I tried to make a dual-band (2m/70cm) dipole this morning by following the instructions on this page: http://www.amateurradio.bz/2m-70cm_vertical_dipole_antenna.html, with the rather major significant exception of placing the antenna's elements in a groove I routed out of a 2x4 and I'm using bare #10 copper wire instead of stainless steel.
I plugged a VNA into it and the antenna's lowest SWR was ~1.09 at ~124MHz. I tried it in a couple different environments and orientations and made sure there wasn't anything conductive right near the antenna (other than the wood, which I'm expecting to reduce the effective velocity factor of the wire.
Since I have no shortage of #10 wire, I started trimming the ends in 1/2" increments. I got the lowest SWR to 1.04 at 148MHz... but only while horizontal! Orienting the antenna vertically causes the lowest SWR point to jump to 1.52 at 153MHz.
What's happening here? Why was the tuning so far off? Is it the wood? Is it the copper wire? Why does orientation affect the tuning so much? I know I've got a lot of variables here, but is there one that is obviously the most important difference?