A monster 18 element 144MHz Loop Feed Array Yagi

The LFA design has a patent pending and all G0KSC designs are copyright. Any ham can build for their own use or those of a friend. Where profit or commerical gain exists, express permission of G0KSC must be sought.

I wanted to present the first on-site model showing some serious advantages over traditional Yagis and this 18 element for 144MHz will not dissapoint. Providing over 18dBi forward gain and in excess of 32dB Front to Back and with no side lobes along with substancial unwanted forward lobe suppression.

To top of the antennas list of fantastic features, a completley flat SWR between 144MHz and 144.6MHz is also provided.

Built from Imperial sized material, our European friends will be disspointed. However, I will add a metric sized version too.

Below is both the Azimuth and Elevation patterns. Have you ever seen a pattern like this in an 18element Yagi?

The above diagram shows that this antenna is fed at the center piece of the frotn element with in the loop feed arrangement.

Construction:

All parasitics elements are made form solid 1/4 inch aluminium rod. The loop itself is made from 2 different sized tubing. The longest elements within the loop are made from 1/2 inch tube while the end pieces (with the bends) are made from 3/8 inch rod or tube and slide into the ends of the 1.2 inch tube. This allows for a trumbone effect for easy fine tuning of the antenna/loop system.

Go to the download page of this site Where you will be able to download the EZNEC design file for this antenna along with 4 bay stacking information too.

Specification:

Forward Gain  -             17.95dBi at 144.300MHz

Front to Back Ratio -      32.16dB at 144.300MHz

Peak Return loss -         -65.062dB at 144.250MHz

Boom Length -              12.987 Metres

Feed Loop Arrangement

The feed loop is made from larger diameter material than the parasitic elements. The largest sections on the loop are made from 12.7/13mm and the smaller sections from 9.6/10mm. All parasitics are made from 6.53/6mm solid aluminium rod. The picture below shows how:

In order to ensure the correct sizing and construction is undertaken, I have included the 4nec2 and EZNEC design files within the download area of this site. This will allow you to make alterations if you wish. However, I think this design is pretty much were it needs to be :)

Stacking and Baying information to be published soon.

Enjoy.

There is a patent pending on the LFA design. For Commercial use, contact G0KSC

NOTE: All information on the LFA (Loop Fed Array) Yagi is now release. DUBUS magazine have kindly given permission for us to allow you to see it. This is available for download in PDF format on my download page.

Revolutionary Yagi Antenna Feed System by G0KSC

The LFA design has a patent pending and all G0KSC designs are copyright. Any ham can build for their own use or those of a friend. Where profit or commerical gain exists, express permission of G0KSC must be sort. 

Justin Johnson G0KSC, has pioneered a revloutionary new feed system for the Yagi directional beam antenna which will see it's Debut in the next isuue of DUBUS - the serious magazine for VHF and up amateur radio www.dubus.org. Whilst there are many benefits seen with the new system, the most important break through is the suppression of the age old problem of unwanted side lobes within longer Yagi designs.

The feed system (which does not consist of a dipole and no, it is not a Quagi!) has 2 different feed points. Whilst the antenna needs re-modeling in order to change from one to the other, different characteristics are seen from each method. The first enhances the bandwidth capabilities of the Yagi and presentes never-seen-before ultra low return loss, excellent gain and front to back ratio, whilst the other provides more traditional OWA (Optimised Wide-band array) style bandwidth with good improvements in forward gain and massive improvements in front to back ratio.

What people say

'This could be a turning point in the development of the Yagi Antenna'  Popa, YU7EF - World renowned Yagi antenna designer/engineer - www.yu7ef.com

'Justin, Indeed this looks like a unique and winning design!' Joel R. Hallas, W1ZR - Technical Editor, QST Magazine

'Hi Justin, Many thanks for the plots, these diagrams look really beautiful! I have never seen such F/B and low sidelobes for a Yagi before' - Joe CT1HZE - DUBUS Magazine

 Benefits of the new design

Whilst going to press, experiments are still being conducted with the new Yagi. However, the new fed adds a number of very interesting characteristics to the Yagi which when focus is applied to optimisation, excellent not-seen-before Yagi performance is the result. The list so far consists of the following:

• Superior Front to Back Ratio with very high supression of unwanted lobes
  
• Extended/alteration of ‘ideal’ boom length
• Exceptional clean pattern performance
• Ultra-wideband characteristics
• 50 Ohm impedance – no matching required
• Easy fine-tuning of feed arrangement itself

Superior Front to Back Ratio

Quad beams are renowned for their superior front to back properties (over Yagi’s) when comparing like to like (number of elements) within small arrays. The New Yagi has picked up this characteristic. In models created so far, front to back figures of between 30-40dB in Yagis of 7 and 8 elements WITHOUT distorting the forward pattern or indeed creating any unwanted side lobes.

Extended/alteration of the ‘ideal’ Boom Length

As any Yagi experimenter will know, there is a boom length for any given frequency/number of elements which provides the ideal balance between forward gain and front to back ratio. If you drop below this point, high front to back and lower forward gain can be achieved (to a point then both drop) and go above this point and forward gain increases whilst front to back drops away.

An interesting characteristic within the new design is the ability to extend this ideal length and as a result, squeeze out additional gain and front to back at the same time. This is in part due to the size of the feed system and the space it takes up on the boom as reflector and first director spacing remains similar to that of traditional Yagi’s although the large feed arrangement is between them. Using 50Mhz as an example, the feed system width (along the boom) is around 45Cms and therefore, on a 5 element Yagi, almost .5 metres length has been added to the boom. I have achieved 11.3dBi forward gain and over 20dB front to back from a 5el 50Mhz beam with a boom length of just under 5 metres.

However, by altering the size of the driven element on the new design (increasing size along the boom) the 'ideal' boom length can be altered and hence new levels of gain and performance achieved. Whilst F/B was compromised slightly, over 12.5dBi forward gain has been achieved from a 5el 70Mhz Yagi with an extremley long boom.

Exceptional Clean Pattern Performance

The new feed system provides an out-of-phase operation along it's Y axis which runs parallel to the boom. It is this side radiation cancellation characteristic which helps the design reduce side and rear unwanted lobes to an absolute minimum. Full details and build information for 2 G0KSC Yagi's will be presented in the next issue of DUBUS Magazine. Both an ultra wide-band 5el 50Mhz Yagi and 11el 144Mhz low temperature Yagi (TanT noise figure below with an antenn using 5mm think elements) will be featured.

When focus is applied to optimising forward pattern along with front to back (in preference to out and out forward gain) very clean patterns with ultra-low noise can be achieved.

This explanation is intentionally non-technical in order that all can understand the basic concepts of the antenna.

Ultra Wideband Characteristics

Another powerful aspect of the new feed system is flexibility of the fed arrangement and how it can alter the performance of the Yagi. Extremely flat curves within minimum reactance and return loss can be achieved.

50 Ohm impedance, no matching unit required

As with all of my designs which keep the amateur antenna builder, novice or professional in mind, no matching unit, coaxial stubs or any other tuning arrangement needs to be added to the antenna. A simple 1:1 balun or coaxial choke balun at the feed point is enough. As stated above, direct feed of the antenna is possible and due to the design of the feed system itself, fine tuning can be achieved easily. Full details in DUBUS! www.dubus.org

Some plots of Yagis created so far:

A preview of the 11el 144Mhz Yagi to be featured in DUBUS below:

Visit www.DUBUS.org for details of the next issue release date.

73

Justin G0KSC