Written by Axel Eng September 1998 Updated
March 2001 (C7 was wrong on the 5W board) - thanks to Syst3m. December 2001 note: I no
longer checks my e-mail regularly and can't promise replies to questions.
If you're unsure about anything, try asking in the Usenet group alt.radio.pirate. -
Axel
The
Veronica 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. It's available in two versions, 1 and 5 watts.
Most of you might want to go for the 5W one, but be ware that the output
transistor is quite expensive ($20) and might be hard to find.
Fig 1:
Wiring diagram for the 1W Veronica
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.
*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.
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" (with facing cathodes)
selcted for the absolutely best sound quality. It's not
critical, almost any varicap(s) will do. In a pinch, even some
regular RF diodes might work (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
Another suitable
antenna is the so-called "Quarter wave Ground plane", as shown in fig. 6C. It
consists of 5 elements of the same type and length as on the dipole. The center
one radiates the signal, while the others make a so-called ground plane. D shows
how the elements are assembled with a chassis-mount BNC socket. The middle rod
is soldered to the center connector, while the lower ones are soldered in the
bolt holes. They could preferable be bent under (an soldered to) the next as
shown, to give mechanical strength. These rods should not point straight out,
but be bent 30-45 degrees downwards. This antenna has 50 ohms impedance, and
should be connected to a 50 ohms cable without a balun. It's important
that the antenna is placed high and free. At FM frequencies, the transmitting
power don't affect the range much, it's more important to have a clear view
towards the listeners. If you put a little effort in finding a good location,
you can get quite far even with a small transmitter. The antenna should be
placed outdoors (preferably on the roof) since both trees and buildings obstruct
FM signals. If someone starts asking, remember that such an antenna is good for
reception too!
The 5W version of the Veronica is pretty similar to the 1W one, but with an
extra amplifier stage (T6). As mentioned, the MRF 237 transistor costs around
$20, and should be handled accordingly.
Parts not mentioned are identical to the 1W version.
R19 10r
R20 22r
R21 1.5k
R22 270r
C23 15p
C24 33p
C28 5.6p
C32 & 34 47p
C33 22p
C35 & 38 1n
C36 220n
C37 100p
L3 3 turns, 6mm dia and
8mm long
L4 Made of an ordinary
2.2k resistor (app
2mm dia) on which you
make 14 turns of 0.2
mm enameled wire, and
solder to each side.
In addition it should
have a ferrite bead
(of the same type as
the RF chokes) on each
pin.
L5 5 turns, 6mm dia and
11mm long
L6 4 turns, 6mm dia and
9mm long
Fig 8: L4, pin position of T6 and dummy-load
The RF chokes should be made of enameled wire on
ferrite beads (app 5mm long, 4mm dia). Make sure you get beads
suited for frequencies around 100MHz, so-called "material 43" should
do the trick. The wire is wound through the hole, so that
through-around-and-through-again is one turn. If you can't get hold
of such beads, you could make the chokes as in the 1W
version.
I got a letter from Dragan
Stojanovic, where he told that he'd used a 2N3926, which I guess
it's cheaper and easier to get hold of tham the MRF237. But note
that if that one is like most RF transistors, the collector is
connected to the casing. You may have to redesign the heatsink a
bit, so it's insulated (important!) from ground, and won't
create too much capacitance.
The output transistor
T6 needs a heatsink. This is made from an L-shaped aluminum profile, app 14cm
long, 2.5cm side width and 3mm material thickness (see fig. 10). The hole for T6
must be made as accurate as possible (measure the transistor itself for exact
dia). If you make a slot as shown, you could carefully bend the heatsink out a
bit to get the transistor in. Use silicone grease if you have it! The heatsink
is mounted with 4 screws to the PCB (the two outermost also to fix it in the
enclosure). You probably need to use one or two washers between PCB and
heatsink, due to the edge of T6. If you're using a dia-cast aluminum enclosure,
you should bolt the heatsink to the inside of this, use silicone grease here,
too. It might be some hassle to get the pins of T6 to fit in the holes in the
PCB (solder in place after assembling the heatsink), The easiest way is to drill
the holes a bit too large (2mm), and bend the pins down to the PCB surface. T4
should have the same type of cooling cap as in the 1W version.