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Free Yagi Antenna Designs for Ham Radio

The New G0KSC 28Ω LFA-Q Quad-style Antenna

 

For some time I have experimented with rectangular quads first introduced by Boban, YU7XL.

However, the limiting factor with these original designs was a mechanical one. First, each loop was a different size which was hard to replicate in the real world. Next, the rgidity was an issue being mounted on one side of each loop meant potential element movement was an issue.

For commerical purposes I developed a much more rigid mechanical version of this. The loop height would be the same on all elements with only the sides varying in size. This would mean twin-booms could be deployed which would result in a much more rigid structure all round. Of cause, the antenna would have a direct 50Ohm feed point to avoid matching issues and further assist in the easy-build of this antenna and ensuring it stays upp in all weathers!

A 2el 28Mhz 50Ω LFA-Q by InnovAntennas

These antennas have proven very popular commercially and also I have been commissioned by certain government entities to design and produce these antennas for a number of different frequencies too. 

One of the big benefits of this antenna is the higher gain it can produce in smaller antennas when comparing against a Yagi. However, this benefit decreases with size as the longer the boom gets (and the more elements added) the less the associated gain advantage over a traditional Yagi antenna. By the time you get to 7 elements, the additional weight size and mechanical complexity starts to out-weigh any performance advantage.

Commercially, I have been developing 50Ohm versions but decided to try with impedance just a little lower. With the help of the DK7ZB 28Ohm coaxial matching device, impedance change could be managed quickly and cheaply and to my delight, improvements in performance worthy of the match resulted. A 3 element 50Mhz LFA-Q with a boom length of 1.9m delivered over 9.5dBi, wow.

Bandwidth suffers slightly with this antenna. However, as the working bandwidth for most 50Mhz enthusiasts is the bottom 300Khz section, this does not present too much of a problem and so far, I have designed 2,3,4 and 5 element versions of this antenna for both 4m and 6m where I believe this antenna with it's short, stubby frame and explosive performance will really excel.

The Azimuth plot of the incredible 1.9m long, 3el 50Mhz G0KSC LFA-Q

 

A very acceptable SWR curve for the bottom end of 50Mhz

It is no just the 3 element version that gives stunning performance. we have discussed the diminishing returns of the quad when compared with a Yagi but still, the 2, 4 and 5el versions give great results too. I have listed the antennas and their respective boom lengths and gain as examples below:

50Mhz 28Ohm LFA-Q performance

Elements - Boom Length - Gain

   2el              39cms           7.5dBi  

   3el             1.91m            9.51dBi

   4el             3.63m           10.71dBi

   5el             5.5m             11.93dBi

 

So, do you want to build one? Below I have a zip file which has EZNEC files for all models so far but please remember IF YOU DO NOT HAVE EZNEC PRO/4 (not EZNEC+ version 5) you WILL NOT see the plots or SWR correctly, NEC2 is not accurate with this type of antenna. Use the file for build dimensions only, do not adjust the model!

I will add a build sheet and correction details shortly so watch this space and Enjoy!

DOWNLOAD EZNEC FILES HERE

Details on the DK7ZB Match can be found HERE

 Any questions, let me know! Justin G0KSC

There are quite a number of 'new design' Yagis out there to self-build that claim to be great but how good are they?

The easiest way to establish what you should build is looks at those serious hams, those that are investing thousands of Euros/Dollars/Pounds into their multi-Yagi systems. These people have done their research and now what makes a good Yagi good. Sure, anyone can talk up their own designs and appear to be neutral in their overview of 'competative' designs but what really makes a good yagi good and why?

There is far more to a modern Yagi that just gain or bandwidth. More than just F/B or F/R. You see there are many things to get wrong when selecting your Yagi as their maybe requirements you need that you are not aware of.

Yagis are not the same form one band to another (or should not be). Simply scalling a good Yagi from HF to VHF will not make a good VHF Yagi, there is so much more in play.

The 432Mhz EME array built by InnovAntennas @ EI8JF


Regardless of how quiet you beleive your location to be, a quiet Yagi will improve your signal to noise ratio and allow you to receive weak signals that you would not otherwise be able to hear. If you antenna is receiving (of sorts) form all directions, your noise floor will be higher. Reducing this noise floor to a minimum (by increasing F/B and minimising side lobes will lead to a much more preferable signal to noise ratio (S/N). This signal to noise ratio is more important at upper VHF and as we move into UHF.

Take a look at services and systems outside of Ham radio and the importance of S/N and minimum unwanted noise. One reason observatories are generally located away from towns and cities is to avoid light polution. This same is the case for radio observatories, absolute minimum noise is required but often, the location for the receive array is more limited and has to be in-town.

W2PU, the radio astronomy site of the Princeton University, New Jersey USA and the home of Joe Taylor, K1JT (the developer behind weak signal programs such as WSJT, JT65, MAP 65 etc.). Joe asked me if it were possible to produce a crossed LFA Yagi for 432Mhz (Radio astronomy is normally carried out on 406Mhz but Joe chose 432Mhz in order that the array could be used for EME (Earth Moon Earth) communication too. I agrred to the task but only on the basis the final antennas would have no compromise electromagnetically as the importance of alignment and external influences (even the coax cable feeding the antenna is an issue on 432Mhz) help decide how well (or not) and array on this band will perform, especially when it is crossed.

A close-up of one of the 432Mhz X-pol LFA Yagis at W2PU - K1JT


While Joe and I will prepare a paper on the extent of the design and how it works so well, some give-aways can be seen in the aboved close-up shot of one of the 432Mhz X-pol LFAs. First is the fact the antennas are all rear mounted (30 elements, 2 x 15el), the square boom supporting the elements is fibreglass to avoid interaction and the drive loops are made from brass which once adjusted, has been solded. Even the coax cable (ultra-thin, low-loss) is soldered directly to the feed point to avoid dismilar metal issues.

Watch out for next year for our joint paper on this subject which will most likely appear first in DUBUS magazine. Keep tunes to these pages as new, latest design antennas will be appearing over the coming weeks, ready for your over-winter projects! These will include a Quad-style LFA-Q antenna for 50Mhx with 3 elements on a 1.9m long boom which produces over 9.1dBi, WOW !!

73 Justin G0KSC

 
 
 
 

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