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Austin Insulators Inc.
suppliers of RF insulators and transformers since 1933

   
       

 

 

 

 

Why do we need insulators?

 

Why do we need insulators?

Towers and masts which act as radiators generally have to be insulated from ground, and in the case of guyed masts also from the guy cables. In addition, guy cables and other masts or towers located close to the radiator(s) must be insulated and broken up into some fraction of the wavelength so as not to re-radiate the signal and cause interference.

There are three types of insulators of different sizes, shapes and methods of construction. Most insulators employ ceramics. This material is very strong in compression, but weak in tension. Therefore it is necessary to configure the insulator so as to take advantage of these properties and achieve the required strength.

Insulator Types

There are three basic types of insulators used in communication towers and masts.

  1. "Ball" or "egg" type insulators - These are porcelain shapes, with holes or grooves, and are inserted into the interlocking loops of guy cables. They have low flashover ratings.
  2. "Compression cone" or "cage" type insulators - These are cones or sleeves of porcelain, which are put into compression through the use of steel rods and castings. They have better flashover ratings than the ball type; but their values are still limited. It is often necessary to put strings of these units together to achieve the required values. There are several forms of this insulator type.
  3. "Oil filled" or "Austin" type insulators. These insulators consist of a continuously wound fibreglass link as its structural and insulating element. The link is then housed in a porcelain sleeve to provide a stable internal environment. Changing the size and length of fibreglass link; and the porcelain sleeve provides various voltage ratings and eliminates the need for strings of multiple units. There are several variations of this principle to suit particular requirements.

Base Insulators

Base insulators are used in the legs of self-supporting towers and at the bottom of guyed masts to insulate the structure from ground or isolate sections of the tower or mast.

For self-supporting towers the assembly can be arranged so the insulator can either transmit tension and compression as well as shear while keeping the porcelain in compression as in the "cage" type insulator. The other type known as the "oil filled" type uses a fibreglass link as the tensile element whilst in compression and shear loads are taken by the porcelain sleeve.

For guyed masts the unit is always in compression. The insulating material is generally porcelain, but other non-conducting materials have been used. These units are of the "compression cone" or "oil-filled" type.

Primary Guy Insulators

Primary insulators are located at the attachment of the guy to the mast or radiating element and therefore, must sustain high flashover voltages. The distribution and magnitude of these voltages, along the radiating mast depends on the type of antenna being supplied. This information is supplied by the radio design engineer. These units may be of any of the constructions.

The ball type units must be used in groups and are only applicable for low power applications. Some consultants feel they are not suitable for this use.

The compression cone units have limited flashover ratings and it is often necessary to connect several units together in "strings" to achieve the required rating. There are several different styles of the compression cone units depending on the manufacturer. The oil-filled units offer the greatest range of flashover voltages and load capacities.

Breakup Guy Insulators

Breakup insulators are used to sectionalize the guy cable and prevent re-radiation of the signal. These are located about 1/8 to 1/10 of the antenna's wavelength apart, although other spacing patterns may be used depending on the application.

For small loads and flashover ratings, the "ball type" units are used. For larger loads the compression cone type are usually used.

Oil-filled types are available; but are generally only used for medium or high power MF, LF, & VLF antennas unless the environmental conditions are particularly severe.

Commentary

Insulators for communication applications are a specialized product and there are only a few designers and manufacturers world wide. There is very little information and data available. Some applicable papers are given in the references of this article.

Porcelain has been the material of choice for most radio frequency insulators. Glass reinforced plastic in the form of rod has been used to some extent, but has not proved to be suitable for long term applications. Ultraviolet radiation and flashover tracking phenomena have made these units suspect.

The term "fail-safe" guy insulators has been a topic of discussion for many years. The pros and cons depend on the manufacturers. Some have claimed that the "oil-filled" units were not "fail-safe" since a failure of the fibreglass band would result in a guy rupture. There were some early problems caused by wartime shortages of quality materials; but these have since been overcome. Recent failures of compression cone cage type insulators indicate that these units are not always as "fail-safe" as was originally claimed.

In medium and high power applications it is often necessary to utilize corona rings at the guy connection ends to reduce the electrical stress at these points.

In areas of dry blowing sand, dust or snow, static charges may build up on the sections and flashover causing noise on the signal. Static drain resistors may need to be installed on the breakups in these cases.

To maintain the insulated characteristic of the mast or tower, ring type isolation transformers are generally used across the base insulator to supply electric power for the obstruction lighting system.

Arcing balls are frequently supplied on the base insulator to take the flashover in case of lightning strikes.

Electric power insulators are tested and operate at 60 Hz. Radio insulators must operate at the higher radio frequencies. Radio insulators are often tested at 60 Hz, but the results may not have any bearing to the insulators performance at the higher frequencies. Tests which can and should be applied to radio insulators involve 60 Hz flash over tests, UC impulse tests and radio frequency tests.

Each type of insulator has its strong and weak points. The radio and structural engineers must work closely together to achieve the best overall performance and safety at the most economical price.

   
         
       
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References:

  1. THE EFFECTIVE INSULATION OF MASTS AND TOWERS FOR AM BROADCASTING (edited)
    - Dennett & Thompson W A.B.E. 1973
  2. ONE VIEW OF INSULATION PROBLEMS FOUND IN BROADCAST ANTENNA SYSTEMS
    - Dennett & Thompson W A.B.E. 1974
  3. PRELIMINARY INVESTIGATION INTO SOME FAILURE MODES OF EGG TYPE INSULATORS
    - Dennett & Thompson W A.B.E. 1975
   
       
 

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