Well, my hopes for last week-end didn't quite work out, however today I did finally got back to building the cheap yagi for 70cm.
I attached the mounting bracket and then thought that before I put the varnish on I should put it up in the air and do some tests. Well, things weren't quite as hoped.
Basically, it looks as though the beam is tuning way down the bottom of the band – essentially just like the version for 432MHz. This is not too surprising, as the radiating element is identical to that for 432MHz, but still I was hoping the changes to the reflector and D1 (of 0.1") would result in the needed improvement in tuning. I'm thinking now I need to model it up and see what NEC can tell me.
But none the less I noted the basic SWR details. This time due to low power I was not able to use my SWR meter (couldn't get enough power through it to calibrate it) so I used the SWR meter on the FT-817 instead. It showed the following:
1:1 from 430MHZ – 433.295
Then it increased quickly with the FT-817 reporting HWSR at 435.895. Now I'm not sure what the FT-817 considers a High SWR, but I'm going to guess it's at least 2:1 – but the SWR meter on the FT-817 is only about about half – so maybe 1.5:1???
Anyway, that tells me it's definitely only useable at the bottom half of the 70cm band, and even at the designed frequency it's still rather high. Really, you'd only use this at 435MHz if you had no choice, and definitely not up at 438MHz like I was aiming for.
So, what could have caused this? Well, I did use an element thickness twice the designed diameter (as that was the smallest aluminium I could find); and as you go higher up the frequencies the tolerances are definitely reduced, so I guess that's it. But to be sure, I'd like to model this up as per the design, and then increase the thickness and see the result. Will keep you posted.
As for what I'll do with the beam… hmmm.. Guess I might as well just finish it anyway and accept it's for 70cm SSB – not 70cm FM as hoped. Plus I still need to figure out the mechanical mounting for the connections – as I'll then use that on the two 2m beams for phasing I have planned.