But what about an element loaded with a coil at the center or at the top?
Would there be advantages to that approach that would come close to the
short verticals with top-hat wires, or any serious disadvantages?
Jon,
The reason I use the hats and do everything I do in the elements is
bandwidth. Even at my quiet rural location on the quietest hour of the
quietest day, almost any element of reasonable height will have more than
enough signal level. This is why I base load and use a large hat. While the
large hat tends to keep current more uniform throughout the element
independent of coil location, and while more uniform current increases
radiation resistance, that effect is meaningless to me. The entire goal for
me is bandwidth, or a stable SWR vs. frequency.
Bandwidth is also why I load the element with a series resistance for
matching, instead of a network. I want to "swamp out" or dilute the effects
of resonance, minimizing element phase shift vs. frequency change at the
element terminals and preventing drastic changes in element feedpoint
impedance from mutual coupling between elements.
The hat is actually the bulk of the loading, and sets the current
distribution. The coil just cancels reactance. Since it is a series network
with the inductor forming a series tank with the termination reactance, the
lower the reactance used (compared to termination resistance) the larger
bandwidth becomes. You want the loading coil to be terminated in the lowest
capacitive reactance possible, and that is at the antenna base.
Because voltage and current are out-of-phase above the coil, even with high
current, the impedance increases. This means the tradeoff in a bottom
inductance is increased voltage above the inductor. The antenna is more
"loss critical" above the coil for anything coupled via the electric field,
including a lossy dielectric.
This is a compromise of two things:
1.) Bandwidth
2.) Sensitivity to dielectrics around the element
Getting rid of the hat while the element is close to a tree does nothing but
bad things to both, but no one can say how much. The last resort for me
would be no "hats". Perhaps you can use T elements with loading wires away
from foliage that might change tuning or losses?
73 Tom
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