Abstract

There are two standard approaches to the transmission of HF signals at current short wave transmitting facilities. The two approaches; coaxial (unbalanced) and open wire (balanced) have been proven to give good results, however, the coaxial approach offers advantages over the open wire designs.

This paper will present a comparison between the two approaches and describe the advantages to using the coaxial approach from a performance, cost and operational standpoint

Historically, transmission line systems for HF Broadcast to overseas audiences have been open wire transmission lines from the transmitter all the way to the antennas.

Open wire systems are well suited to handle the tremendously high powers of stations rated at 500 KW and above. The fact that the open wire system requires large tracts of real estate was not consequential until recently.

With the demand for similar or greater coverage with less space used as well as personnel safety concerns and the need to protect these installations from attack, both political and personal, makes the consideration of coaxial approaches highly desirable.

The coaxial installation includes coaxial transmission line with high power EIA type flanges. Coaxial lines will be run from each individual transmitter into and throughout the switch matrix. No considerations needs to be given for clearance between transmission lines other than that of interference fit; isolations is not a problem with coaxial switching.

The output of the switch matrix may exit the building through a wall, the floor or ceiling depending on construction of the building and the local terrain. In addition to the basic system, multiplexing power monitors and combiners may be added.

OPEN WIRE INSTALLATION (BASIC SYSTEM)

PERFORMANCE

The following chart gives a summary of performance characteristics.

Coaxial switches are designed for indoor installation. Open wire switches by Dielectric Communications are designed for either indoor or outdoor installation.

For purposes of a cost comparison of certain significant cost factors, it is assumed that the switching matrix will be designed to receive the input of 5 transmits and feed out from any of the 5 transmitters to any one of 10 antennas.

Initial cost of:

Other costs must be considered. Among these cost is the cost of building and in the case of the open wire switch, if located outdoors, the cost of the structural support and framework.

The cost of stanchions for carrying above ground either open wire or coaxial lines from the switch to the antennas must also be considered. In the case of open wire lines, the lines should be at least 6 meters above the ground to minimize ground loss and to offer protection to workers from downward radiation. Coaxial lines can be mounted at any height convenient to the site layout.

The case of the unbalanced lines, rigid coax or flexible coax, these may also be buried underground. This underground installation has the advantage of making use of some 10 acres of land area for any other purposes advantageous to local requirements.

When the switch matrix is located indoors, the support structure can be made from Unistrut or similar materials, however when located outdoors, the structure should be constructed from heavy steel.

Bowl ceramic insulators, used with open wire systems, are heavy pieces. Indoors they can be installed within the framework of the building; but in the case of an outdoor switching matrix, the bowls must be built into short duct sections for mechanical support and for weather shielding.

A coaxial type of switching matrix is compact, because of this, it lends itself quite well to being located indoors in a building which probably also houses the transmitters. Indoor location is desirable especially in climates where there is a great deal of rain or blowing sand or where it is difficult to work outside 12 months of the year because of weather. Additionally, where industrial or agricultural fall-out products that are extremely corrosive to exposed open wire transmission lines is encountered closed coaxial lines offer a maintenance free operation.

An outdoor switching matrix needs rugged supporting structures as it will have to withstand high winds, blowing sand etc. In contrast, a switch located within a building can be RF shielded with panels and supported on a lighter framework.

A further advantage, with reference to the environment specifically, is the freedom of the indoor switch from the requirement for insect proofing

An indoor installation, not subject to hostile environmental factors is more easily maintained.

In the case of the coaxial system, there are fewer moving parts and less assembly hardware than in the case of open wire switches.

In the coaxial switches the insulating material between the outer and inner conductor (Teflon) is confined, therefore, considerably less subject to damage than the steatite or alumna insulators of ceramic material used in the open wire systems.

The cost of insulator is not insignificant. In the case of the open wire switch the cost of spare insulators is much higher than the cost of Teflon insulators. Also due to fragile nature of ceramic insulators, they are much more prone to damage than the insulators used in the coaxial switches and transmission lines.

Because the frame used for the indoor matrices are not subjected to the sometimes harsh environment outdoors, there is virtually no maintenance require. Outdoor frames however require periodic inspections and maintenance to assure proper structural integrity.

Open wire switches and transmission lines have served well in the past, however they tend to be much more expensive to purchase, install and maintain than the coaxial choice. The use of coaxial switch matrices, offer space savings while also offering superior electrical performance. Lower VSWR, insertion loss and much better isolation, lead to more and better signal getting to your audience.

Coaxial transmission lines allow for a safer working environment while taking up much less space. Because the coax lines can be installed close together and low to the ground, installation is quick, inexpensive and easily accomplished without special equipment. Because of the rigid outer conductor, the transmission line is safe from the environment as well as personnel damage. The nature of coaxial line and switches make them safe to work around and under without fear of the RF radiation hazard associated with open wire lines.