Topband: Short receiving verticals question

[Doug Renwick]

I have used this same setup for my 4-square 160m receive array for years.
Since I have to take down and put up this array every spring/fall, I have to
re-tune each element for the 160m band.  I have found that the base loading
does not have to be exact for the system to 'work'.  Last year I decided to
make inductor substitution box for each element to easily tune each element
close to 1.830 MHz.  The biggest problem with the top hat is deer catching
the wire/string and bending the element or some rodent eating the string.
It's amazing how forgiving aluminum tubing is as I can straighten it many
times without breaking.  At the base I use a 2 ft ground rod and 4 short
radials.  I found the use of the ground rod makes a large change in the
tuning of the element.
Doug

-----Original Message-----

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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