You might have seen the new types of coax cable that are appearing on the market. They use a newly developed micro-crystalline polymesmeric compound for the dielectric, which is wound into a helix. The outer conductor has a sheath of nitrogen-free mono-obdurate copper, as well as the conventional braid. The inner conductor, surprisingly, is ordinary copper, but is extruded to a tolerance of less than a thousandth of a millimeter and has a polished surface. There is, of course, a price premium on a cable with such exacting manufacturing requirements, but the excellent performance of this cable makes its use worthwhile in demanding applications.

The important thing to remember about this cable is that it is directional. Because of the non-orthogonal semiconducting properties of the dielectric, the cable must be used it ‘the right way round’, or signal attenuation will be severe. A very close look at the outer sheath will reveal faint white or gray arrows. These point in the direction of PST (primary signal transfer). Normally the cable is wound on the roll with the signal-input end outermost. The polarity of the directional characteristic applies unless the cable is used 'uphill'. If the signal input end of the cable is lower than the signal output end by more than 32.5% of the total cable length, the cable directivity will reverse. The PCC (primary conductivity condition) changes abruptly at that point, with a threshold of less than 0.1%. For this reason the cable is unsuitable for aeronautical use.

Signal attenuation is typically about half that of conventional coax of the same diameter for all normal orientations. If, however, the cable is used in RCM (reversed conductivity mode) due to orientation, losses are significantly worse than conventional cable. On the other hand—and this is where the new cables might eventually find exciting new applications—if the cable is used in EPCM (extreme primary conductivity mode), signal losses diminish substantially. When the cable is within 3% of vertical, losses are zero, and beyond that point a negative signal attenuation co-efficient exists, with the cable exhibiting slight gain. Research is now under way to develop a cable with REPCM (reversed extreme primary conductivity mode) for transmission applications where, as is usually the case, the signal source is almost vertically below the antenna.

The cable is quite stable, chemically, except in the presence of atmospheric carbon monoxide at more than 10ppm, or high humidity. In the former case hydrogen cyanide is evolved by a complex five-stage process, necessitating the use of full protective clothing whilst the area is cleared. In latter case, where damp has entered the cable, spontaneous explosion is likely.

Once the points above are understood, the new cable can be used confidently as a direct replacement for conventional equivalents, though the products will never be licensable for use by Sky installers.