Components list:
R1 = 100K Ohms
R2 = 10 Ohms
R3 = 47K Ohms
R4 = 220 Ohms
C1 = 4.7uF/16V
C2, C5 = 1nF
C3 = 10 – 40 pF trimmer capacitor
C4 = 4.7pF

Q1 = 2N3906 PNP transistor
Q2, Q3, Q4 = 2N3904 NPN transistor
L1 = 4-5 turns of 22 ga. magnet wire close wound around 1/8″ non-conductive core
Antenna = 10 to 12″ hookup wire

With wide voltage supply (3-9v), this FM tracking transmitter can used 3V or 9V battery to operate.

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The coil L1 was consist of 7 turns on a quarter inch plastic former with a tuning slug. The tuning slug is adjusted to tune the transmitter. Actual range on this prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work.

Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.

One final point, don’t hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations.

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Components List:

The MK484 we use is a Japanese copy of the original ZN414. It contains an RF amplifier, active detector and automatic gain control (AGC to improve sensitivity) all in a 3-pin package. The input impedence is typically 4M ohm. It operates over a range of 150kHz to 3MHz. DC supply of 1.1V to 1.8V & 0.3mA current drain makes it ideal for battery operation. The output is typically 40 – 60 mV of audio signal. Optimal AGC is provided by R3 and C2 (see Figure 2). R3 (the AGC resistor) should be in the range 100R to 1.5K. A bandwidth of about 4kHz is achieved.

Download the MK484 datasheet:
[wpdm_file id=52]

The MK484 is now a discontinued IC. It was made by New Japan Radio although it does not appear anywhere on their website(s). You may try to find the equivalent IC for MK484.

Download the complete explanation of this AM radio receiver circuit:
[wpdm_file id=53]

Note: The kit for this circuit is available at kitsrus.com.

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This circuit uses a combination of transistors SA101 and SA103, as the modulation signal amplifier.

Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz and in North America an extended MW broadcast band goes from 535 kHz to 1705 kHz.

Wavelengths in this band are long enough that radio waves are not blocked by buildings and hills and can propagate beyond the horizon following the curvature of the Earth; this is called the groundwave. Practical groundwave reception typically extends to 200-300 miles, with longer distances over terrain with higher ground conductivity, and greatest distances over salt water. Most broadcast stations use groundwave to cover their listening area.

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This is the FM transmitter circuit which apply 4 radio frequency stages, that are a VHF oscillator designed around transistor BF494 (T1), a preamplifier designed around transistor BF200 (T2), a driver designed around transistor 2N2219 (T3) and also a power amplifier designed around transistor 2N3866 (T4). A condenser microphone is wired at the input of the oscillator.

The Operational of this FM transmitter circuit is very simple. At the time you speak close to the microphone, frequency-modulated signals are produced at the collector of oscillator transistor T1. The FM signals are amplified by the VHF preamplifier and also the pre-driver stage. You’ll be able to also use transistor 2N5109 as a placement of 2N2219.

The preamplifier works as a tuned class-A RF amplifier while the driver works as a class-C amplifier. Signals are finally fed towards the class-C RF power amplifier, which delivers RF power to a 50-ohm horizontal dipole or ground plane antenna.

Regulator IC 78C09 gives you stable 9V supply to the oscillator, so variation in the supply voltage is not going to have an impact on the frequency produced. You are able to also use a 12V battery to power the circuit.

Use a heat-sink with transistor 2N3866 for heat dissipation. Alter the trimmer VC1 connected across L1 to produce frequency within 88-108 MHz with care. Also alter trimmers VC2 through VC7 to obtain maximum output at maximum assortment.

Assemble the circuit on a generalpurpose PCB. Set up the antenna correctly for optimum range. Coils L1 through L5 are built with 20 SWG copper-enamelled wire wound over air-cores that have 8mm diameter. They will have 4, 6, 6, 5 and 7 turns of wire, respectively.

