Many beginner (and not only) radio amateurs sooner or later become interested in the topic of transmitters. Indeed, the construction of VHF transmitters for the 88-108 MHz range is a fascinating and useful topic. Radio microphones, bugs and other devices can be assembled based on FM radio transmitters. There are many schemes for such devices, but finding a simple, powerful and at the same time stable UHF generator is a problem. After a long search, the choice fell on the following scheme.
The block was built on the basis of well-known circuits, but several modifications were added. The system works almost perfectly, the range is large, and the sound quality is good. BF240 transistors are used, but others can be installed here from the list below. The frequency is changed using a potentiometer.
There is only one, very easy to wind, reel. Many people have problems with this, but anyone can wind 5 turns of 1 mm wire on a 5 mm mandrel.
As for shielding, the tin does its job. When tests were done without a screen, the frequency floated and responded to the approach of a hand. After applying the shielding, the circuit worked stably and no longer reacts to the approach of a hand.
Capacitors and power supply chokes can be useful to prevent self-excitation. This did not occur during testing, so the decoupling was not installed.
In addition to the output power level of the radio transmitter, a lot depends on the antenna. You can even receive a signal from it at a distance of up to 1 km if you place a long pin a couple of meters away.
The goal of the work was to create a stable transmitter to broadcast music within a house within a radius of approximately 50 meters. In this case, a clear signal is required, since the reception was going to be carried out on a regular Chinese receiver. Ideally, the following was required: to turn on music on the computer at home and go to the garage to work (it will soon get warmer and I will spend a lot of time there, the weather, as they say, is just whispering). So, I chose a classic and proven circuit - at 88-108 MHz, but slightly modernized it.
First of all, I decided to make it powered from the mains, so as not to bother with the battery and its constant charge.
For power supply, a 220V power supply is used, which was on hand - this is a factory-made 12 volt 4 A pulse generator. Of course, you can take a less powerful one, this one just happened to be closer. The output without load is 12.7 V. So, in order to make the transmitter more stable, because as we know, such simple radio transmitters change in frequency if the power supply is changed, it was decided to make a stabilizer based on a microcircuit - 78L05 - a linear voltage stabilizer of 5 volts. When the input is 9 to 20 volts, the output will be a stable 5 V. The stabilizer in my device is made on a separate PCB board, which is glued inside the case.
The stabilizer board also has 2 0.22 uF capacitors, which prevent self-excitation of the microcircuit circuits. Thus, we get a device that can be connected to various units, regardless of their voltage (within reasonable limits). The stabilizer current is up to 100 mA, which is quite suitable. The device consumption was about 40 mA. Of these, 22 mA is the transmitter board itself, the rest is on the LED - an operating mode indicator.
The device is mounted in a purchased plastic case (40 rubles), the audio input is a standard 3.5 mm audio plug. Modulation using a UHF kt368am varicap, a generator using the same transistor. 100 µH choke. Regarding the power supply, it is necessary to install a 47 uF capacitor in order to significantly reduce the ripple - I can only hear it slightly when the receiver is almost turned up to maximum volume and there is no signal at the input.
I took a metal plug, it is better in quality. The connection is more reliable, there is less noise and background noise. The antenna is ready, steel with a tip.
Connected to the collector of the UHF transistor through a 22 pf capacitor to protect against frequency drift due to the influences of movement and accidental touches.
When checking for stability, the FM transmitter worked for more than 12 hours. The departure was not noticed, the receiver operates stably at the same frequency. The deviation was selected to be about 0.2 MHz. Comrade was with you. Redmoon
Discuss the article STABLE RADIO TRANSMITTER AT 88-108 MHz
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The radio transmitter, the diagram of which is shown in the figure below, operates at a frequency of 88-108 MHz, the radio signal transmission range is from 1 to 5 kilometers, depending on the design of the circuit.
The circuit uses widely available radio-electronic components. The circuit is powered from any 9V power source, it can be a KRONA battery or an AC power supply.
The first transistor contains a master oscillator and a modulator. High power of the radio transmitter is achieved through the use of an additional RF power amplification stage assembled on the KT610 transistor and the preceding RF amplification stage assembled on the KT315 transistor.
If such transmitter power is not needed, then the circuit can be significantly simplified by eliminating the RF signal amplification stage; in the circuit, this stage is highlighted in a blue block. In this case, we connect the antenna to the middle tap of the L3 coil. Thus, the power of the radio transmitter will decrease and its range will be 800m - 1km.
