I have never used any F12 antennas but from your description, it
sounds like this one is: a vertically polarized off center dipole
loaded in two ways--coils and horizontal rods perpendicular to the
ends.
Such an antenna will be fussy and tricky; for example it should be
away from other large metal objects. you have this problem: the
unbalanced feed is feeding a balanced antenna that isn't really
balanced because it is asymmetric. this means the feedline is going
to affect the antenna and your measurements so you have to isolate it
from the antenna as much as possible. two methods are firstly to
employ a choke at the feedpoint with some sort of balun, ferrite cores
on the coax and/or a wound coax choke coil with sufficient inductive
reactance at 3.5 MHz (max reactance is when the no. of turns gives you
a coil length equal to its diameter) and secondly, to suspend the coax
away from the antenna feedpoint for a distance at 90 degrees to the
antenna, instead of allowing it to drape down from the feedpoint
alongside the bottom portion of the dipole. Disclaimer: I don't
guarantee any of this will fix your problem because I have not seen
the site where the antenna is located but these ideas should give you
a starting point so you have some things to try and experiment with.
As usual, getting an antenna to work involves a certain amount of
messing around (the part where we learn things so ham radio isn't all
dull operating) until you hit on something and everything clicks.
73
rob / k5uj
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