It was five years ago when I did an attempt to build my first
fm-transmitter. It ended in a giant faillure. The only thing it did was
interferring with our tv-set. Looking back it was due to the lack of
information I had. A schematic was my only help. Now, five years later, I
know a lot more about electro-technics. So I searched for a schematic
of a stable, tested fm-transmitter with a far reach. I will put all
information you'll have to know in my page. I made drawings to make
things clearer. As said before: I'm still building it, so I will add
information every time I made progress. It would be wise for you out
there not to start building untill I'm ready and have tested it. It has
been succesfully built before, but my succes will give you a double
security. I remind you of the fact that I can also fail.
Intro
Building a good fm-transmitter(88-110Mhz) begins with getting a good
schematic. You don't have to understand the precise working of the
transmitter to build it. But some basic information won't harm. A
transmitter alone is, as you probably know, is not enough to start your
radio-station. In the simplest form you need 4 things. First an input
device such as an amplifiler you also use with your home-stereo.
You can also use a walkman. Details about input-devices in the page:
"Input". Second you need a regulated power-supply. In this case a 14-18
Volts/2,5-3,5 Ampere. One of the most influencial things you need is
antenna and coax-cable. More about this later on. And finally the
transmitter itself. You can devide the transmitter in two main parts:
the oscilator and the amplifiler. The oscilator converts electric sound
information into electromagnetic waves. The amplifiler gives these waves
a bigger amplitude.
Building
It's stable and has output of 15-18 watts. This enough to terrorize your wide surroundings at the fm-band.
The most often used technique to connect the components to each other is
soldering them on a double sided copper-board. Another way is
connecting the components floating. It is cheaper but very tricky. Below
you see the copper-board layout(PCB). I designed it looking closely at
the root scheme.
To get this pattern in copper surface you use a acid bath. Use a
water-resistant permanent marker to paint your own copper-board black in
the pattern the shown above. Color the back side ompletely black. The
grid-squares are 0,5*0,5 cm each.
When the acid has eaten the non-painted copper away you must remove
the complet thin layer of black paint with sandpaper. Don't remove too
much copper with it.
So, now you have the surface to solder the electric components on.
Now a few basic rules for good soldering:
1. Use a special electronics-solderingrod with a slim top.
2. Use soldering-metal with an anti-oxidant-fluid core.
3. Don't heat the components! Heat the connection-point on your PCB.
4. Make sure that the surface is not too smooth.
5. Don't use too much metal.
6. Don't let the soldering metal form a bridge beetween two copper-surfaces.
7. If you're smart you start from the middle of your prepaired board.
In this way you'll have enough space.
Below the schematic. The yellow lines are pieces of copperboard that
devide the transmitter in 3 parts. This is essential. Without them,
internal interferrence will ruin your signal.
Parts
There are some components that need extra attention. Transistors usually have 3 or 4 different
wires comin' out. If you connect these wires in the wrong way the
transmitter won't work. It may even explode. The picture below shows how
to prevent from such an event.
You can find the numbers and letters back in the soldering schematic.
Coils also require extra attention. You can buy the coils trough
ferrite in the shop, but the other ones have to be made yourself. Use
1mm AgCu wire. A coil like 7x/d=10mm/l=15mm, goes round 7 times, has an
diameter of 10 millimeter and is long 15 millimeters. The best way to
make a coil is to bend it around a pencil or other cilindrical shaped
object tight. The diameter of the object is always d-coil minus 1 mm. In
this case 9mm. As I said: bend the wire round (in this case 7times)
with the revolves tight together. To get the desired length stretch the
coil when still around the pencil
If you decide to build the transmitter and buy the parts, this list will be handy:
compon.doc
READ THIS E-MAIL I RECEIVED
Hello,
just to give some input: I have built the 15W FM transmitter you describe about 4.5 years ago.
The PCB lay out and component selection is still the same as it was
then and after some modifications, I had an average output power of
16.8W @ 98.6 MHz (measured with Rhode and Schwarz equipement). You will
need additional filtering on the power lines otherwise a stable power
supply for the modulating circuit cannot be guaranteed. The legs of the
modulating diode are, at best, kept long for extra capacitance. This to
make sure you fall within the FM band because before I did that, I had
problems falling withing the 88-108 MHz. I was actually interfering
with the police and fire brigade radio bands (Belgium). Of course, this
is not the intention. I also advice you and readers to carefully check
the orientation of the BLY88 because my professor blew one up due to
lack of specification and inclarities in the datasheets (the actual pin
out of the component changed a few years ago, resulting in a swapped
emitter and collector - no good if you position it wrong!!! (the white
cap flies of)). You will also need to play with the spacing between the
windings of the different coils in order to get a good coupling between
the different stages. I short circuited parts of the coils and made them
smaller than specified to have near-optimal coupling. I also added
extra ferrite bead coils for extra decoupling of the power lines, and
used a very good shielding. Above 16.8W there is coupling (primarily
through the air) between the output and the modulating/input stage and
oscillation occurs. So for I have not found any other solution than
lower the output power. Both extra decoupling
and extra shielding had no effect (my transmitter is built into a
fully closed aluminium box with seperating plates that are fully
connected to the case or ground plane on the PCB, except from where
tracks run (0.5mm spacing provided)). Also, use a good heat sink for the
last power stage!!!
I hope this information will be usefull. If you have any questions, please ask.
Kind regards,
Erwin Huybreghts
Electronic Engineer
Space applications and space instruments division
Verhaert D&D
Belgium
Author:
website: http://www.geocities.com/SouthBeach/3433/transmit.htm
