Take care with transmitter circuits. It is illegal in most countries to operate radio transmitters without a license. Although only low power this circuit may be tuned to operate over the range 87-108MHz with a range of 20 or 30 metres.
Notes
I have used a pair of BC548 transistors in this circuit. Although not strictly RF transistors, they still give good results. I have used an ECM Mic insert from Maplin Electronics, order code FS43W. It is a two terminal ECM, but ordinary dynamic mic inserts can also be used, simply omit the front 10k resistor. The coil L1 was again from Maplin, part no. UF68Y and consists of 7 turns on a quarter inch plastic former with a tuning slug. The tuning slug is adjusted to tune the transmitter. Actual range on my prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work. Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.
One final point, don’t hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations. I have had some first hand experience of this problem
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And an amp!:
Parts ListReference Description
- C1, C2, C4 5.5 – 50p miniature ceramic trimmer (green)
- C3 100p ceramic disc 50V NP0 dielectric
- C5, C6, C7 100n multilayer ceramic 50V X7R dielectric
- C8 100u 35V electrolytic radial capacitor
- C9 500p metal clad capacitor 500V
- C10 1n ceramic lead through capacitor capacitor
- C11 16 – 100p mica compression trimmer capacitor (Arco 424)
- C12 25 – 150p mica compression trimmer capacitor (Arco 423 or Sprague GMA30300)
- C14, C17 25p metal clad capacitor 500V
- C15, C16 50p metal clad capacitor 500V
- L1 64nH inductor – 4 turns 18 SWG tinned Cu wire on 6.5mm dia. former, turns length 8mm 1
- L2 25nH inductor – 2 turns 18 SWG tinned Cu wire on 6.5mm dia. former, turns length 4mm 1
- L3 6 hole ferrite bead threaded with 2.5 turns 22 SWG tinned Cu Wire to form wideband choke 219-850 1
- L4 210nH inductor – 8 turns 18 SWG enamelled Cu wire on 6.5mm dia. former, turns length 12mm 1
- L5 21nH inductor – 3 turns 18 SWG tinned Cu wire on 4mm dia. former, turns length 10mm 1
- L6 41nH inductor – 4 turns 22 SWG tinned Cu wire on 4mm dia. former, turns length 6mm 1
- L7 2 ferrite beads threaded onto lead of C10
- L8, L10 100nH inductor – 5 turns 18 SWG tinned Cu wire on 6.5mm dia. former, turns length 8mm 2
- L9 115nH inductor – 6 turns 18 SWG tinned Cu wire on 6.5mm dia. former, turns length 12mm 1
- R1 10K cermet potentiometer 0.5W
- R2 1K8 metal film resistor 0.5W
- R3 33R metal film resistor 0.5W
- D1, D2 BZX79C5V6 400mW Zener Diode
- TR1 MRF171A (Motorola)
- SK1 BNC bulkhead socket 583-509
- SK2 N type panel socket, square flange
- Diecast Box 29830PSL 38 x 120 x 95mm
- Heatsink 16 x 60 x 89mm 3.4°C/W (Redpoint Thermalloy 3.5Y1)
- Double sided Cu clad PCB material 1.6mm thick A/R
- Copper Tape or Foil 152-659 A/R
- M3 nut, bolt, crinkly washer set 16
- Non-Silicone Heat Transfer Paste 317-950 A/R
Notes
- Farnell Part Numbers are for guide only – other equivalent parts can be substituted.
- Metal clad capacitors are either Semco MCM series, Unelco J101 series, Underwood, or Arco MCJ-101 series available from, amongst other places, RF Parts.
- MRF171A available from BFI (UK), Richardson or RF Parts (US)
- Arco or Sprague trimmers are available from Communication Concepts (US)
- 18 SWG (standard wire gauge) is approximately 1.2mm diameter
- 22 SWG (standard wire gauge) is approximately 0.7mm diameter
- To make the inductors – wind the required number of turns round an appropriately sized former, initially use one wire diameter spacing between each turn. Then pull the turns apart to get the length required in the parts list table. Finally check the value using a network analyser and adjust accordingly.
- The exception to the above spacing rule is L4, which is close wound.
- Copper foil is available from craft shops (used in stained glass making)
- A/R = as required
Or you can buy a kit off ebay and do that and hook your computer to the tx make a simple vertical dipole and no more having to listen to crappy radio stations when in range. go pirate radio!
note: If you do this in a big city you WILL get busted.FCC has listening posts scattered throughout the US.
