Topband: Tophat? Does it have to be at the top?

[Tom Rauch]

> How does the "hat" on a Cushcraft 402-CD work? You have
those crossed
> aluminum rods after the coil, then extended aluminum after
that. What
> happens if you take off the crossed rods?


Tony,

If they just needed to tweak the tuning very slightly and
found that adding a hat was cheaper than retooling a coil
form, then the hat is probably OK. If they added the hats to
decrease loss or increase efficiency or if the capacitance
of the hat is large compared to the antenna impedance
extended beyond the hat, then they made a really bad
decision.

A small-sized hat, when placed near the coil, increases
current in the coil **without** really decreasing current in
the area of antenna extended out beyond the hat. In a large
antenna with a small coil, the increased coil current makes
little difference. In a large coil like a mobile antenna on
low bands, it can noticeably degrade coil ESR. The increase
in effective series resistance of the coil *can be* as
severe as proportional to the reduction in inductance
required by using the hat. In other words if coil reactance
is 500 ohms and a hat makes the system only require 250 (by
adding stray C across the coil), it COULD as much as ~double
the effective series resistance of the coil!

The really wild thing is this occurs without an increase in
bandwidth, so you may think the efficiency is still good. In
general any capacitance across the coil is bad news, but
ONLY when the coil is somewhat large in reactance. If it's a
low reactance coil it has so little loss even doubling loss
won't hurt.

If the hat is large and has a low impedance compared to what
is above the hat, then you might as well throw away whatever
is above (or beyond) the hat.

I've started to put some stuff on my web page about the
effects of current distribution on radiation resistance and
about capacitance hats. It was also the subject of my talk
at Dayton, that should be on K3LR.com

73 Tom