The Transmitter Palace : The Place of RF Transmitter

Wireless FM Transmitter

This wireless FM transmitter is designed to use an input from another sound source and transmits on the commercial FM band. This low power fm radio transmitter it is actually quite powerful. The first stage is the oscillator, and is tuned with the variable capacitor. Select an unused frequency, and carefully adjust C3 until the background noise stops (you have to disable the FM receiver’s mute circuit to hear this).

When assembling the wireless transmitter circuit, make sure the rotor of C3 is connected to the +9V supply. This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft. You can use a small piece of non copper-clad circuit board to make a screwdriver - this will not alter the frequency.

Q1 is a conventional Colpitts oscillator design. The audio signal applied to the base of Q1 causes the frequency to change, as the transistor’s collector current is modulated by the audio. This provides the frequency modulation (FM) that can be received on any standard FM band receiver.

The inductors are 9.5 turns of 1mm diameter enamelled copper wire. They are close wound on a 3mm diameter former, which is removed after the coils are wound. The output is a low power of 100 mW, but for some of you this fm transmitter can delivers the desired power for broadcasting on your street or with a proper antenna you can cover a small neighborhood. If you need a power transmitter use the above menu, you can find transmitters starting with low fm power up to high power fm transmitters.

Source: Low power fm transmitter

Check this out Voice Transmitter

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Power Supply with Over Voltage Protection 10A 13.8V

This power supply circuit will give 10 amps (12 amps surge) with performance that equals or exceeds any commercial unit. The circuit even has a current limiting feature which is a more reliable system than most commercial units have.

Just like other commercial units, this circuit uses the LM723 IC which gives us excellent voltage regulation. The circuit uses 3 pass transistors which must be heat sinked. Resistor R9 allows the fine tuning of the voltage to exactly 13.8 volts and the resistor network formed by resistors R4 through R7 controls the current limiting. The LM723 limits the current when the voltage drop across R5 approaches .7 volts. To reduce costs, most commercial units rely on the HFE of the pass transistors to determine the current limiting. The fault in that system is that the HFE of the pass transistors actually increases when the transistors heat up and risks a thermal runaway condition causing a possible failure of the pass transistors. Because this circuit samples the collector current of the pass transistors, thermal runaway is not a problem in this circuit making it a much more reliable power supply.

The only adjustment required is setting R9 to the desired output voltage of anywhere between 10 and 14 volts. You may use a front panel mounted 1K potentiometer for this purpose if desired. Resistor R1 only enhances temperature stability and can be eliminated if desired by connecting pins 5 and 6 of IC-1 together. Although it really isn't needed due to the type of current limiting circuit used, over voltage protection can be added to the circuit by connecting the circuit of Figure 2 to Vout. The only way over voltage could occur is if transistors Q2 or Q3 were to fail with a collector to emitter short. Although collector to emitter shorts do happen, it is more much more likely that the transistors will open up when they fail. I actually tested this and purposely destroyed several 2N3055's by shorting the emitters to ground. In all cases the transistors opened up and no collector to emitter short occurred in any transistor. In any event, the optional circuit in Figure 2 will give you that extra peace of mind when a very expensive radio is used with the power supply.

The circuit in Figure 2 senses when the voltage exceeds 15 volts and causes the zener diode to conduct. When the zener diode conducts, the gate of the SCR is turned on and causes the SCR to short which blows the 15 amp fuse and shuts off the output voltage. A 2N6399 was used for the SCR in the prototype but any suitable SCR can be used. While over voltage protection is a good idea, it should not be considered a substitute for large heat sinks. I personally feel the best protection from over voltage is the use of large heat sinks and a reliable current limiting circuit. Be sure to use large heat sinks along with heat sink grease for the 2N3055 transistors.

You will used this power supply on all kinds of transceivers from HF, VHF to UHF with excellent results and absolutely no hum. This power supply will be a welcome addition to your shack and will greatly enhance your knowledge of power supplies. Check out 2 Transistors Voice Transmitter

Check this out Build A 10 Amp 13.8 Volt Power Supply

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900 MHz Directional Antenna

This is the Japanese Yagi antenna that said Yagi antenna. It is a simple system of high-gain directional antenna, Yagi antenna classical structure, the following chart.