This 4 stage FM transmitter tested by Electronics For You mag and should be work. You can download this circuit in PDF version from the following link:

Four Stage FM Transmitter Circuit Project

1 file(s) 241.67 KB

Download

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Figure A

Figure A is the schematic of the microstrip single stage RF amplifier. The amplifier is based on the M/A-Com LF2810A MOSFET. The transistor is actually a 10 watt, 28 volt part, but provides adequate gain for this application at 12 VDC. The amplifier provides greater than 40% efficiency at the desired output power. Trimmer capacitors are used for input and output matching. Output power is adjusted by a trimpot which sets the gate bias voltage.

Figure B

Figure B is the layout for the microstrip circuit board. The board material is 0.030 inch Duroid. The circuit board is compression soldered onto a similarly sized copper heat spreader. The board and spreader are milled and drilled to accept the flange-mount transistor.

About LF2810A:

LF2810A is a RF Power MOSFET Transistor 10W, 500-1000MHz, 28V.

LF2810A Features:

• N-Channel enhancement mode device
• DMOS structure
• Lower capacitances for broadband operation
• Common source configuration
• Lower noise floor
• Applications – Broadband linear operation 500 MHz to 1200 MHz

Download LF2810A RF Power MOSFET Datasheet:

LF2810A Datasheet

1 file(s) 136.42 KB

Download Datasheet

source: http://www.ham-radio.com/wb6zsu/components/exciter/exciter_mods.html

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The circuit is composed of a mic amplifier circuit, a variable frequency oscillator, and modulation amplifier stages. Transistor T1 (BF195) is put to use as a simple RF oscillator. Resistors R6 and R7 determine base bias, while resistor R9 is utilized for stability. Feedback is provided by 150pF capacitor C11 to maintain oscillations. The primary of shortwave oscillator coil and variable condenser VC1 (365pF, 1/2J gang) form the frequency determining network.

By altering the coil inductance or the capacitance of gang condenser, the frequency of oscillation can be modified. The carrier RF signal from the oscillator is inductively coupled through the secondary of transformer X1 to the next RF amplifier-cum-modulation stage assembled around transistor T2 that is run in class “A” mode. Audio signal from the audio amplifier assembled around IC BEL1895 is coupled to the emitter of transistor 2N2222 (T2) for RF modulation.

IC BEL1895 is a monolithic audio power amplifier intendeded for sensitive AM radio applications. It can deliver 1W power to 4 ohms at 9V power supply, with low distortion and noise characteristics. Since the amplifier’s voltage gain is of the order of 600, the signal from condenser mic can be straightly linked to its input without any amplification.

The SW transmitter’s stability is managed by the quality of the tuned circuit parts as well as the degree of regulation of the supply voltage. A 9V regulated power source is required. RF output to the aerial consists of harmonics, because transistor T2 doesn’t have tuned coil in its collector circuit. However, for short-range communication, it does not create any trouble. The harmonic content of the output may be lowered by means of a high-Q L-C filter or resonant L-C traps tuned to each of the prominent harmonics. The power output of this transmitter is about 100 milliwatts.

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Here the short wave AM Transmitter circuit design diagram. The circuit is quite simple and easy to build since it applies only a few electronic components. The primary feature of this transmitter is that it really is absolutely free from the LC (inductor, capacitor) tuned circuit and runs using a fixed frequency of 12 MHz that is very stable. An LC based tuned circuit is inherently unstable because of drift of resonant frequency due to temperature and humidity variations.

Resistors R1 and R2 are utilized for DC biasing of transistor T1. The capacitor C1 gives coupling in between the condenser microphone and the base of transistor T1. In the same way, resistors R3, R4 and R5 give DC biasing to transistor T2.

The oscillator segment is a combination of transistor T2, crystal XTAL, capacitor C2, C3 and resistors R3, R4 and R5. The crystal is excited by a portion of energy from the collector of transistor T2 via the feedback capacitor C2. The crystal vibrates at its essential frequency and the oscillations happening because of the crystal are placed to the base of transistor T2 across resistor R4. Using this method, continuous undamped oscillations are acquired. Any crystal having the frequency in short wave range could be substituted in this circuit, even though the operation was tried using a 12 MHz crystal.

