Topband: Effect of trees- tree appreciation

[Jim Brown]

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