Some of my Yagis are on masts and some are on rotating rings so there is
some side offset, perhaps 50cm depending on the ring diameter. I have not
modeled this offset however my stacks appear to work well. For a while I
had a pair of homebrew 6el OWA 18m boom antennas that I had to shift
backwards to obtain tower clearance for a director element. I shifted the
mast mounted beam so they were both aligned front to back. I also added
weight to keep them balanced after the shift. You could model your shifted
stack to see the effect but if it's not too difficult I would just slide the
top antenna forward or backward so it lines up with the lower one(s).
GL
John KK9A
HA3LN wrote:
Dear All,
Turning into the final stage lifting the stacks for 10m and 15m before
ARRL-DX contests
I need a bit of guidance regarding the feed-line setup.
I read in the ARRL Antenna book that if 2x vertically stacked Yagi's
are not exactly in the same vertical plane feed-line length compensation
is needed to avoid stacking performance degradation.
Both band setups will have similar OWA Yagi types via coax chokes + WX0B
like stackers.
However, due to the tower geometry it is not possible to position the
lower antennas to the proper vertical position "below" the upper one.
...for 15m (crank-up tower), is OK to have the lower one "underneath"
the upper one but with a ~60cm offset to the side direction.
...for 10m (freestanding tower), not only the side ~80cm offset but due
to the element positions it needs to be pulled a bit back with ~80cm as
well. So in this case the offset is two dimensional.
- are these less than 1m offsets are serious enough the compensate (with
aiming for max performance)?
- is it possible/needed to compensate the "side only" offset in 15m case
as the driven elements will be in vertically same plane?
- these calculation formulas can be used to determine the extra coax
needed?
(Driver element offset length) / WL x 360 = degrees of delay required
(Driver element offset length) / VF = extra coax length required
Thanks in advance.
73!
Csaba HA3LN / HG3N
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