The Transistor T1 has 3 capabilities:

1. The transistor features the DC path for extending +VCC source to transistor T2.
2. It amplifies the audio signals which is generated by condenser mic.
3. It injects the audio signal into the high frequency carrier signal for modulation.

The condenser microphone transforms the voice message into the electrical signal that is amplified by transistor T1. This amplified audio signal modulates the carrier frequency produced by transistor T2. The amplitude modulated output is acquired at the collector of transistor T2 and is transmitted by a loop antenna into space in the form of electromagnetic waves. The antenna could be tuned to a specific frequency by fine-tuning the trimmer C5 and also by modifying the length of ferrite rod into the coil.

The transmitted signals could be received on any short wave receiver without having distortion and noise. The range of this transmitter is 25 to 30 metres and may be expanded even more in case the length of the antenna wire is suitably extended together with good matching.

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This the Good Quality FM transmitter for your stereo or any other amplifier gives you a pretty good signal strength up to a range of 500 metres having a power output of about 200 mW. This circuit can be operated with a 9V battery.

The audio-frequency modulation stage is constructed close to transistor BF494 (T1), that is wired as a VHF oscillator and modulates the audio signal present at the base. Working with preset VR1, you’ll be able to alter the audio signal level.

The VHF frequency is decided by coil L1 and variable capacitor VC1. Decrease the value of VR2 to obtain a higher power output.

The next stage is designed close to transistor BC548 (T2), which serves to be a Class-A power amplifier. This stage is inductively coupled towards the audio-frequency modulation stage. The antenna matching network contains variable capacitor VC2 and capacitor C9. Fine-tune VC2 for the optimum transmission of power or signal strength at the receiver.

For frequency stability, use a regulated DC power supply and house the transmitter inside of a metallic cabinet. For better antenna gain, utilize a telescopic antenna in place of the simple wire. Coils L1 and L2 are to be wound over the same air core such that windings for coil L2 start from the end point for coil L1.

Here the coil winding specification:

• L1: 5 turns of 24 SWG wire closely wound over a 5mm diameter air core
• L2: 2 turns of 24 SWG wire closely wound over the 5mm diameter air core
• L3: 7 turns of 24 SWG wire closely wound over a 4mm diameter air core
• L4: 5 turns of 28 SWG wire on an intermediate-frequency transmitter (IFT) ferrite core

This good quality 500M FM transmitter circuit absolutely works fine since it already tested.

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Above diagram is a very easy and efficient receiver for actuating garage doors, starter motors, alarms, warning systems and many some other possibilities. The SCR, which has a extremely low trigger current of 30 uA is typical — it demands an input electricity of just 30 uW to activate the relay. A high Q tuned antenna circuit assures rejection of spurious signals. A whip or wire antenna is adequate as much as 100 feet from a low power transistor transmitter. A momentary-off switch resets the circuit.

The circuit specifies a whip or wire antenna which just means a solid piece of wire 6-12 inches lengthy (15-30cm). The antenna coil is experimental, however you are able to begin with ten to 12 turns of #22 (0.7mm) magnet wire, and 5/16″ (8mm) coil diameter. Antenna wire is soldered at 1/2 turn of the coil and the gate of the BRY35 is soldered about halfway the coil. This circuit will transmit as much as 100-feet with the above specifications @ 30uA.

The relay coil is specify’s as much as 200 ohm but that’s only the one I had in stock. Any low-ohm relay, even at 9V or so, should work. And a remaining note, do not count on too much with this circuit. The “Simplest R/C Circuit” is just that; Easy!

The BRY35 is really an classic semiconductor made by Philips. A good alternative could be the EC103D1, also produced by Philips (see pin-out picture for this device). NTE Semiconductors gives an alternative of NTE5405, but I’ve not attempted it. I believe, a NTE5400 (30V) or NTE5401 (60V) will work just also. The NTE5405 is a bit overkill at 400V.

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