1w sändare

The Ve**nica is an easy to build and tune transmitter for the FM band. It's known for it's stability and clean signal, does not use any IC's or similar specialized parts, and it has a built-in tuning aid that makes it possible to tune it with no extra equipment.

Fig 1: Wiring diagram for the 1W Ve**nica

The TX has a built-in "mini-mixer" which makes it possible to use it without any external mixing table. This consists of the transistor T1, which amplify the mike signal before it gets combined with the CD (or tape) one. R1 and 2 are adjustable resistors (potentiometers) which is used to set the audio level (see the tuning section).

The components between R8 and C21 is the oscillator, the part which generates the radio signal. The diode D1 is a so-called "varicap", which can be seen as an variable capacitor, controlled by the audio signal. That, C12, 13, and the coil L1 decides the frequency. The oscillator is actually two oscillators, operating in antiphase around 50MHz. When the two signals are combined, they make a 100MHz signal. This setup is generally much more stable than one oscillator operating directly at 100MHz. The signal is then amplified up to 1W in T4.

To the far right is a the tuning aid, which rectifies a part of the output and controls the LED D5. The higher output, the brighter is D5.

Parts list

Resistors:
R1+2 10k pot R3 820k R4 4.7k R5-7+19 220r R8 1.5k R9 15k R10+11 1k R12 33k R13+14 56r R15+16 68k R17 47r R18 270r R20 10k
Capacitors: If nothing else is stated, use ceramic or mica types.
C1, 2, 7, 16, 17, 19, 24, 29 & 31 1n C3-5 & 8 10u elect. C6, 18 & 30 220u elect. C9, 10 & 20 10n C11 22p* C12 47p* C13 22p trimmer C14 & 15 15p* C21, C25 & 26 65p trimmer C22 100p C23 5.6p C27 & 28 1.8p
*C11, 12, 14 and 15 affects the frequency, for best results use high-quality capacitors.
Coils: Free-standing air-core types, wound of 1mm cu wire. Wind closely on a pin or similar with the right diameter, then carefully pull them out to the right length. Make sure the ends are as in fig. 2.
Coils
Fig. 2: Proper winding
L1 6 coils, each of 2 turns, 5mm internal dia and 5mm length L2 3 turns, 7mm dia and 7mm length L3 4 turns, 5mm dia and 7mm length L4 6 turns, 5mm dia and 10mm length
RF choke (H1): This can be made of a resistor of 33k, which you wind app 1/2m of 0.2mm enameled wire around, and solder to each end.
Diodes: D1 is a "dual varicap". If you can't get that, try two single ones, with the cathodes facing each other. In worst case, try different ordinary RF diodes, they might show some varicap effect. If you do this, you might have to increase the value of C11 a bit.
D1 KV1310 D2+3 1N4148 D4 Standard LED D5 1N4001
Transistors:
T1+5 BC548 T2+3 BF494 T4 2N4427

Fig. 3: Pin positions of transistors and regulator, seen from below
I1 is a standard 5 volts stabilizer, which gives a constant voltage to D1, even if the supply varies. This keeps the TX better on frequency.
I1: 78L05 (or ordinary 7805)
Misc:
Enclosure BNC socket 2 X 3.5mm jack sockets Power supply socket Power supply Antenna Mike CD/tape player

Assembly

Fig 4: PCB layout for the 1W Ve**nica

The transmitter is to be built on the PCB in fig. 4. RF stuff is pretty sensitive to poor PCB layout, avoid using veroboard or similar.

The components should be placed as flat as possible on the board, with short wires. The transmitter should be mounted in a shielded metal enclosure (connected to circuit ground), like these die-cast aluminum "block" enclosures. Use 3mm bolts with 5-10mm spacers, to create a good electrical connection.

The transistor T4 needs a little heatsink to cool it off. This could be made from a 2cm long piece of metal pipe, with slightly smaller diameter than the transistor. Cut a lengthways slot in the pipe, so it can be bent a bit out, and put on. Make some holes in the lid above it, to ensure air circulation. The heatsink is connected directly to the collector of T4, so make sure it doesn't get too close to the lid (app 5mm distance). The mike and CD connections could be done with 3.5mm jack plugs, while the power could be done with a such a plug that's often used with power supplies. For the antenna output I would recommend BNC plugs (like those used with some computer networks). The sockets' ground side should have a good connection to the enclosure, and internal wire should be made as short as possible (especially important with the antenna connection!). It's wise to mount D5 in the lid, so you constantly can see if the transmitter is operating properly.

Fig 5: Component placement

Tuning

In order to make the transmitter operate properly, it needs to be tuned. To this I'll recommend you to make a so-called dummy-load, which makes it easier to distinguish the main signal from weaker ones. This is a resistor of 47 or 68 ohms (corresponding to the antenna you intend to use), which is soldered to an antenna plug. Make sure the resistors can handle the power from the transmitter, and that they're not wire-wound. Don't ever turn on a transmitter without antenna or dummy, or else the output stage might blow.

Set all the trimmer capacitors to middle position (the upper plates cover half the lower ones), connect the dummy to the antenna output and a CD player to the CD input. When you switch on, the LED should light a bit (if not, try adjusting C21) and the transmitter operate around 98MHz. Use a small screwdriver with insulated handle (to not affect the circuit) and tune C21, 25 and 26 for maximum LED brightness. Then adjust C13 carefully up or down (depends on which frequency you intend to use) until the LED dims, but not goes completely out. Then tune the other trimmers until the LED is bright again. Continue this way until you get the frequency where you want it. Check with a radio to ensure that you're only transmitting on a single place in the band, if not you might have to re-tune from the beginning. If you have trouble reaching the ends of the band, L1 must be changed a bit. Carefully squeeze the turns a bit closer together to go down in frequency, increase the spacin! ! g to go up.

Now adjust R2 until the CD player sound as loud as the other stations. Be ware that many stations uses "compression" in order to make the sound seem louder than it actually is, and if you set it that loud, you might over-modulate. This might cause noise in neighbor channels, and must be avoided at all times. Similarly you must be careful not to set the mike (or talk) too loud, the best is to have an external mixer with some kinda ALC (automatic level control).

Don't get pissed if it doesn't work at first, transmitters generally requires a bit of fiddling and patience...