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Crash Course : FM transmitter for Micro Power Broadcasting

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from: Crash Media [crashmedia@yourserver.co.uk]
date:15 May 98 - 14h:41m

message:

Attention: this is the obligatory note that the below transmitter is for educational purposes only -ö GCSE level Physics. If (illegally) used with an antenna this kit will transmit between 87 and 108 MHz...

... i.e. transmit within the FM band used by commercial stations. Technically speaking this transmitter will not reach any further into public space than voices yelling at Speaker's Corner - which raises the question of what exactly the difference is between acoustic and electro-acoustic oscillation, or: "Is media space public space?" The transmitter doesn't reach far but some might find it extremely useful to bridge the gap between outdoor events and the nearest phone connection for web audio streaming.

Figure 1 shows the circuit diagram and figure 2 displays the finished kit with the components on the PCB from above. Figure 3 contains the track layout and needs to be enlarged to 45mm x 70mm before photocopied (or printed) onto OHP film with a laser copier / printer. OHP might be the easiest way to etch the copper clad board, but you can use any other method. The board contains a coil which is essential to the transmitter, so if you decide against etching you will have to build it manually (use your imagination).

This transmitter works with 9-14 V DC. You should experiment with the length of the antenna (between 30 and 40cm). The circuit has been taken from the instruction sheet of a kit purchased in London, so if you want to spend time on searching for (rather than building) a kit, good luck.

figure 1:

clicking on this picture will download a scalable EPS version of the image


figure 2:




figure 3:

clicking on this picture will download a scalable EPS version of the image



List of the parts:

  • C1 5pF ceramic
  • C2 6pF ceramic
  • C3 15pF ceramic
  • C4 22pF trimmer cap
  • C5 15pF ceramic
  • C6 1nF ceramic
  • C7 100uF electrolytic
  • C8 4.7uF electrolytic
  • C9 100pF ceramic
  • C10 1nF ceramic
  • D1 Varicap diode (eg. BB119)
  • D2 1N4148
  • R1 100K
  • R2 220K
  • R3 22R
  • R4 1K trimmer
  • R5 1K
  • R6 56K
  • R7 1M
  • R8 1K2
  • T1 BF244A or BF245A FET
  • T2 2N3819 FET
  • T3 BC307/8/9 or BC557/8/9 PNP


see also: [http://www.freeradio.org]
[http://www.irational.org/sic/radio]

Crash Media [crashmedia@yourserver.co.uk]