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!
Details on the DK7ZB Match can be found HERE
Any questions, let me know! Justin G0KSC