Radio Transmitter

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In radio electronics and broadcasting, a transmitter usually has a power supply, an oscillator, a modulator, and amplifiers for audio frequency (AF) and radio frequency (RF). The modulator is the device which piggybacks (or modulates) the signal information onto the carrier frequency, which is then broadcast. Sometimes a device (for example, a cell phone) contains both a transmitter and a radio receiver, with the combined unit referred to as a transceiver. In amateur radio, a transmitter can be a separate piece of electronic gear or a subset of a transceiver, and often referred to using an abbreviated form; "XMTR".

FM Broadcast Radio

A type of radio broadcasting that uses frequency modulation (FM) to provide high-fidelity sound over broadcast radio. The broadcast band falls within the VHF part of the radio spectrum, specifically 88 to 108 MHz. Odd numbered multiples of 100kHz are used for channel separation.

With FM the voice is expressed over a carrier wave by varying its frequency, which causes FM modulation to consume more of the spectrum than AM broadcasts, which merely modulate the amplitude on a single frequency.

Stereo FM broadcasting was approved by the FCC in 1961 when a system developed independently both by GE and Zenith were accepted. Although Stereo FM offered the Left and Right separation already used in recording, Stereo FM signals are more susceptible to noise and multipath distortion than are mono FM signals. This is why FM stereo receivers traditionally include a mono switch, which is useful as the signal-to-noise ratio worsens when a station becomes more distant or signal fades.

Low Power FM (LPFM) radio service

The FCC launched the Low Power FM (LPFM) radio service in 2000 after grassroots pressure demanded community control of the airwaves. The service is entirely commercial-free, and licenses were only granted to nonprofit organizations. Low power FM stations can operate at a maximum power of 100 watts, which generally provides coverage within three to five miles.

Unfortunately, as the FCC was starting to license new stations, Congress put a hault to it after a lobbying campaign by corporate broadcasters, which restricted new licensing opportunities.

NPR - National Public Radio was one major opponent trying to stop LPFM.

The Communications Act of 1934 replaced the FRC with the broader Federal Communications Commission (FCC), noting that their role was to“regulate the public airwaves in the public interest,” but still offering no specific requirements for soliciting the public’s input.

The first well-known micro-broadcaster was Mbanna Kantako, who used a 1 watt transmitter to broadcast to his neighbors in a Springfield, Illinois housing project starting in 1987. Kantako used his radio station to discuss police brutality and other issues affecting his neighborhood. Kantako refused to shut down his transmitter after multiple requests by the FCC, insisting that he had a right to have a voice on the airwaves.

Some in the grassroots resorted to broadcasting without obtaining a license. The flashpoint of the grassroots radio movement was the court challenges made by Stephen Dunifer. Lower courts justified these unlicensed operations.

Micro Broadcaster

Microbroadcasting, in radio terms, is the use of low-power transmitters (FCC Part 15) to broadcast a radio signal over the space of a neighborhood or small town. Similarly to pirate radio, microbroadcasters generally operate without a license from the local regulation body, but sacrifice range in favor of using legal power limits (for example, 100 mW for medium wave broadcasts). Higher power levels can be achieved using carrier current techniques, which are widely used in colleges and universities. Both AM and FM bands are used, although AM tends to have better propagation characteristics at low power.

Microbroadcasting is also used by schools and businesses to serve just the immediate campus of the operation; well-known uses include audio tour guide systems, airport information services, and drive-in theaters, which often provide movie audio over the driver's car audio system. It has also been adopted as an advertising technique, particularly by car dealers and real estate agents.

Consumer Personal FM Transmitter

This is for the guy wanting to listen to his iPod through his car stereo, which has not audio input jacks. He plugs his personal FM transmitter into his iPod, and tunes his car FM radio to a select channel which allows him to listen to the music from his iPod. This works for "her" as well as "him."

These transmitters should operate between 88 to 108 MHz.

Low end short range

These are cheap pieces of junk that can be hand for under $20.00 and are barely suitable for use in your car.

  • SONY DCC-FMT3 Car FM Stereo Transmitter
  • Radio Shack 12-2054 FM Transmitter

High end short range

This equipment serves for providing good quality FM radio from your audio source to receivers throughout a home or office. The top end of these can serve a college campus, or an entire golf course. Some of these options are also suitable for setting up your own FCC Part 15 FM broadcast station.

FM Transmitter Circuit

Low powered FM transmitter with a range of 300ft at 9v or 400ft at 12v.

Wiki-fm-xmitter-schem.gif

Part Total Qty. Description Substitutions
C1 1 0.001uf Disc Capacitor
C2 1 5.6pf Disc Capacitor
C3,C4 2 10uf Electrolytic Capacitor
C5 1 3-18pf Adjustable Cap
R1 1 270 Ohm 1/8W Resistor 270 Ohm 1/4W Resistor
R2,R5,R6 3 4.7k 1/8W Resistor 4.7K 1/4W Resistor
R3 1 10k 1/8W Resistor 10K 1/4W Resistor
R4 1 100k 1/8W Resistor 100K 1/4W Resistor
Q1, Q2 2 2N2222A NPN Transistor 2N3904, NTE123A
L1, L2 2 5 Turn Air Core Coil
MIC 1 Electret Microphone
MISC 1 9V Battery Snap, PC Board, Wire For Antenna

Pirate Radio

The funny thing is that, at its core, radio is an incredibly simple technology. With just a couple of electronic components that cost at most a d­ollar or two, you can build simple radio transmitters and receivers.

The FCC limits the broadcast power of an unlicensed individual's radio. The official regulation that handles non-licensed FM Transmitters (among other things) is FCC Part 15 and based on that regulation, the official rule is 250 micro volts measured at 3 meters (actually written as 250 µV/meter @ 3 meters). However when testing FM Transmitters they convert it to be measured as 47.95 dBµV/m, but it is usually rounded off to just 48 dBµV/m. The FCC put out a Public Notice that says the approximate distance you can transmit without needing a license is around a 200 feet.

Avoiding Interference

Complaints to the FCC against micro-broadcasters are typically lodged due to interference caused by the unlicensed transmitter, especially that on a licensed station. Select a frequency that is not being used by a station already. Refer to this radio locator to find what licensed stations are in your area.