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

[Tom Rauch]

Watching frequency shift to determine current distribution
is a great deal like the misplaced notion we can grab the
jacket of coax and watch SWR shift to tell if we need a
balun.

Every Yagi would appear as a capacitance on 1.8MHz, the
value of which doesn't change much with height. Where you
place the capacitance most certainly has an effect on
current distribution in the tower. The amount of effect
depends on the ratio of the impedance of the tower system
where the Yagi is placed to the impedance of the capacitance
of the Yagi.

In some cases the Yagi will make "change for the good". In
some cases it will make a "change for the worse". Sometimes
it will do nothing noticeable. The best solution is to model
the system as closely as possible with real losses (as real
as a model can get) and see if the Yagi's are helping or
hurting field strength by adding and removing them or
repositioning them.

There are very few hats that 100% stop all current beyond
the hat, since it is really an impedance ratio problem and
the hat is almost never near zero impedance and the area
above the hat almost never near infinite impedance. Adding a
hat can do about anything from the range of positive to
negative effects depending on many separate issues.

73 Tom