How to build your own FM Radio Station

 Building 'practical models' of R.F circuit designs is a test of knowledge, skill +experience, when compared to other types of analogue circuitry designs. The level of difficulty becomes higher with its operational frequency. However with proper instructions and guidance you too can make your own, home brewed "FM Radio Station"

Where to begin?

Basic Function of the Transmitter

When Building a FM Transmitter system, number of basic individual stages has to be considered. The diagram below gives a broad impression of a transmitter.

Exciter/Modulator 

 The RF Oscillator, Frequency modulator and the Buffer stage.  

The Oscillator

Function of the RF oscillator is to produce a constant RF 'Carrier Signal' on transmitting frequency.  In this case, somewhere around FM range of frequencies (88-108MHz).  The values used for L1 and C in the tank circuit determines its frequency. The tank circuit is the basic oscillator, consists of an inductor and a capacitor network which beats on a resonant frequency

 Modulator

The primary purpose of a modulator is to somehow insert / feed the information to the carrier signal of the transmitter where then it’s possible to retrieve the same info from a receiver working on the same frequency.

There are three systems used in RF technology. Namely, Amplitude Modulation, Phase Modulation And Frequency Modulation.

Here it's frequency modulation. Modulating signal creates variations on frequency amplitude. Say if the amplitude of the modulating signal goes high, then the frequency of the carrier signal goes high and vice - versa.  In practice F.M is obtained using a device called the  'vari- cap' diode. Short for Variable Capacitor.  It reacts to voltage swings as variations in its capacitance.  These voltage 'swings' or variations come in a range of around 1/10 a Volt as amplitude variations of the modulating signal source.   Ex: audio, video, digital, etc..  Since the vari-cap is connected to the  tank circuit,  these 'capacitance-swing'  directly  effects the carrier frequency thus, produces "Frequency Modulation". The frequency swing (Also called the deviation) is  between +/-75 kHz on the mean carrier frequency when modulation is @100%.

Buffer Amplifier

The Buffer Stage basically provides two functions.  First it amplifies the incoming RF signal from the oscillator. Secondly,  it functions as a shield between the RF oscillator and the Rear stages. This stabilizes the Oscillator and reduces harmonics. The output from the RF Oscillator is fed to  Buffer via a capacitor / resistor network as an impedance matching technique for the two sections.

Power Output Section  

For long distance transmission greater power radiation is required and hence more RF amplification stages are required in between the aerial. As first amplifier after the oscillator is called the Buffer amplifier those between the buffer amp and the aerial are called the intermediate power amplifiers.

The basic functions of all these stages are the same. That is to amplify the FM signal produced by the Exciter Stage to a higher level, where then it is possible to achieve a worthy transmitting distance. Transistors were used for all stages. BLY 88 is the end Power Amp and works in Class C mode. It produces a RF out put of 15 -20W(P.E.P) and lot of heat at same time. To operate the transmitter safely, proper heat sinking should apply to the transistor.

PSU  

PSU or the powers supply unit supplies the correct voltages to the transmitter. Each Stage requires a stabilized supply of 15V while the Rear Stage 15V with 3-4 Amps at max power. The step down transformer T1 lowers the AC mains supply to a usable voltage of 18V. It must be 100W or more for safe operation.  Four-rectifier diodes (1N 3001)  rectifies AC to DC. The Voltage stabilizer IC (7815) stabilizes the voltage for the Front Stage and a hi-power transistor (2N 3055) works in combination with it to supply power to the Rear Stage, which requires a greater power.  You may build the PSU on the given PCB.  

On the PCB design, Mains Transformer and the power transistor (2N 3055) are placed away from the PSU board for easy installation. They're connected to the PSU via wires. As the metallic casing of the transistor is automatically connected to the collector pin on manufacturing process, to avoid short-circuiting while mounting to the Heatsink, place a mica washer between transistor and the Heat sink. (When the heat-sink is grounded.)

This diagram shows how to mount the power-transistor to a bracket (Heatsink) preventing short-circuiting.

Transmitting Antenna  

The Final Stage of any transmitter is the antenna. Where RF energy is converted to electromagnetic waves. When radio waves leaves a transmitting Aerial they travel out in all directions. By using a combination arrey, of simple aerials, the radiation may be made stronger in one direction than the others, and if this direction is chosen to be that of the receiver, communication is made more efficient.

Example: GP (Ground Plane) Antenna for Band II  [ 88-108 MHz.]

This type of antenna is really the most simple and easy you can build, perfect for the first time attempt. As a ground plane you use 4 rods pointing out in a cross from the plug, in 45° angle. In drawing from the side, you only see 2 of the four rods connected to earth. The length of rods depends on the transmitting frequency. Use the formula given below to determine the length of the rods.

Length of rod = Wavelength / 4

Ex; If transmitter is working @ 100MHz then, wavelength  = 3 0000  / 100

Which is 300. To determine the length of a pole divide 300 by 4 where you get 75.

