Topband: Element grounding

[Tom Rauch]

I have one in my shop right now.  There is no electrical connection to the
boom whatsoever.>>

That is really something you need to look into for two reasons:

1.) Shunt feeding a tower.

2.) Nearby or direct lightning strikes.

In both cases, there has to be some safe undamaging path for currents
between the extended elements (even if they are inside insulated tubes) and
the boom.

Of course there could be very large well-insulated gaps and everything still
be fine. For example a fiberglass spreader quad is not a problem because the
spreaders are very long. Neither nearby lightning nor shunt feeding would
cause a dielectric failure in such a system.

The KLM antenna element insulation or Force 12's Yagi element mounting are
example of systems with potential issues. Arcing is very possible because
voltage would always be across a somewhat close gap. The antenna needs
several inches of good arc-path length and very reliable insulation with
insulated elements or a proper direct grounding path.

You'd also always want any dielectric punch-through, should it happen, to be
in a spot that won't cause a system failure.

With shunt feeding we can NEVER permit arcs even if they are in a "harmless
spot". All elements more than a few feet long should have a low common mode
impedance path capable of sustaining high currents for brief periods, and
moderate currents for long periods when used on a shunt fed tower.

To give you an idea of the voltages involved with nearby lightning strikes,
I know of a commercial yagi using vacuum relays switching an element tuning
system in an antenna with insulated elements. The first nearby lightning
strike wiped out many relays in one antenna. (The same effect could easily
occur from shunt feeding.)

Shunt feeding is one of those tricky applications, as is lightning, where
the tower and everything else on and near the tower affect the voltage and
current applied in common mode to the antenna components.

Anything that changes current and voltage at the antenna, even ground
systems and how far the metallic parts of the elements are extended, affects
reliability. What might work OK one day can suddenly fail the next day.

DC floated elements in Yagis are generally a bad idea for both lightning and
shunt feeding. I'd stay away from them.

73 Tom