For coaxial cables, the insulation method of the main conductor is a critical factor. The manufacturers currently use physically or chemically expanded dielectric materials. The difference consists in the expansion method used.
In the chemically expanded dielectrics, when the material is still plastic, its components react under temperature and emit gas products. At the stage of material curing in the cable moulds, the bubbles are formed creating the foam structure. Similar technology is applied in the physically expanded dielectrics, however, the gas (usually nitrogen) is injected directly to the plasticized plastic.
The technology significantly affects the cable attenuation. The tests show that vapour slowly condenses inside the chemical bubbles on the entire length of the cable. It is due to the internal dielectric structure with non-uniform bubbles linked with channels transferring the water particles. It increases the cable attenuation up to 60% at 860 MHz and up to 80% at higher frequencies. For comparison, a cable with physically expanded dielectric can increase its attenuation up to 5% at the same frequency due to uniform and closed dielectric bubbles. The water does not ingress inside the cable.
The type of dielectric used is a critical factor in signal transmission quality. The coaxial cable manufacturers more often use physically expanded dielectrics, however, make sure what type of insulation is used before purchase.
Physical expansion process is more expensive, however, it is worth the cost. Insulation type is not the only parameter affecting the cable quality. To make sure the signal level does not drop rapidly with time, we need to allow for the type of dielectric used in the antenna cable.