If you need a range of about 50-200 meters, then you can eliminate both RF amplification stages on transistors KT610 and KT315, leaving only the master oscillator on the first transistor (circled in a gray rectangle). In this case, coil L2 is no longer needed; we connect the antenna through a 5-10 pF capacitor to the collector of the transistor in the master oscillator.
#24 Andrey March 17 2015
Is there a scheme specifically for round-the-clock broadcasting over 3-5 km, but with a clearly recorded wave (so that it doesn’t wander and there are no problems with the signal on the receivers)?
#25 Konstantin June 08 2015
Is there a circuit for a transmitter of similar power, but more stable, with a varicap?
I’m broadcasting from home to my summer cottage, I’m tired of running around and making adjustments. The neighbors approve of the idea and also ask for stability. It turns out funny: they adjust the receiver at their place, I dance around the transmitter with a tambourine, and together we all adjust our receivers again. After a while, again in a circle.
#26 root June 09 2015
Here is a radio transmitter with an output power of 100-200 mW and with a varicap: Diagram of a powerful radio transmitter with FM at 65-108 MHz.
Let’s also add that in order for the frequency not to float and the transmitter to work stably, you need a high-quality, well-stabilized power source.
#27 NULL June 16 2015
Hello, I'm looking for advice
I assembled this transmitter in a version with the first two stages, and it “worked” almost immediately.
First, a question about the design: two coils of 3 turns that form L3, how should they be positioned? On the same axis next to each other or parallel to each other? I placed it on one axis.
Now a question about work: how to check the functionality of the second cascade? The problem is that the transmitter works, but very weakly, the range is 1-2 meters, then there is interference. Frequency adjustment is excellent. I use a smartphone with headphones as a receiver.
Because the source is a linear output, I threw out the 2k resistor, replaced the 5 uF capacitor with 0.22 uF ceramics, replaced the 100k resistor with 75k, and from it 100k to ground.
Instead of 120pf capacitors I installed 100pf.
An important point: all capacitors are permanent. I adjust the frequency by screwing the core into the plastic frame L1.
I installed the transistors I found with a frequency of more than 100 MHz: 1st stage - 2SC1740, 2nd stage - 2SD667. Antenna - 30cm piece of wire. Power supply - 12V battery.
Observations are as follows: the total consumption of the circuit turned out to be 7-8 mA, which seems to be not enough. If you touch the antenna with your hand, the generation stops, and I don’t understand this, because the antenna is connected to the second stage, but it doesn’t seem to show signs of life. The resistor in the second stage is variable up to 1 MΩ, rotating it does nothing. The transistor in it is cold. Before soldering it was 100% working with hfe 130.
Something like that. Since the first cascade, if you don’t touch it with your hands, generates stably, then, I think, you need to dig in the direction of the second. What advice would you give? Why was the range of 1-2m even for the first stage so short? Is it because the antenna is connected to the second?
It's a shame, but I don't understand how the second cascade works. What affects the capacitance of the substring capacitor in it? So I’m almost a complete 0 in these _radio_ matters.
#28 root June 17 2015
Both parts of the L3 coil are located on the same axis, you did everything correctly.
Before you start setting up the second stage, turn it off completely and set up the first stage with the generator so that the signal from it is transmitted over several tens of meters.
Connecting to the line output, as you wrote, may cause interference and loss of radiated power. You need to achieve stable operation of the generator by selecting the resistors that you connected to the base.
You can try to assemble the first stage according to this diagram and connect the second stage to it to increase the RF power.
Also, to improve the situation, you can try to assemble an additional low-frequency stage on a transistor, and connect the signal source to it.
Screwing the core into the L1 frame is not a good idea; try to get a tuning capacitor somewhere and check the operation with tuning through it.
When powered by 12V, try increasing the resistance of the resistor in the generator power circuit (380 Ohms).
Check the transistor in the second stage - it may have already burned out, for experiments you can solder a new one and connect a resistor with a resistance of approximately 200-300 Ohms into the emitter gap. When the second stage starts working, you can select the most suitable resistance.
#29 NULL June 17 2015
Thanks for your comments.
Yes, I’m kind of confused, you’re right about the separation of the first cascade - I’ll start with that. Quite a long time ago I assembled a similar 1-transistor transmitter, as per your link, it worked within the apartment and I used it, but when I took it to a private house, it turned out that the power was insufficient: on the site, outside the walls of the house, the signal was already with interference. Recently I needed a transmitter again and I decided to try this 2-3 transistor circuit.
As soon as I have time, I’ll try to experiment: I’ll unscrew the core, solder in a loop capacitor of a larger capacity (without the core the frequency is higher than 108 MHz). I forgot to write that instead of 300 and 380 ohm resistors, I used 330 ohm. In the emitter, I think it’s not critical, but I’ll try to increase it in terms of power supply. Well, I’ll play with high-resistance ones.