So the length of a  pole = 75 cm

Each rod is attached with a screw to the female contact. At the top you solder the rod. The rods I used were thin with a copper Colour, used for welding. They are perfect for this use, they are stiff and long. Be sure to make the top rod a bit longer, and cut off two or three millimeters at a time to get the lowest SWR reading for your frequency.

Why impedance matching is essential

It's very important that the output impedance of the antenna matches cable and antenna.

If a radio signal travels from one impedance to another without matching, you will get a reflection in that point. The signal will then travel in both directions, some signal to the antenna, and some signal back to the transmitter.

If your antenna is totally wrong made, this reflection can destroy the transmitter since it's normally not dimensioned to handle this extra power.

 BUILDING THE TRANSMITTER

As now you're familiar with the schematics and the basic operating principals of the Transmitter, you could give try to build it. To begin with you should convert the initial 'Paper design' to a practical circuit.

There're many ways of building  a good electronic circuit. Most common and popular is the PCB technique. (Printed Circuit Board)   I recommend you use the PCB patterns to make this transmitter. PCB technique is a neat and professional method and makes life much easier during troubleshooting.

PCB  

PCB is the most common and professional way of building commercial type of circuitry. It's comprised from a copper layer bonded to an epoxy glass or synthetic resin bounded on paper. (SRBP)  The result is a neat and professional looking circuit, which is ideal for prototype as well as production quantities which is easily available in most electronics components selling shops.

Interconnectors or the electrical paths on design is obtained through removing unnecessary copper by a chemical reaction ( 'the etching process'). An 'etch resistive' positive is used to draw the design, which eventually 'eats away' the unprotected area. The end result is a neat copper pattern where you can assemble the 'practical circuit'.

PCB designs.

Exciter PCB

Exciter/ Power Amp PCB

PCBs Made Easy

The two methods described below explains easy and cheap ways of building  prototype for small-scale PCBs. (This is only for the novices)

The maker pen technique 

Use a 'water resist' marker pen to draw  PCB pattern.( Be sure the pen your using is a "Water resistive" one.)

Draw  the  pattern as close as possible to the original draft layout. Use mm scaled ruler to keep up with the 'aspect ratio' with the original PCB pattern.  If you have a double-sided board, be sure to completely paint the 'plate' side to prevent it from etching away. Check for possible 'mishaps'.

Sticker paper technique

The sticker paper should  be water resistant. Buy 'printable surface' sticker paper. Get the PCB pattern printed on it, using sticker paper as the printers default paper. Then carefully paste it on the copper board. Avoid wrinkling.  You have to cut the edges of the pcb pattern using a sharp pencil cutter,  peel off the unwanted portions leaving the initial track layout on the board. Remove if there's any 'gluing' left on the board, otherwise it'll act as a barrier to the FeCl³ thus, could result in unnecessary copper patches on the finished design.

Using FeCl3 to remove excess Copper  

Although there are many type of chemicals on the market that could be used as a 'copper etchant', (copper remover) the most common  and effective is the FeCl³.( Ferric chloride)  It's available in different forms (liquid, solid, powder).  Water (@25°C)  is added following the instruction given by the supplier. If no instructions are given the normal quantities involved are 750 ml of water to 500g of Ferric chloride. Etching times will also be very much dependent on temperature and concentration but, for a fresh solution warmed around 40°C the time taken should typically be ten to fifteen minutes. During this time the board should regularly agitated and check to ascertain the state of etching.  

CAUTION: Whatever the chemical you may use,  exercise great care while handling. Remember these are highly corrosive chemicals. Prevent any spills or splashes. Avoid contact with skin or eyes. Wear rubber gloves.  Use glass or plastic vessels when using/storing. Prevent contact  with metallic  stuff, woods, clothing etc..  If you don't have any prior experience of handling chemicals, seek assistance/advise of an experienced person.

Put  FeCl3+Water solution to a tray like apparatus and drown the PCB in it. Leave it for about 10 - 15   minutes. Regularly  check and agitate the board to ascertain the sate of itching.

When etching process is completed, carefully take the board out from the solution and wash it  Thouraly under a running tap. Check for 'unwanted copper traces' still on to the board. If so, put it back in the chemical. Else, wash it thoroughly with addition of   little soap. When it's done, wipe off the moisture with a dry cloth.

Clearing the Etched PCB  

Now you need to prepare copper surface for the soldering process.

If you used the felt pen technique, remove the ink using a strong solvent like thinner or turpentine.

 Peel off blocks of sticker paper. If the board feels sticky, use thinner or other type of a solvent to remove it. If all went as you've planned, you should have your home made PCB ready to use.

Soldering components to the PCB

The component layout PCBs

The necessary components for the transmitter construction. You may need to build some inductors yourself. Component values and coil construction info are given below.

