[TowerTalk] a question about using EZNEC5

[StellarCAT]

I’m sure this is in the manual somewhere – forgive me for not digging for it... 

I’ve modeled a 6 element 15... and I was wondering how one best models the boom bracket. I know for YW it uses a lump – for example a 6” length of 3” equivalent diameter “tubing”... 

so I had a model for a 6 element 15 M2 ... both the standard (manual provided) 15M6DX and the 15M6-125. The results using only the element diameters and lengths seem to agree quite well with the manual published data on gain, FB as well as SWR. 

So I changed the –125 model to reflect adding a 3” long 3” diameter ‘piece’ at the boom. Of course appropriately shortening the next lengths going out by the proper amount (1.5” each)... when I change the reflector adding this very little happens (only reflector changed). The gain is (still) at 17.1 dbi... exactly what the manual says... but as soon as I change the driven element BAM! It all of a sudden is a SUPER yagi with 1.3 db more gain (18.4dbi)!! Adding the rest of the element ‘brackets’ keeps this ‘super’ gain increase. So it’s the addition of the driven element center ‘bracket’ that causes it to jump. 

I assume something about having the source connected at the mid point of that small 3” long segment at the center as well as having the hairpin there. I had been implementing the element centers as such:   60, –60 (inches)... this new method is –60, –1.5; –1.5, 1.5; 1.5, 60.

anyone have an explanation?

Gary 
K9RX