By the way, what is the function of the 120 pf capacitor that is connected to the base of the first transistor? Is it needed in the version with a linear output as a signal source?
#30 Andrey August 23 2015
I assembled the transmitter with only a generator. The power is pleasing - >=30m taking into account the walls. But harmonics were noticed (even at the stated range). I was looking for the true frequency for noise immunity and power. I found about three such frequencies (I searched at a distance) in the range of 64-108 MHz (the most stable and possibly the true one was below the frequency stated in the description). I tried to rotate the capacitors and resistor, put the generator in a box with metal soldered to the negative (screen) and without. The harmonics remain. There are no parts nearby near the coil except for the interline capacitor. The power supply is a 10V battery (with mains power, although with a simple stabilizer, the background is strong), although with the battery you can hear a little background when the power cord is nearby. The input capacitor is 0.33 microns mica. The 2k resistor was removed (as a linear input). Mounting on a board with cut tracks (the gap between them is about 0.5mm. What are your recommendations?
#31 novel November 14 2015
good schematic, can anyone send me the board and parts?
#32 andr March 01 2016
I soldered the transmitter on the breadboard on the first two stages of this circuit.
More precisely, the circuit of the first stage (oscillator) is taken for the linear input option, and not for the microphone. Almost all the denominations of the elements are slightly different. But that's not the point.
In the first stage there are 2n3904. First I set it up. The best we managed to achieve was reliable reception through 1-2 walls. Current consumption 8 mA.
Next, I installed and configured the second stage, the KT603B transistor. Reliable reception was established throughout the entire apartment (through 4 walls).
And now the question. The circuit consumption was immediately 150mA (with a 90kOhm resistor in the base), powered by a 12V battery. This is 1.8W of power. I perfectly understand what 1.8 watts of power is and I understand that the KT603 should boil and die. But this doesn't happen. His temperature is about 40C. Question: is it really that most of the power goes into radiation? It turns out that the output power of my transmitter is around 1-1.5W? Somehow unexpectedly a lot for such a simple scheme.
I did not check the range, because... required only within the apartment.
And also another question: how to choose the optimal antenna length? I tried different ones from 15 cm to 1 m and noticed that the length slightly affects the heating of the transistor.
#33 root March 01 2016
For convenient setup, you can assemble a wave meter circuit. Bring the wave meter antenna close to the radio transmitter antenna at a short distance and adjust the P-circuit of the transmitter or the matching device for the antenna, achieving maximum values in the wave meter readings.
In the diagram (Fig. 1), we adjust the matching with the antenna using a capacitor that is connected to the coils L7, L8, as well as by changing the distance between the turns of these coils.
The transmitter cannot be turned on without a load (antenna or its equivalent) - the output transistor may burn out.
In your case, the current consumption is quite acceptable; just in case, you can install a small radiator on the transistor. The power consumed by the circuit is not equal to the power radiated into the antenna; this is facilitated by heating losses, transistor operating mode, antenna type, etc.
#34 andr March 01 2016
Thanks for the answer! Is KD522 suitable instead of KD510? Or is it better to immediately look for 1n4148?
About power - well, I figured that if the total consumption is 1.8 W, and the only powerful element heats up weakly, then most of it (1-1.5 W) goes into radiation, because there is nothing left to bask there, but we have to go somewhere. By the way, the body of the KT603 is similar to the old MPsheks, so you can only solder the radiator to it.
Another question. In most cases, it is recommended to use a piece of coaxial wire as an antenna. Why? I use pieces of simple wires - why are they worse?
#35 POPS March 07 2016
Tell me, how critical is the capacitance of the separating capacitor in the base of the second transistor, which is 120 pf in the circuit, what causes it?
If you use 1nf or even 10nf film, will the sound be better? it's kind of wooden
#36 Alexey January 06 2017
Can the microphone be replaced with a km 70??????, or a Chinese polar one?
#37 root January 06 2017
You can use any electret or condenser microphone (with a built-in transistor amplifier). The Chinese polar one from a tape recorder is an electret microphone.
#38 Alexander Compromister October 09 2017
I came up with an idea for the first scheme: to combine transistors VT1 and VT2 into one transistor assembly 1HT591. And additionally hang a powerful cascade on the same KT610, so that the butt does not crack across from the strain.
