The proposed circuit is a straightforward to construct 5 km range FM Transmitter. It includes 3 blocks; modulator / oscillator, 2 stage 500mW VHF amplifier and LED based power meter. The circuit enables transmiting music signals to FM band with frequencies from 88 MHz to 108 MHz.
As a result of surprisingly low power usage of under 100mA the circuit could be flawlessly driven by applying 9-12V battery or power supply in case you choose. The circuit may be split up into individual stages in order that it is be more beneficial for all to comprehend exactly how every single part functions individually.
Selecting the Transistors
Observe that each of the transmitter blocks happen to be developed utilizing 4 low noise general purpose NPN transistors. These types of transistors should be readily available. The majority of the typical NPN transistors must function great and these are simply a few of the examples: BC549 (low noise), 2n2222, BC109 (low noise), BC107, etc.
Please be mindful that audio power transistors like BD140, 2n3055, TIP3055, 2SC5200 and the like will not likely operate simply because they cannot deal with bigger frequencies. Appreciate it the truth that the normal NPN can function at 100MHz and over we are able to make use of them in all of the lower power modules of FM transmitters. Should you choose you may even make use of RF transistors like BF199, MPSH10, BF240, etc.
How the 5 km FM Transmitter Works
Stabilizing Stage
The 1st module is structured around transistor Q1 - BC549 which works like a basic modulator / oscillator. Most of the affordable FM transmitters utilize much the same circuit to create radio signals however this oscillator have been thoroughly enhanced to supply far better stability. Right off the bat which will make a terrific difference may be the utilization of a tunable coil for L1 instead of a normal air coil.
The application of this type of coil permits accurate tuning of a preferred frequency which can be specifically essential on jampacked FM bands. A problem having an air based coils is that it is significantly more challenging to tune-in to a particular frequency and generally considerably more time is required to set this kind of oscillator to a expected frequency.
An additional benefit of tunable coil is that its coil is incredibly strong and therefore a lot more resistive to frequency drifts triggered by an temperature variations.
These kind of oscillators will also be vulnerable to frequency drifts brought on by unstable supply voltages. This pertains both to batteries as well as DC power supplies. To enable dealing with this issue an IC1 - 5V regulator is utilized to power an oscillator.
Oscillator
The carrier frequency of an oscillator depends upon the capacity of C4, C5 along with inductance of the coil L1. 3.5 turns are essential and 27pF for C5 to tune to some lower portion of FM band (88-100MHz). In order to tune to a higher portion (100-108MHz) you may use a 10pF capacitor.
C6 is critical for Q1 to maintain oscillation and C7 works well for frequency tuning. R1 - 100 Ohm may be chosen in order to get a crystal clear oscillation of a frequency. Do not get enticed to make use of substantially reduced resistance or to take it off totally since that could result in lack of stability and creation of undesirable frequencies across the main carrier frequency.
C8 may be the ultimate and incredibly essential element. Its work is to both isolate the oscillator from the remaining modules to circumvent frequency drifts and 2.2pF is simply sufficient to move an oscillator's signal for an amplifier. Yet again, do not get persuaded to utilize a bigger capacitance simply because that wont improve an output power of the transmitter, it might actually do the reverse and trigger unwelcome outcomes.
VHF Amplifier
These are some images from an early tests with VHF amplifier.
This is actually the module which will offer you a great deal of enjoyment and educate you on the fundamental idea of VHF amplifiers, specifically if you have never developed one. You should not get disheartened if the schematic appears like it includes numerous coils since they are quite simple to build. From the above image of an earlier prototype you can observe one of the achievable methods to develop the VHF amplifier.
By carrying out it this way you are going to enormously reduce the external noise which could usually happen to be amplified combined with the inbound signal from the oscillator. What you will require is copper PC board as the major board and little pieces of PC bard which act as interconnections between the components.
Since you can easily spot the driver is additionally split up by a metal plate which is soldered to the ground. Each one of these improvements increases the overall amplifier's quality for the reason that amplified signal from the output stage won't be rebounded back to the driver's stage. An additional extremely critical point that you need to always remember while developing a good amplifier is that the coils must not be inserted too near to one another, always offer some nominal air break up.
Power Meter
Using only a couple of parts you may even develop a LED based RF meter which is really helpful to test for any existence and of a RF signal. The power meter could be constructed on a tiny board 0.5"x1" as demonstrated beneath. Just requires 3 connectors; RF input, voltage supply and a ground.
This meter is only going to detect high frequencies and also the energy of their transmission. When there is simply no RF activity the LED is not going to light up. You may also hook up its RF input to 12V and find out that nothing is showing due to the fact an input of the meter is isolated by a couple of 1.8pF capacitors which will barely allow any RF signals. To check and observe if the meter functions, just link the first block (oscillator) to the power supply, after that attach the input of the meter to an output of an oscillator and LED should light up at about 20% out its complete brightness.
With regard to efficiency 2 stage amplifier are actually employed here. Transistor Q2 performs like a driver and Q3 as an output stage of the amplifier. You could construct an amplifier utilizing only one transistor however in that situation the transistor may not have the ability to offer its optimum 500mW output power. For this reason the majority of amplifiers make use of both a driver and an output transistors.
The driver's function here is to consider the weak transmission released from an oscillator (Q1) and boost it to some necessary degree prior to transferring it to an output transistor. Driver could be constructed with only one transistor and it could use 2, 3 or as many as required. The explanation for this is that each transistor which is used within an output stage has to have a small element of input power to accomplish its maximum output power.