Figure in a straight line oscillator Φ10mm with aluminum tubes, or oscillator used as a cross-section of 10 × 3mm aluminum production. One reflector 200mm, lead-130mm, equivalent oscillator 150mm.

The 800-900MHz antenna in the use of the best, the ceiling can be extended to mobile phone spectrum, the effect can be, 970MHz gain later sharply reduced.Lower limit to be 780MHz, 750MHz lower following a sharp gain.

As the number of lead-based on the actual situation in the selection, is better, but too many gains after raising small antenna with a total length has increased a lot. It has been measured, 7 lead when the antenna gain of about 10dB, when the figure of about 15dB. See 4W FM Transmitter Circuit

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Simple VHF SWR Protection Circuit

Here is a simple SWR protection circuit you can easily build. The directional coupler and detector components are from an old VHF SWR meter.You may want to replace the meter's existing RF bypass Capacitors with higher quality (mica) ones.

You may also want to replace the line coupler's termination Resistors which higher wattage ones. Be sure to use the exact same value, or the SWR meter will be inaccurate. Also, be sure to use non-inductive Resistors (carbon-film or metal-oxide will work).

This then feeds to all LM3914 bargraph display to light the LEDs, indicating the current SWR ratio. SWR When the ratio reaches approximately 3, it will engage the relay, cutting off the RF input to the power amplifier . You could also just a LED light or sound an alarm if you want to.

You should compare the circuit schematic above to this one below :

Visit this links for more, 4W FM Transmitter Directional Coupler with RF Filter for 88-108 MHz - 100w LCD PIC16F88 SWR Meter for VHF- UHF and click this link for building: The Windows ® program for drawing analog meter scales.

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Making 1 to 4 Balun for Matching Impedance

Here's the way to match 75 Ohm to 300 Ohm impedance. In case of making balun for 200-300 Ohm folded dipole antenna to 75 Ohm transmission cable.

Please see table on image above to determine velocity factor for different cable type.

Please check out my 4 Watt FM transmitter!

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40W Broadband FM RF Power Amplifier

Here's broadband RF power amplifier for FM amplifier design is based on Motorola MRF171A MOSFET. The amplifier was constructed in a small aluminium diecast box. RF input from transmitter and output connections are made by coaxial sockets. The power supply is routed through a ceramic feedthrough capacitor bolted in the wall of the box.

This constructional techniques results in excellent shielding, preventing RF radiation escaping from the amplifier. Without it, significant amounts of RF radiation could be radiated, interfering with other sensitive circuits such as VCOs and audio stages, also significant amounts of harmonic radiation could occur.

Any RF power amplifier must be followed by a low pass filter (LPF) to reduce the harmonics to an acceptable level. What this level is in a unlicensed application is a moot point, but as the output power is increased, more attention must be be paid to the harmonic suppression.

In this RF power amplifier design used a 7 pole Chebyshev low pass filter. A Chebyshev was chosen as the phase and amplitude ripple within the passband was not critical, and the Chebyshev gives a better stop band attenuation than compared to say, a Butterworth. The design stopband was chosen to 113MHz, giving a 5MHz implementation margin from the highest desired passband frequency at 108MHz and the start of the stopband at 113MHz.

The next critical design parameter was the passband ripple. For a single frequency design it is normal practice to choose a large passband ripple, for example 1dB, and tune the peak of the last passband maxima to the desired output frequency. This gives the best stopband attenuation because greater passband ripple results in more rapid stopband attenuation. A seven pole filter has 7 reactive elements, in this design four capacitors and three inductors. The more poles, the better the stopband attenuation, at the expense of increased complexity and more passband insertion loss. An odd number of poles is required as both the input and output impedance was designed to be 50R.

40W Broadband FM RF Power Amplifier

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250W Power Amplifier with the BLF548 for 100 - 450 MHz

The BLF548 is a balanced N-channel enhancement mode vertical D-MOS transistor in a SOT262 package, especially designed for use in wideband amplifiers up to 500 MHz. The transistor is capable to deliver 150 W nominal output power at a supply voltage of 28 Volts. Due to the low output capacitance the attainable bandwidth will exceed 300 MHz.

A balanced transistor was chosen in order to reduce the second harmonic from transmitter (due to the push-pull effect) and to reduce the number of required components. The criteria for chosen MOSFETs over bipolar transistors are; high power gain, high load mismatch capabilities, low noise and easy biasing.

Schematic Circuit