#39 Alexander Compromister October 09 2017
Re: #25 Andrey March 10, 2015 Try to make a diagram [Shustov M.A. Practical circuit design: 450 useful circuits for radio amateurs: Book 1. Altex-A: Moscow, 2001. - P.125. Figure 13.11], or [ibid. - P.128. Figure 13.16] for video broadcast. More details: [f. Radio. 10/96-19] and [f. Radio amateur. 3/99-8], respectively.
#40 Danila January 17 2019
Hello, I apologize for such a stupid question. What can replace KT610? Can I install the KT9180, will it be more powerful?
#41 root January 17, 2019
Danila, this question has already been asked in the comments. The KT9180 has a cutoff frequency of the current transfer coefficient of approximately 100 MHz; it is not suitable for use in this circuit.
#42 Danila February 05, 2019
Thank you very much, I did not look at the frequency of the kt9180 and did not expect to receive an answer at all. But I have a few more questions:
1. What to do with the earth, I used to think that the earth = -, but after Googling, I realized that this is not so. I read somewhere in the comments that the ground needs to be connected to the housing for screening. I'm completely confused what's what.
2. the same question about KT610, can it be replaced with BFG135? This is a microwave n-n-n SMD. If so, will it be necessary to mount it on the radiator?
3. in the comments you advised that in order to use the audio input, assemble 1 cascade according to this circuit, and then I had a question - how to connect it to this circuit? Thank you very much for your concern and attention.
#43 root February 06 2019
It is better to install this circuit immediately, taking into account complete shielding and separation of its parts by shielding partitions. You can assemble the circuit on the “patch” according to the method of S. Zhutyaev; descriptions and examples with photos are in the articles and comments to them:
With this installation, all connections are made on patches and mounted. The remaining foil lining, isolated from the patches, is connected to the minus of the circuit, it serves as a screen and the leads of the components that should go to the minus, as well as the partitions between the cascades, are connected to it. This foil surface of fiberglass and the screen will be the ground of the circuit.
Installation of the transmitter with cascades shielded by partitions:
As for the BFG135 - a high-frequency SMD transistor (up to 7000 MHz) with a collector current of 150 mA. You can try using it in the output stage, but it needs a heatsink.
The transistor lining is a collector, and in the diagram the emitter goes to the minus, for this reason it will not be possible to solder it to the fiberglass foil. But you can cut out a separate pad under the collector on the board and solder the transistor pad there - heat will be transferred through it to the printed circuit board.
To use the generator circuit from another article, it is enough to connect coil L2 to coil L1, which is connected to the RF power amplification stages:
What to do if you have a personal computer, laptop or tablet, but don’t have the necessary external speaker systems? You don't have to run to the store at all. You can see what you have at home.
If you have a radio, radio, music center, or any other audio equipment with a built-in FM receiver, you don’t have to spend money on purchasing acoustics. The signal from the sound card output can be sent to any of these devices. Moreover, there is no need to redo the audio equipment itself, and there is no need to lay extra wires either.
The whole secret is to transmit the audio signal taken from the output of the sound card via radio frequency in the FM range 88-108 MHz. Then the signal can be received by any FM receiver, be it stand-alone or as part of any audio equipment.
For this you need a special transmitter, or as they say now, a transmitter. The signal to its modulating input can be supplied from the audio output of the computer, and it can be powered with a voltage of 5V from the USB port, which is found in any computer, laptop or tablet.
The transmitter itself is made on transistor VT1. This is an RF signal generator at a frequency in the range of 88-108 MHz. The specific tuning frequency depends on the oscillating circuit consisting of coil L2 and capacitor C4. This circuit is included in the collector circuit of the transistor.
At high frequencies, the transistor operates according to a common base circuit. In order for it to generate, a capacitor C5 is connected between the collector and the emitter, through which the positive feedback necessary to excite the cascade and generate generation occurs.
Fig.1. Schematic diagram of a simple VHF-FM radio transmitter based on the KT3102 transistor.
Through capacitor C6, the signal from the collector of the transistor enters antenna W1. A piece of mounting wire of any length will do just fine. It will act as an antenna and emit an RF signal that can be received by any FM broadcast receiver.
To transmit an audio signal, they need to frequency modulate the RF signal emitted by the transmitter. To do this, the audio signal is supplied to the input of this transmitter via a cable with an X2 connector. This is a standard plug cable for connecting to the audio output of your computer.
It has two shielded audio wires. The wires of the stereo channels are separated and connected to resistors R1 and R2 forming the mixer. And the braids are connected together and soldered to point “G”, that is, to the common negative. Resistors R1 and R2 convert the stereo signal to monophonic. Yes, this is a minus of the circuit that the sound will be monophonic.
