Radio Components – Radio Transmissions

This animation shows a half-wave dipole antenna receiving power from a radio wave. The antenna consists of two metal rods each one-quarter of the wavelength long, attached through a parallel transmission line to a resistance R equal to the characteristic impedance of the antenna, representing the receiver.

The electromagnetic wave, coming from the right, is represented by its electric field (E, green arrows) (it should be kept in mind that the drawing only shows the field along one line, while the radio wave is actually a plane wave and the electric field is actually the same at every point on a plane perpendicular to the direction of motion). The wave’s magnetic field is not shown.

The oscillating electric field exerts force on the electrons in the antenna rods , causing them to move back and forth in currents (black arrows) between the ends of the antenna rods, charging the ends of the antenna alternately positive (+) and negative (−). Since the antenna is a half-wavelength long at the radio wave’s frequency, it excites standing waves of voltage (V, red) and current in the antenna.

Amplitude Modulation (AM)

Frequency Modulation (FM)

Pulse Modulation (PM)

• The aim of pulse modulation methods is to transfer a narrowband analogue signal, (for example a phone call), over a wideband baseband channel or, in some of the schemes, as a bit stream over another digital transmission system.
• In music synthesizers, modulation may be used to synthesise waveforms with an extensive overtone spectrum using a small number of oscillators. In this case the carrier frequency is typically in the same order or much lower than the modulating waveform.
• In terms of what we are looking at – Pulse Modulation can be something as ‘simple’ as ON/OFF encoding!

The squelch controls the strength at which a received signal must be before it is passed to the audio amplifier.

For maximum reception sensitivity you would ordinarily turn the squelch control all the way down at which point you will hear a hissing sound from your receiver (also ignition and alternator noises, if present). This setting, with the squelch disabled, allows you to receive the weakest signals.

It also causes that hiss when no signal is being received . . . it is not an atmospheric noise being picked up by the antenna. As you turn the squelch control up, there will be a point where the hissing noise disappears. This is the correct setting for your squelch control. Turning the squelch any further than this will prevent the receiver from picking up weaker signals.