Nautel XR12, Руководство по предварительной установке

Страница: 48 / 76

XR12 Transmitter Pre-installation Manual RF output requirements
Page 8-2 Issue 0.17 2005-08-31
Three-material junctions:
A frequent cause of breakdowns at an insulator surface is the junction
of three materials: a metal conductor, a solid insulator, and a gas insulator. The insulators form a
capacitive RF divider between the metal conductor and the grounded periphery. Since the dielectric
constant of the solid is higher than the gas (air), the high voltage stress is concentrated on the air.
It is very important to minimize the junction's stress gradient and to avoid triple junctions wherever
possible.
Bowl type bushings.
Use bowl type bushing insulators to increase the surface tracking distance
from the central conductor to the grounded periphery or wall.
Insulator Breakdown/Damage.
An insulator can be damaged during gas breakdown flashovers.
When current flows across the surface of an insulator, especially when it is coated with a conductive
contaminant that is slightly damp, carbon tracks can form. Once this occurs, the track provides a
conductive path and reduces the effectiveness of the insulator.
Pitting and erosion of the insulator may also occur. To reduce the possibility of tracking damage,
clean all insulator surfaces periodically, and use stress control techniques at the conductor/insulator
junction.
Other causes.
Other causes of breakdowns may include improperly adjusted spark balls or the
disruption of air gaps by rain, snow, insects, birds, grass, or an accumulation of pollution (soot) on
insulator surfaces.
Small radius conductors
Be careful when using small radius conductors to carry high RF voltages and currents. If the radius is
too small it may cause a local corona, which can lead to a breakdown.
Feedline testing
Test the RF transmission system from the transmitter flange to the antenna before putting the
transmitter into service. This is very important, especially if you are re-using an existing site with a
previously installed feedline. Ensure that the entire system can tolerate the expected peak voltages of
normal operation, especially those occurring during modulation peaks, without breaking down.
WARNING: FAILURE TO PROVIDE CORRECT STRESS CONTROL AT HIGH RF
VOLTAGE CONDUCTOR/INSULATOR JUNCTIONS CAN CAUSE INSULATOR
BREAKDOWNS OR EVEN FIRES.