Even with very low power levels, the surface fields on an active antenna can still be substantial. This issue has arisen in connection with the common practice of doing tower rigging, beacon replacement and painting on active (hot) AM radio broadcast towers. A project conducted for the Federal Communications Commission (FCC) took the approach of measuring the current flowing between the tower and the body of a climber as an initial way of exploring the critical parameters involved in hot tower climbing. This study included climbing several different hot towers, including different electrical height towers, and comparing the observed body currents to computed surface near-fields, in particular, the radial component of the electric field on the tower.
Interestingly, the measured currents were found to track almost exactly the method-of-moments calculated distribution of the radial electric field component. This is the component that exists on the surface of the tower and provides for most coupling with the extended arm of a tower climber grasping the tower. The study made estimates of the power delivered to the tower that would prevent the induced body currents from exceeding recommended limits. In a separate study, the effectiveness of different kinds of work gloves in reducing the current flowing into the climber was investigated and showed that common cotton gloves can provide for at least a factor of two reduction in bare-handed currents.
Strong electric and magnetic fields are also associated with the 'dog house' structure used to house the impedance matching circuit for coupling the transmitter output to the tower. While this is not normally an issue for those climbing the tower, maintenance activities, such as tower base current readings, may present the possibility of high exposure levels. In the photo below, electric fields are being measured near the matching circuit inductors.
