On 8/9/2013 9:31 PM, Gary and Kathleen Pearse wrote:
Where are the high voltage points in a "T" antenna?
The end of an antenna (for example, at the end insulator of a wire
antenna) MUST be a current minimum (near zero) because there is no place
for it to go except by capacitance to space or surrounding objects. A
mathematician or engineer would call this a "boundary condition."
Where there is a current minimum, there is a voltage maximum, and
because impedance is the ratio of voltage to current, it is a high
impedance point. If the antenna is a half wave or longer, there will
usually be another high impedance (high voltage) point one half wave
from the end.
The Tee would seems the better solution, because it puts the high
current parts of the antenna farther from the tree, but I would try to
avoid having the ends in close contact with the tree(s). There are
several ways this might be avoided.
1) Because the horizontal wire(s) that form the top of the tee are, in
essence, providing capacitance to lengthen the antenna. The top wires
can be shorter, yet be equally effective, if there are more of them. So
if your real estate, or your trees allow that, it's a very good solution.
2) The primary function of the horizontal wires (top loading) is to make
the antenna resonant, so that it loads more easily. But there's nothing
wrong with letting it be a bit short -- that's why there are antenna
tuners, and all sorts of possible matching networks.
Another important point associated with #2 is that the loss in coax is
quite low on 160M, even with a fairly high VSWR. Loss will be a lot
lower with bigger coax (like RG8).
3) The third option is a loading coil at the base.
73, Jim K9YC
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Topband Reflector
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