Short-wave Broadcasting

International broadcasting in the high-frequency (short-wave) spectrum generally employs high transmitter power, commonly in the 50 kilowatt to 200 kW range, or greater, with either curtain antenna arrays or rhombic antennas fed with open-wire transmission lines from the transmitter. This photo shows the antenna array at a short-wave broadcast station studied in Guam. These systems present the possibility of strong RF fields associated not only with the antenna itself, but also with the switching circuits used to control which antenna is connected to a given transmitter. The open-wire feeders are typically supported about 8-10 feet above the ground.




An interesting aspect of these HF facilities is the presence of vertically polarized electric fields beneath the large, horizontally polarized antenna arrays. These vertically polarized fields come about because of the high RF potential between the elements and the ground. This phenomenon leads to the existence of electric fields that can induce substantial currents in a standing person. In some cases, these induced body currents can exceed the Maximum Permissible Exposure (MPE) limits well before either the electric or magnetic fields exceed their corresponding MPEs. A convenient rule of thumb for computing the induced body current in a bare-footed, adult in good electrical contact with ground is:




I(mA) = 0.38xE(V/m)f(MHz)

where: E is the electric field strength parallel to the body. It's easy to see that at 25 MHz, for example, an electric field of only 10.5 V/m is required to induce 100 mA of body current! The photo below shows a single axis electric field sensor commonly used for measuring HF transmitter site fields. A short element on the top of the meter case permits direct coupling to the internal electronics, avoiding commonly encountered cable pick-up effects at lower frequencies.




At HF broadcast stations using multiple transmitters and multiple antennas, an antenna switch room (matrix room) provides for the necessary relays to interconnect any transmitter to any antenna. Because the transmission lines are all open structures, high RF fields may be present within the room and both electric and magnetic fields must be measured to properly evaluate exposure.





Induced body currents may be measured with different types of equipment. In the case shown here, body current to ground is being measured using a narrow-band receiver to permit separating the current according to frequency. It is important to note that the one-legged current can be almost as great as when standing with both feet in electrical contact with ground. This is because the body current still exits the body through the available conductive contact. A ground rod driven into the ground beneath the platform electrode surface provides the other connection to earth.





Other instrumentation can be also be used for measuring induced body currents. In the photo below, a 'bath room scales' type of meter is used to stand on and permits direct indication of the current.






The most effective way of measuring induced body currents, however, is via the use of the clamp-on type of current transformer with a remote readout module that permits direct measurement of current as one walks or stands on a site. In this photograph, ankle current is being measured at a major VHF-UHF broadcast site. Such instruments provide a measurement of the actual current flowing through the legs, feet and shoes to ground without any of the potential for stray displacement currents that are inherently possible with separated plate type electrode foot current meters.