Concept

Broadcasting at very low power and low cost, to a small community area.

In Canada, there are legal restrictions -- you have to get licenses from both Industry Canada (technicalities of spectrum space and interference) and the CRTC (for making sure you're transmitting regulation content).

Canada allows 10W and 50W LPFM radio stations.

(More on the concept, possibilities, and legalities here: low-power broadcasting)

FM Transmission Basics

FM signals can be generated using either direct or indirect frequency modulation.

• Direct FM modulation can be achieved by directly feeding the message into the input of a VCO.
• For indirect FM modulation, the message signal is integrated to generate a phase modulated signal. This is used to modulate a crystal controlled oscillator, and the result is passed through a frequency multiplier to give an FM signal[1].

To amplify the signal, use a switching amplifier.

To broadcast in stereo, you have to multiplex the audio input in a special way.

Antennas are important, so here are some basics.

Range: a 50W transmitter can go 5-30km, depending on terrain

FM Reception

A common FM detector circuit:

• Foster-Seeley discriminator

It uses a tuned RF transformer to convert frequency changes into amplitude changes.

Circuit Components

• http://www.minicircuits.com/

  • ZOS-150 (75-150mhz VCO, $120)

  • ZFL-1000LN (20dB gain low-noise amplifier, 3 dBm power output, $90)

  • ZHL-100W-52-S (100 watt high-power amplifier, 50dB gain, 50dBm power output, $2000)

  • AMP-77 (16 dBm amplifier, 2.0 standing wave ratio, $60)

  • 300W FM amplifier with MOSFET technology (300 watt output, $380 euros)

• http://www.pcs-electronics.com

  • All-in-one Transmitter (50W, 880 euros)

  • MRF173 (FM radio amplifier, 80W, no low-pass filter, 140 euros)

  • MAX PRO 2000+ (15W all-in-one FM transmitter, 140 euros)

• http://www.intertechfm.com/

  • CyberMax MICRO v3 Stereo FM Transmitter (2 watt, $500)

  • PCI MAX FM Transmitter (1 watt, $270)

  • 25 watt booster (designed for use with PCI MAX, $400)

• MRF136 (50 watt pallet amplifier, needs heat sink, $90)

• PCI MAX 2007 transmitter ($230)

• http://www.electrokits.com/

  • 150 Watt FM Power Amplifier

  • 40 watt FM Amplifier schematic/circuit design

Resources

• What is low-power FM? (howstuffworks)

• How Radio Works

• Frequency Modulation (wikipedia)

• FM Broadcasting (wikipedia)

• Circuits

  • FM Transmitter (very simple)

  • FM Transmitter (9-12V, 400 feet)

  • FM Transmitter (3-20 watt)

  • Voltage-controlled Oscillator (important FM element)

• VGASIG - FM radio transmitter using a VGA graphics card

• How Oscillators Work

• Rogue Radio (lots of links -- equipment, other stations, etc.)