On Fri,9/23/2016 5:08 AM, Mark Pride via TowerTalk wrote:
Moving soon to a new QTH where I plan to use wires hung from trees as verticals
and it will be over a wetland area. I will try the single radial approach
first to see how that works and eventually put in more radials (ground, in
water) later.
More observations. Fundamentals, really.
1) Nulls in directional patterns are produced by precise cancelling of
multiple elements or multiple antennas. Deep nulls are produced when the
elements are more precisely equal and more precisely 180 degrees out of
phase. Antennas which are not precisely the same will not null very
well, and the nulls may not be where you expect them to be.
2) Things that contribute to antennas not being precisely the same
include differences in the radial system, differences in the soil under
the radials, conductors, especially conductors in parallel with the
antennas, coax electrical length (physical length and VF), and antenna
length.
3) With trees, all the literature I can find, as well as my own
experience, says that it is the TRUNK of trees that most strongly
affects antennas below VHF, that it mostly affects verticals. In
general, trees increase loss, and the loss increases with frequency. In
addition, loss is greatest when the antenna is very close to the trunk.
I would not, for example, suspend a vertical wire from a limb close to
the tree trunk, but a wire suspended at some distance from the trunk
will probably not have enough loss to matter. I would expect nulls in
arrays of vertical wires very close to trees to be a bit skewed or not
as deep.
4) In an excellent presentation at Contest University several years ago
on RX antenna arrays, W3LPL noted that some types of arrays are quite
sensitive to vegetation, while one or more is not. That talk is on line,
both audio and slides. Maybe even video.
5) Gain from arrays is not nearly as "tweaky" as the nulls. That is,
small differences between elements of the array have far less effect on
the main lobe than on the nulls. If your primary concern is being
LOUDER, an array doesn't have to be nearly as perfect.
73, Jim K9YC
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