[Antennas] Capacity Hat Information

[David Kelley]

Juan,

You've had some good responses to this question already.  I would just
add that you can also think of a capacity hat as a "source" or "sink"
(or "reservoir") of charge.  That is, charge can flow from the antenna wire
into the capacity hat, or it can be drawn out of it.  In a basic vertical with
no capacity hat, the current has to be zero at the end because there is
no more wire along which it can flow.  With a capacity hat on the end
there can be non-zero current at the end because the current can simply
flow into and out of the hat.

You can visualize a vertical as a water pipe.  A basic vertical is like a
pipe that's capped at its end.  You can push water into the pipe, but it
won't flow past the end cap -- the (water) current is zero there.  But if
you put a big tank at the end of the pipe, water can then continue to
flow past the end of the pipe into the tank.  You can also draw water
out of the tank through the pipe.  The tank acts like a capacity hat on
a vertical antenna.

There are lots of ways to construct capacity hats.  One way is to use
a solid disk.  But another (and perhaps easier) way is to use two or
more wires attached to the top of the vertical.  The wires should run
directly away from the vertical and should be spaced evenly around it.
For example, if you use only two wires, they should be spaced 180
degrees apart (This creates the classic "T" antenna).  Three wires should
be spaced 120 degrees; four wires 90 degrees; and so forth.  The equal
spacing reduces the radiation from the capacity hat wires.  This is
important if you want most of your radiated energy to have vertical
polarization.  Also, a horizontal radiator close (a small fraction of a
wavelength) to the ground can reduce the efficiency.

If you make the wires long enough so that the length of each one plus
the height of the vertical add up to a quarter wavelength, you may not
need a loading coil (although you may still need an impedance-matching
coil at the base).  If you can't run the capacity hat wires directly away
from the antenna or space them perfectly, don't worry about it; the
antenna should still work very well.  Many people run the wires from the
top of the vertical to the ground.  Basically, the wires serve double-duty
as the capacity hat and as guy wires in this case.

I hope you find this information helpful.

73,
Dave NB4J

>Following with my trying of improving the performance of MP-1 (I already
>added better radials, a counterpoise to the rig, built my own
>inducti-match,..) and observing an improvement I would like to try with the
>capacity Hat so many times mentioned in different articles of vertical
>antennas.
>
>I've found some instructions about how to build one but ... I would like to
>know where to find the theory of this stuff in order to understand better
>how it is working, the basis and why it is improving the performance,
>etc... I was looking for in internet but most of the sites (I'm still
>checking out from the 5360 found) just mention the capacity hat as a
>feature of the antenna shown but not the theory.
>
>I will appreciate any help on that
>
>73 Juan EA5XQ