[TowerTalk] dipole question

[Tom Rauch]

>  So knowing that, how does the side of my dipole that is
connected to the shield do anything if the outside of the
sparkgap is grounded?

In an ideally operating unbalanced feedline, current
entering or leaving the shield at either end would always be
equal and opposite to the current entering or leaving the
center conductor. The only thing unbalanced is the cable
voltage with respect to the environment around the cable. A
perfectly unbalanced line would have zero volts between the
shield and anything near the line. That means all current,
voltage, and all radiation stays inside the shield.

That's why you can ground the shield, because in theory an
ideally operating line has zero current and voltage on the
OUTSIDE of the shield.

The problem is up at the dipole. The dipole would have to
have equal and opposite terminal voltages in order to have
equal currents in each dipole half. Since you attach the
grounded shield to one half, the outside of the cable
elevates in voltage in an attempt to get current flowing in
the grounded half of the dipole. That causes some current to
flow down the outside of the shield, and the cable to
radiate. The outside of the shield actually becomes part of
the antenna when the feedpoint isn't properly designed!

Certain  shield-to-ground connection lengths minimize this
unwanted shield current because that make a high impedance
at the dipole. Some lengths will aggravate the problem. A
proper balun greatly reduces the effect of the shield
grounding on a balanced antenna.

It actually is a bit more complex than this, but this is a
good rough idea of what is happening.

By the way, I hope you bonded the three ground rods to your
utility company service entrance ground. With all that
grounding work you probably want to do things correctly!

73 Tom