Tuning in the radio..

[laho pia]

Why is it that when you are trying to tune in to a radio station, the reception can be clearer when you either touch the radio or dont touch it at all? Its totally wierd!!

[heathfield]

It's because you are affecting the tuning circuits of the receiver. These are made up of inductive and capacitive components, and you are acting as an additional capacitor that increases in value as you approach the radio. The 'Theramin' musical instrument uses this effect to produce a varying musical note, controlled by the nearness of the player to the instrument. (See Google for Theramin).

[mibn2cweus]

Radios use an antenna to capture the electromagnetic energy that is coupled to space through a broadcast antenna. The directionality and sensitivity of a radio receivers antenna can be effected in a variety of ways.

An antenna is essentially a two terminal device. A car battery will not supply current by connecting only the positive lead. The negative lead typically uses the chassis as the other conductor to complete the circuit.
In a radio the chassis often serves as one conductor for many of the connections, including the antenna. When you touch the chassis of the radio you may be increasing the coupling of the chassis to earth ground through capacitance.

A capacitor has the ability to store a small charge electro-statically between two plates that are separated by a small gap that prohibits current from flowing through it. When a voltage difference is applied to both plates a current will flow into it until it has a charge that equals the applied voltage. Switching the polarity of the applied voltage will cause the capacitor to discharge and recharge to the opposite polarity. This make a capacitor a good conductor of alternating current; the higher the frequency of the alternating current the better a capacitor conducts.

The signal picked up by an antenna is a modulated radio frequency electro-magnetic wave. A properly grounded antenna provides a path through which more of the radio frequency energy will flow thereby improving reception.

The ability of an antenna to absorb electromagnetic energy also depends on the direction from which the signal is coming. Touching the radio can disturb the directionality of the receiving antenna reducing its ability to absorb electromagnetic energy from a certain direction. You may become an shield conducting the signal from the broadcast to the receivers ground.

Weird, ain't it?