Answer to Question #5794 Submitted to "Ask the Experts"

When a metal object makes contact with the antenna of a shortwave band transistor radio set, there is usually a cracking noise from the radio's speaker. What is the scientific explanation between the metal object and the noise from the radio? I am a teacher and my students also want to know this. If possible, please also send to us the schematic diagram of this circuit.

While this is not my area of special expertise, I do have some interest in it and will attempt, based on my limited knowledge and reading on the subject, to provide at least a partial answer to your question.

Any conductor brought into contact with the antenna of the shortwave radio acts, itself, as an indiscriminate antenna and injects a myriad of signals to the input of the tuning circuits (the so-called "front end" of the electronics). Often these overload the input tuning circuits, and this results in an output that consists of very harsh and unintelligible noise. This also often happens if one tries to enhance signal strength by connecting a long wire to an existing telescope-type antenna using a simple alligator clip. People who are into shortwave listening sometimes connect a long wire antenna by inductive coupling, rather than by direct connection, in order to enhance signal strength and reduce this noise effect. This is done by taking one end of the wire antenna and wrapping it in a dozen or so continuous loops around a length of cardboard tubing that can then be slid over the telescopic radio antenna. The free end of the wire is often connected to ground. The cardboard tube is then moved slowly up and down the telescope antenna until the best signal is obtained.

If the radio has an antenna socket (which many portable radios do not), enthusiasts claim that a better way to enhance the signal while suppressing the noise is to use a device called an antenna tuning unit (ATU) that plugs into the socket. One of the problems with connecting a long-wire-type (or most other types) antenna to the input is that the input stage of the radio wants to see an impedance of about 50 ohms, and the actual impedance of a wire antenna may vary dramatically, depending on the frequency of the signal of interest. The ATU unit does a better job of matching the impedance at the input stage of the radio; it also can be equipped with an attenuator that can be adjusted to reduce signal strength as necessary for best reception. I'm not sure exactly what circuitry you are interested in, but here is a link to a Web site that shows how to build a relatively simple ATU. You can also find much information on designing and building shortwave receivers on the Internet. Simply input shortwave circuit design or shortwave radio into Google^_TM or another search engine, and numerous references will be displayed. Good luck to you and your students.

George Chabot, PhD, CHP