You may not simply plug-in an oscillator to one 1W, 10W or 100W transistor and anticipate it to present its full RF power. Each and every datasheet of a assigned transistor must express its least power that is required to drive that specified transistor. For instance 1W transistor such as 2n4427 may require 150-300mW input signal, 2n3866 5W transistor a minimum of 500mW, 10W transistor the very least 1W and 100W transistor no less than 10W. It is best to remember that while constructing a VHF amplifier.
If you have a peek at amplifier's schematic you might realize that driver and output transistor had been established to give the optimum gain since there is simply no resistor between the emitter of Q2, Q3 and the ground. It was executed to optimize an output power of NPN transistors and consequently these transistors could get slightly warm although not very hot).
In case instead of Q2 and Q3 you choose to extra strong transistors in that case you can certainly must make use of a smaller resistor to safeguard all of them and to reduce the heat emission. Driver must have a resistor of bigger resistance such as 100 Ohms based on the voltage supply and also the kind of transistor being employed. Output transistor must have a resistor having a lower resistance.
Any time amplifier is employed and in case where no inbound RF signal present it is basically in the "resting stage" and power meter's LED probably should not light up. As soon as the oscillator or small power transmitter is hooked up power meter's LED will probably light up suggesting the existence of RF signal and providing you with responses that amplifier is functioning effectively.
Prior to utilizing a RF amplifier minor tuning has to be carried out to ensure that amplifier provides the utmost output power. Fine-tune trimmers C12 and C13 to ensure that LED lights up in its maximum brightness. Accomplish the same for C17 and C18 trimmers. In case you hook up input of the power meter with the capacitor C14 through an antenna output edge LED must light up at 50 - 60 PER CENT of its total brightness. At this point examine an oscillator's output and check how RF signal is increased.
It is extremely simple to grasp that RF signal is increased progressively and this may be the method it should usually be achieved. In more powerful transmitters when driver is required to offer excess RF power, for example 1W, the signal is initially amplified through small signal transistors and transferred towards the stronger RF transistors.
You must also realize that RF amplifier may amplify particular choice of frequencies. By means of selected volume of turns in the coils, amplifier may boost the specified frequencies (within our situation frequencies close to 80-120MHz). Having a small adjustments a similar amplifier could be additionally accustomed to amplify an antenna's signal of your own FM radio.
How to Make the Coils
Necessary Resources
The following are the resources which have been employed to make coils for the proposed transmitter. In case spare super enamled wires is not vailable with you check out many online shops. These people offer about three spools of magnet wire for approximately $5. The coil package bought from these shops consists of 14 meters of 22AWG (.065mm) wire, 25 meters of 26AWG (0.4mm) wire and 65 meters of 30AWG (0.25mm) wire.
Tiny ferrite beads are utilized for amplifier coils.For those who have problems acquiring ferrite beads you could substitute L4, L5, L7 coils with standard air coils however remember that ferrite based coils offer increased output gain.
Below are the substitutes:
L4, L5 - 4 turns / 5mm diameter / 0.5-1mm super enameled coppper wire (precisely the same as L2, L8 )
L5 - is equivalent to L4, L5 although must have 5 turns rather than 4.
Trim the super enameled copper wires to recommended size.
L1 Coil
Employ 3.5 turns superior grade variable coil (0.5mm / 24AWG super enameled copper wire on 5mm diameter). Tunable RF coils are perfect for highly accurate frequency tuning of FM transmitter over the complete FM band (88 - 108MHz). The super enameled copper wire is midway inserted inside the plastic delivering outstanding frequency balance.
For those who have absolutely no access to variable coils you could attempt to employ a normal air coil. The wire thickness must be approximately 1 mm and ensure to work with resin in order to help it become solid. Remember air coils are generally not encouraged for oscillator coils due to the fact temperature alterations may cause frequency deviations.
L2, L8 Coils
4 turns of 0.65mm / 22AWG super enameled copper wire on 5mm diameter.
- Trim 9cm (3.5") of wire (1mm - 0.2m / 18 - 30AWG wire is great too).
2. Create 4 finished turns a demonstrated in the images.
L3 Coil
4 turns of 0.65mm / 22AWG super enameled copper wire on 9mm diameter.
- Trim 13cm (5") of super enameled copper wire (1mm - 0.2m / 18 - 30AWG wire is okay too).
2. Create 4 finished turns (as in L2, L8) over a 9mm dimension or a pencil.
3. Stretch it to around 0.6mm
L4, L7 Coils
Wind 4 turns of 0.25mm / 30AWG super enameled copper wire on two small ferrite beads.
- Trim 9cm (3.5") of super enemeled copper wire
2. Create 4 turns. The very first image exhibits one full turn, 4 turns is going to be accomplished once you notice four wires within the top section.
These little beads tend to be ideal to work with in the transmitters ranging upto 1W. Bigger ferrite beads and a heavier enameled copper wire must be employed for higher wattage output transmitters.
L5 Coil
4 turns over a solitary ferrite bead.
- Trim 10cm of 0.25mm / 30AWG super enameled copper
wire.
2. Create 4 turns over a single tiny ferrite bead as displayed within the images.
L6 Choke
20 turns of 0.65mm / 22AWG super enameled copper wire
over a ferrite ring.
This is the 1.5 cm ferrite ring obtainable from computer power supplies or motherboards. They could be available in an alternative shades nevertheless they will certainly carry out exactly the same task.
Pictures of completed coils which were applied to proposed 5 km long range transmitter.
Developing the PC Board
It's advocated to utilize a couple of 1" x 3" PCBs, one for oscillator and the other one for VHF Amplifier. This method will help you to test out the different oscillators / transmitters. When building RF amplifier attention should be given to shield each oscillator and amplifier within the metal cases to reduce outside frequency disturbance.