I'm new to yagi building and so far have made a four element test LFA yagi for 70 cm using a wooden boom and thick copper wire. Worked as expected. I scaled it up and build a 10 element version very closely to the specs of the InnovAntenna version (thanks KB1PVH for the help there). You can find an photo here: http://bx2abt.com/main/data/_uploaded/image/20180924-yagi.jpg The main differences are the tubing (InnovAntenna uses 1/2", I use 8 mm) and the width of the loop (InnovAntenna uses 46 mm, the smallest my tube bender can do is 68 mm). Apart from these two the InnovAntenna has through-boom elements and I use on-boom elements.
According to the lenght of my loop the resonant frequency is 436 MHz (300/436 = 68.8 cm). Using a HT and my Daiwa SWR meter I only get resonance (SWR 1:1) at 412 MHz. Making the loop shorter in length doesn't make much difference. I understand that the reflector and directors bring the high impedance of the loop down to the desired 50 ohms, hence they have a noticable influence on the resonant frequency.
How to go on from here? What am I doing wrong?
My first thought was to start trimming the elements up to 95% of their current lengths (412/436*100 = 94.5%), then adjust the element spacing to 95% of the original distance.
Second thought: there is (still) a feedline mismatch and the coax throws off the resonant frequency. On the G0KSC website there is a design for a 1:1 symetrical coax balun and talk about the Pawsey stub. Worth trying?
I would appreciate input from people who have successfully homebrew LFA yagis and who could point me in the right direction. Also if you have experience with modelling LFAs using MMANA I would like to hear from you. Cheers,
Hans BX2ABT