Radio Broadcast Technical Brief

Comment: Several years ago, I was the leader of an operationalized telecommunication cell. The purpose of the cell was to monitor the effectiveness and readiness of the telecommunications in support of the ongoing operations. The staff regularly turned over due to the operational tempo and I had to train new staff quickly. I did so by preparing a series of technical briefs on topics the cell dealt with. This brief was dealing with radio and broadcast signals. 

Radio Broadcast Technical Brief

Radio has been used by governments, industry, and political/ideological actors who desire to get their message out to large populations of people.  In the United States there has been a few instances of pirate radio and even TV when Captain Midnight captured control of the broadcasting satellite for Home Box Office, HBO. Few people understand radio and broadcasted signals. This brief is to provide the basics behind radio broadcasting equipment. 

The purpose radio waves is to send information over wide area and accomplishes this by broadcasted frequency signals encoded with audio and/or video information.  FM operates in the VHF range from 88 MHz to 108 MHz and there are operators in the Medium Wave range, 540KHz to 1600KHz.  Two kinds of FM exist: Narrow band and wide band. The maximum allowable carrier frequency shift occurs when fully modulated and known as the deviation differential. Narrow band deviation is 8KHz and wide band deviation is 75 KHz. Most FM broadcasting stations will use the wide band for one way broadcast and narrow band is used for fully duplexed communication radios such as citizens band radio. A typical FM station is setup as shown in figure 1. 

Figure 1: Typical FM Station Setup

The studio setup is simple. Either a computer or recording equipment input into a mixer. Then a Dynamic Noise Limiter (DNL) is used to reduce the tape hiss during the quiet parts of a recorded broadcast. Mono transmissions do not sound professional and stereo listeners may not tune in when their red stereo light does not illuminate on the receiver.  A stereo encoder will be used to achieve higher quality.  Then a cable will run from the remote studio equipment to the transmission point. 

An oscillator is at the heart of the system and generates the broadcast signal. There are three types of oscillators: Variable Frequency (VFO), Crystal, and Phase Locked Loop (PLL).  VFO’s are sensitive to transistor temperatures and minor temperature changes can lead to enormous transmission frequency variances. Thus they are unsuitable for serious use.  Crystal oscillators incorporate a crystal into the frequency determining circuit and are very stable. In fact, this has a shortfall that precludes the use of stereo and higher frequencies. PLL oscillators are the proper method of generating the VHF signal. It uses the crystal to establish a reference frequency and a VFO is used to provide the output frequency. Through its circuitry the PLL autocorrects for frequency drift and deviations taking advantage of the best of the two oscillators. Another advantage of the PLL is that frequencies can be easily changed because a VFO divider can be software programmable.

Buffers always follow the oscillator to protect it from changes in the load as the following stages are tuned. This is usually one or two transistors operating in class A mode.  Class A mode means the same transistors produce both halves of the signal which is very inefficient and generally runs very hot even when there is no audio output.  The RF amplifier increases the power output of the transmitter. Please note that radio frequencies power or amperage is not audio amperage. There are several basic concepts to amplification. They follow:

• Bandwidth – The range of frequencies that will amplify properly as limited by the amplifying device.
• Gain & Maximum output – The measure of an amplifiers ability to make its input bigger and often measured in decibels.
• Input & Output Impedance - Alternating current version of resistance and is measured in OHMs.

The Harmonic filters purpose is to remove harmful multiples of the frequency that result as side effects in the circuit. These harmful frequency multiples are also known as harmonics and produce radio frequency interference.

Broadcasting: The broadcast signal is line-of-sight and the distance to the most furthest point is based on the transmitter power and antenna height. The curvature of the earth creates a horizon at 12 miles when on the surface. Height can increase the horizon's distance.  Generally, with a 50 ft tall antenna tower on flat land:

• 1 watt < 3 miles
• 20 watts < 10 miles
• 100 watts < 25 miles
• 300 watts < 35 miles
• 500 watts < 50 miles

If producing unlicensed or illegal transmissions, the broadcast can be detected in real time. Authorities will react quickly raiding the broadcast site.  In the United States, human rights such as the freedom of speech and freedom of the press have permitted unregulated speech lessening the desire for unlicensed or illegal transmissions. However, under tyrannical rule and oppressive regimes pirate radio has been a tool used to transmit ideologies to the people in order to gain legitimacy.