Next, the audio signal is sent to variable resistor R3, which can be used to adjust the modulation depth and thus achieve the best sound. The signal arrives at the base of transistor VT1 and changes its operating point within small limits, which leads to a change in the capacitances of the transistor junctions, and this leads to a change in the frequency of the output RF signal.
This is how frequency modulation occurs. With this method, of course, amplitude modulation is also present, but it is effectively suppressed by the frequency detector of the radio receiver receiving the signal.
The transmitter is powered by 5V voltage from the USB port of the device, from which it receives an audio signal. To do this, the supply voltage is supplied to it via a cable with connector X1. The type of X1 connector (USB, miniUSB, microUSB) depends on which connector is on the device with which this transmitter will work.
Choke L1 with capacitors C1 and C2 suppresses noise that can penetrate the power circuit.
By the way, it is not at all necessary that the transmitter be powered by the device that serves as the source of the audio signal. The 5V voltage can also be taken from another source, for example, from a charger and power supply for a cell phone; now such devices have a standard USB connector.
Now about the details. Transistor KT3102, in a plastic case, with any letter index. The printed circuit board is made just for it, for its pin layout.
Variable resistor R3 type SPZ-4. This variable resistor has leads made in the form of loops. You need to solder three hard copper wires to them and install a resistor on them on the board.
It will rise above the board and its shaft will be parallel to the board. The shaft will point towards the places where the wires from the X2 connector cable are soldered. The nominal resistance of resistor R3 does not have to be exactly 10K, it can be anything from 10K to 30K. By the way, this also applies to all other resistors; the circuit will work quite well even if their resistances differ by up to 30% from those indicated on the circuit. But it should be noted that resistors R1 and R2 must be the same.
When purchasing capacitors, it is important not to get confused in the designations. C4, C5 and C6 capacitances must be in picofarads. If you take the same numbers by mistake, but in nanofarads the circuit will not work.
To wind inductor L1, you can take any ferrite ring with an outer diameter of 6 to 10 mm. You need to take a PEV or PEL winding wire with a diameter of 0.1-0.2 mm and fold it in half. Then fold it like this and wind 20-30 turns.
After this, separate the ends and find the ends using an ohmmeter or a continuity tester, for example, a multimeter in continuity mode. It is necessary to solder the ends of the windings of inductor L1 with special care, since an error here can lead to both the failure of transistor VT1 due to the incorrect polarity of the voltage applied to it, and damage to the USB port if, for example, one winding is connected in parallel with C1.
In general, when you turn it on for the first time, it is advisable, and even mandatory, to supply the power supply voltage to the X1 not from the computer’s USB port, but from a similar connector on a cell phone charger or USB hub. Because damage to a computer's USB port is a very big nuisance. Only after you are sure that everything is working correctly can you connect it to the USB port of your computer.
Coil L1 is wound with thick PEV or PEL winding wire with a diameter of 0.4 to 1.0 mm. Only 8 turns. The L1 coil is frameless. First you need to take some round object with a diameter of 5-6 mm, for example, the shank of a drill of the corresponding diameter.
Then wind 8 turns on it. Then separate the leads and remove the resulting “spring” from this round object. The reel is ready.
The installation is carried out on a printed circuit board. The board layout is shown in Figure 2 in full size (view from the side of the printed tracks).
Rice. 2. Printed circuit board for a radio transmitter based on the KT3102 transistor.
Figure 3 shows the wiring diagram (view from the parts side). Transistor VT1 is shown as it will look from above when installed on the board. Throttle L1 is installed vertically and glued with a drop of BF-4 glue.
Rice. 3. Layout of components on the transmitter board and its connections.
For a cable with connector X1, you need to buy any USB cable with the appropriate plug (USB, miniUSB, microUSB) and simply cut off the second connector. Then cut the wire. There will be four wires, you need black and red.
ATTENTIVELY! If these wires are mixed up, you can damage the transistor VT1 or the device to which X1 will be connected.
For X2 you need to buy any cable with a plug that matches the audio output of your device. Cut off the connector (or connectors) at the other end. There will be two shielded wires. They are usually red and white.
They come in yellow and red or the same. The braids need to be twisted together and soldered to point “G”. And two wires to resistors R1 and R2, in what order it does not matter.
First of all, you need to check everything thoroughly. Then connect and put R3 in the middle position. Try to catch the signal with the receiver. If you can’t catch it, move the coils L2 a little (compress, spread). As soon as the signal is received, you need, again, by compressing or stretching L2, to move it along the receiver scale to a place free from broadcast stations.
Use resistor R3 and the volume control of the audio source to achieve the best sound quality.