Components List

Exciter/Modulator

PSU

Accessories

Heatsink / Cu, Zn / 2N3924 (r=20mm / h=10mm.)

Heatsink / Cu, Zn plate BLY88 (75 x 100mm.)

Cu board(s)

FeCl2 bottle.

Water resist Marker pen./ Sticker paper

Heatsink Compound., Mica insulator

Electric Soldering iron (30W).

Circuit wires

50 Ohm  Co-ax cable

Solder.

Soldering Components to the PCB

• Too little solder: solder has failed to flow around entire joint and some of the lead   or pad remains exposed.
• Too much solder: The solder has formed in to a large 'blob' the majority of which is not direct contact with either with the lead or the pad.  
• The joint is 'dry':  temperature of the joint was insufficient to permit the solder to flow adequately or if the joint was disturb during cooling.

Some basic tools  you must have

1. Transistors are 'heat sensitive'. Excess heat could damage the devices.
2. Mind polarity (+/-) of capacitors and pin configuration (E, C, B) when soldering transistors
3. Lookout for accidental 'solder-splashes' that may go unnoticed which can cause short-circuiting during operation.
4. When placing heat sinks take care to avoid accidental contacts with leads.
5. When your unsure of  contact  points, check for continuity using a multimeter.
6. Begin from left side and work through to right.

Before Operation,

Safety measurements for the novices

First check the basics. Those are: make sure  that you built it correctly and according to the instructions given. Soldered the components and transistors in correct configuration. (E, C, B +/- etc..)

Check the power supply unit  for correct voltages. Connect your A.F source to the  I/P of the Front Stage. The source could be anything from Headphone-out of a Walkman, CD player and LD player.. or other 'pre' type audio out.   The input signal is normally 200 mV  too much of AF could result in distorted transmission signal. If so adjust the  audio level (Modulation %) ( If you have more advanced A.F/ R.F measuring devices like the SWR, Oscilloscope?, RF Power meter make use of them for a much easier and better job.)

To align "Exciter", Connect the antenna or 50 ohm dummy load (This will act as a temporary antenna for the testing session) to the Tx antenna.   Turn your FM receiver( portable) /tuner on. Scan FM frequency spectrum for the transmission, where you should hear the AF produce by your signal source through the FM receiver.

No reception of the signal usually means the transmitter is not working properly or the signal is out of the FM band . If so adjust   Trimmer(CV1) and if needed L1. If the signal is out of FM range you should be able to resolve that problem with it.

If you can hear the transmission, Be sure the signal is 'genuine' and not the  harmonics. Verify this  by monitoring the strength of the signal. Monitor  the distance of your broadcast. The genuine signal ,when the tank is near resonance should at least cover a good distance  of about1Km.(with a proper antenna)   Adjust the  CV1 capacitor to obtain the correct frequency spot where you want operate the transmitter.
  

 Exciter Modulator/ Power Amp.  

This stage has power amplifier section that  amplifies the incoming 1W RF signal to higher level of power thus making long distance broadcasting possible. You need to have a RF power meter for a 'good' alignment on this stage to make a proper alignment.  Placing a metallic plates (shield)  between stages can minimize interference and feedback and stable the tx. When placing shields be careful not to short circuit with other components as transistor heat sinks.  Connect the  Antenna /Dummy load to R.F out. Never Operate the power section without a proper antenna or RF load as it'll 'destroy' the power transistors without a hesitation ant they are expensive to replace!

Monitor the TX output on   makeshift/RF power meter. Adjust each trimmers while monitoring the output power from max to min. don't always adjust for full power. Try to keep the tx stable on a lower power than going berserk on "all" frequencies at full power!

Some of the factors which effect the frequency stability of all Oscillators and methods of cure are,

Variation of Load.

The oscillator is designed to deliver power for a useful E.g.: to drive an aerial. Any changes in the load causes the frequency to shift a little. the effect can be reduced by using an amplifier - to separate the tuned circuit from the load.  

Changes in Collector Voltage.  

Collector Voltage can be kept constant by using voltage stabilizers.  

Vibration and shock.  

Shock absorbers and bonding on the circuit reduces vibrations.  

Hand Capacitance.  

the proximity of operators hand or other parts of the body may introduce an extra capacitance. This can be reduced by earthing one side of the tuned circuit by screening the whole circuit.  

Temperature.  

Coils and capacitors alter their size, and therefore there values changes with the temperature. Sometimes temperature is compensated for by design. e.g.: by using two capacitors in parallel , one which increases its capacitance with the temperature and the other of which decreases it. or by placing the tuned circuit in a thermo statically controlled compartment.
 
  

Trimming Hints

  Mount all boards in an appropriate enclosure or box. (You may have to make little adjustments after this.) put a power on/off switch, led indicator, I/P O/P connectors, controls, etc.. You're now ready to go on air with your.