving the ends of the loop in or out (very much as one would to tune a folded dipole). Figure 8 shows another example of the LFA’s excellent VSWR bandwidth. Note that the narrow dip which is so characteristic of the dipole fed Yagi has gone. For the practical version of the FD0605H 50MHz 5el Yagi (Figure 3) I tried a 1:1 voltage balun (branch feed with ¼ wave and ¾ wavelengths of coax). This was successful but the ends of the balun must terminate upon the loop itself. Any connecting wires will de-tune the antenna and increase the VSWR.

Optimized Loop Dimensions

As already noted, the loop dimensions offer more design variables that can be used to help optimize the entire Yagi array. There are still many more tests to be conducted. The loop circumference is close to a full wavelength, but modelling experiments have already established that by altering the shape of the loop, the F/B ratio and forward gain can be balanced one against the other. The typical width of a loop (the Y dimension, parallel to the parasitic elements) for 50MHz would be around 10cm shorter than the first director and the loop length (X axis, along the boom) between 35cm and 60cm for best results. When starting a model for 50MHz, I typically use 2.5m width and around 45-50cm length, although with certain boom lengths a smaller loop length of 35cm has also returned good results. Scaling those dimensions for other bands will give a good starting point. For example, with 5mm wires at 144MHz, the loop length I am using is around 16-18cm along the boom.

To fine tune the antenna once it is in place, I recommend making the end sections of the loop from a smaller size of telescoping tubing, so it can be slid in and out to alter the total circumference of the loop. However, it is important to note that part of the side lobe suppression is delivered by the anti-phase currents along the ends of the loop. To reproduce the computer models as accurately as possible, the corners of the loop should be formed as small radius bends. If these ends are formed as half circles (to look like a large folded dipole), the side lobe suppression will not be anywhere near as good.

Ultra low temperature antennas for EME

Another break-through is the very low sky temperature of these antennas which is of particular importance for EME work. The popular VE7BQH list (Google search) provides a list of 4 stacked arrays from around the global and compares attributes such as gain, G/T, sky temperature (measured in degrees Kelvin) and more.
The LFA Yagis, while not appearing on this list as yet, show a much lower temperature than comparable arrays. In fact, a 4 by 13 element array for 144MHz gives a lower sky temperature than any other array listed. Considering that an antenna's sky temperature drops the larger the array becomes and that the VE7BQH list has antennas over 8 wavelengths long, this attribute is quite an achievement.

[http://www.southgatearc.org/news/may2009/images/13el.jpg]
The incredible pattern of a 13 element 144MHz Yagi with extremely tight F/R ensuring low QRM pick up in city locations

New Low-Noise Antennas for 'Urban' locations

One problem that affects many Hams is the tendency for antennas to pick up man-made noise in city locations. One of the new breed of LFAs are an ultra low noise Yagi which drastically reduces noise pick up form any direction other than the direction the antenna is facing.

This is achieved by modelling the massive amount of Front to Back possible with an LFA (without generating unwanted side lobes) Fig2 displays the plot of a 13 element LFA showing the characteristics of the noise noise LFA

Conclusion

I believe I have only scratched the surface of the LFA Yagi concept. In a relatively short time it has enabled me to create some very good models which are now beginning to show proven practical results, so I am sure that with more time and many more minds, the LFA concept can produce some remarkable antennas. I am also working on a number of models for 70cm and 23cm that have equally impressive predicted performance – perhaps these will be the topic of a later article!

By offering more design variables than a traditional Yagi (the width, length, positioning and feed point of the driven loop), the LFA Yagi offers much more flexibility in design and optimization, so that more of the desirable performance objectives can be achieved simultaneously. Overall, this will result in superior performance.

At last it seems possible to achieve more of the characteristics of the legendary quad antenna within the compact and more visually pleasing outline of a Yagi, while also losing some of the less desirable properties of traditional Yagis. I am sure that, like me, many hams will see the LFA Yagi as a viable alternative to both the quad and traditional Yagi, and equally I believe the LFA will become a common feature within DX and EME stations.

If you have any comments, suggestions or findings relating to the LFA, please let me know and I will be happy to share these results on my website, which in the near future will have a section dedicated to the LFA designs.



Below is a link to my download page where the original DUBUS article can be downloaded. http://www.g0ksc.co.uk/file-download/category/1-antenna-design-files.html

Justin, G0KSC

E-mail: justin  g0ksc.co.uk<mailto:justin  g0ksc.co.uk>
Website: www.g0ksc.co.uk<http://www.g0ksc.co.uk/>
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