1. Field of the Invention
The present invention relates to the field of leakage coaxial cables which are used for radio communication in closed in areas such as the interior of buildings, tunnels or underground markets, where no ordinary radio waves can be received.
2. Description of the Prior Art
A leakage coaxial cable typically has slots formed in the exterior thereof which are spaced at predetermined intervals along the conductor, so that electromagnetic waves propagating inside the coaxial cable are partially radiated from the conductor into an external space through the slots.
When a fixed signal source is connected to the leakage coaxial cable, the signal is radiated into the external space and may be received by a mobile station running near the leakage coaxial cable. In addition, a signal transmitted by the mobile station may be received by the fixed station through the leakage coaxial cable.
A common application for leakage coaxial cables has been in disaster warning and prevention system for use in building, tunnels and underground markets. It is important in such applications to make certain that the cables are fireproof. The ability of prior art leakage coaxial cables to withstand high heat has been limited. Consequently, there has existed a long and unfilled need in the prior art for a method of making leakage coaxial cables that have transmission characteristics which will not degrade during a fire related emergency.
A conventional leakage coaxial cable has an external conductor with slots formed therein to radiate an electromagnetic wave which is propagating inside the cable outwardly into an external space. The external conductor is disposed coaxially around an internal conductor with an insulating member therebetween. The external conductor is covered by a protective sheath. In order to minimize the transmission loss of the electromagnetic wave, the insulating member is preferably made of a low-loss plastic material, such as polyethylene or polystyrene. The external conductor is preferably made of high-conductivity material such as alminum or copper. A polyester film is laminated on the external conductor with adhesive to compensate for the decrease of the mechanical strength of the conductor which is caused by the formation of the slots. The protective sheath is preferably made of polyethylene or polyvinyl chloride.
If a leakage coaxial cable thus constructed encounters a fire, the protective sheath will burn away and the external conductor will be directly exposed to the flames. The polyester will burn, and the plastic insulating member will melt. The molten plastic will flow through the slots formed in the external conductor, ignite and drop from the cable while burning. The burning molten plastic may actually contribute to the spread of the fire and may burn the skins or clothes of persons fighting the fire or running away from the fire.
One example of a prior art method of making a leakage coaxial cable refractory has been disclosed in Japanese Utility Model Application Publication No. 16682/1977. In that method, a heat-resistant tape which is made of an inorganic material such as asbestos is spirally wound between a polyethylene insulating member and an external conductor. The heat-resistant tape prevents the melting of the polyethylene insulating material for a long time. In addition, the heat-resistant tape maintains the insulation between the internal and external conductors even after the polyethylene insulating member has been molten. Therefore, the radio communication properties of the cable can be maintained unchanged for some time after the occurrence of the fire.
However, the leakage coaxial cable disclosed in Japanese Utility Model Application Publication No. 16682/1977 is still not an ideal solution to the problem discussed above, since the polyethylene insulating material will eventually ooze out through the stitch lines of the non-organic tape or the seams of the spirally wound tape and flow out through the slots. In addition, since the tape is relatively thick, on the order of 0.25 to 0.5 mm, the dielectric value between the internal and external conductors is large, causing large transmission losses.
Another example of a prior art conventional leakage coaxial cable is disclosed in Japanese Utility Model Application (OPI) No. 3537/1980. In that cable, a heat-resisting tape of polyimide resin is spirally wound between an external conductor and a polyethylene insulating member. Because of the presence of the heat-resisting tape, the internal and external conductors are not short-circuited even if the polyethylene insulating member is molten.
However, that leakage coaxial cable also has the problem of the polyethylene insulating member oozing out through the seams of the spirally wound tape, flowing out through the slots and dropping from the cable while burning.