More on 435MHz WA5VJB – High SWR

As previously mentioned, after completing the basic build of an 8el version of the WA5VJB Cheap Yagi for 435MHz, I found the SWR to be so high as to be unsable (for that part of the band). I've now gone away and done an NEC2 model to see if the issue was the design or the build.
Well, all I can say (and I add the disclaimer here that at best I'm a beginner NEC modeller) that it appears to be an issue with the design. From what I can see from the model, this beam was doomed to fail from the start with an SWR approaching 3:1 at 435MHz.
Anyway, I've attached the model and a capture of the SWR graph for your perusal. Feel free to have a play, and if I've got it wrong let me know. But so far, the model confirms the physical implementation – which I'll now simply use for SSB at the bottom of the 70cm band, and as a practice platform for adding a socket.
I might now go away and do a model of the Cheap Yagi targetted at 432MHz…
Quick Note: I did find if I changed the length of the DE from 13.0" to 12.7" that I could get the SWR down around 1.6:1 at 435MHz and another dip at 441MHz with an SWR of around 1.4:1. More playing with the optimizer might yield even more improvements…. Ahhh, so it does… Changing the reflector from 13.4" to 13.06" resulted in an SWR around 1.36:1 from 435MHz to 441.3MHz where it drops to it's lowest of 1:31:1!! If I was to do this, then the beam would cover the FM portion of 70cm like I was after!! I wonder how feasible this is now that I've varnished it… (Oh, and it still has a reasonable pattern – albeit reduced F/B ratio – with a gain a bit over 11dBi.)



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