Choke Coil in VHF Antenna Coaxial Cable

Choke Coil in VHF Antenna Coaxial Cable

This picture shows the choke coil in the coaxial cable when I mounted the CP22E VHF antenna. Manufacturers have suggested one turn choke coil immediately below the antenna, of about 6 to 10 inches diameter. They mention that it will act as an isolation choke coil and release strain on the connection. According to them, the coil diameter is not critical. They added that some cables may require a larger diameter coil to prevent movement of the center conductor. The coil is tied to the mast.

Choke is used to prevent the coax shield from becoming part of the antenna system and causing unintentional radiation. Unintentional radiation from the coax shield can cause interference to nearby devices. Coax shield can also work as a receiving antenna and pick up interference from the mains and other nearby sources. The number of turns for the common mode current choke increases gradually for lower bands with higher wavelengths. PVC former is used when there are larger number of turns. Alternate method is to wind it on a large ferrite toroid.