RF Power Amplifier 80W 2SC2782

This is a conventional FM RF power amplifier design, using bipolar transistors in a tuned class C circuit. Thanks to the use of two stages, the amplifier can be driven to full power with less than 1 watt driving power, so that a large gain margin results in this FM transmitter.

 

Bipolar VHF power transistors have a severe affinity for low frequency self- oscillation. To obtain stability in this amplifier, I employed several techniques, such as placing the resonances of base and collector chokes far apart, damping the chokes with resistors, using RC combinations for absorption of unwanted frequencies, using feedtrough capacitors for bypassing on the board, etc. It took some tweaking, but the amplifier ended up unconditionally stable.

The impedance matching network between the two transistors calls for such a low inductance, that it would be impractical to make it with actual wire. So I used a micro stripline etched on the PCB. Also, the power and SWR sensor at the output was made with micro striplines.

This amplifier has a low pass filter at the output, resulting in a signal clean enough to be directly connected to an antenna. The SWR meter was placed before the filter, in order to clean out the harmonics produced by its diodes. In any case, while the signal is clean enough to easily satisfy usual legal and technical requirements, this transmitter should not be used at a multi-transmitter site without further narrowband filtering! This is so because any other strong signals on nearby frequencies would be picked up by the antenna and coupled to the power transistor, which would mix it up with the own signal, creating a wide array of intermodulation products, some of which would be re-radiated! This is a common and very big problem in many multitransmitter sites. In such places, NOT EVEN ONE FM transmitter should be allowed on the air without narrowband filtering! Such filtering is easily accomplished by means of a single tuned cavity, which can be constructed from copper tubing or sheet.


RF Power Amplifier 80W parts overlay without parts identification!

Here is the PCB layout, including the microstrips. The board is 20cm long and is double-sided, with the backside being a continuous groundplane except for two small pads at the driver transistor base and collector. I cut out these pads with a knife, rather than making a whole computer drawing for that!

 

Source: 80 Watt FM stereo broadcast transmitter

See more: RF TV Modulator - Audio Video RF Modulator - TV Modulator

Read more ...

High Fidelity FM Stereo Modulator

This circuit is a stereo encoder for FM transmitter. It is used for generating a high quality FM stereo multiplex signal that is suitable for driving mono FM transmitters. This version features improvements to the audio mixer stage, better gain adjustments in the audio filter stage and a rework of the multiplexer stage. When it is adjusted properly, the Rev E design produces an FM stereo signal with excellent fidelity.

In addition, It is a good idea to have a stereo compressor between the signal source and the transmitter, when properly set, it can save the operator from having to "ride" the volume level between the point of good modulation and the point of distortion. This circuit does not perform pre-emphasis (high frequency boosting), a graphic equalizer in front of the compressor can be used if desired.

This is a fairly high-level project, it should only be attempted by someone with a fair amount of electronics experience. A decent oscilloscope and an audio signal generator are necessary for aligning this circuit.

It is the operator's responsibility to run the transmitter in accordance with the frequency regulating authority of their country, it may be necessary to use an output attenuator on some fm ransmitters.

High Fidelity FM Stereo Modulator authorized by Forrest Cook

See more : 2.4 GHz Power Amplifier - RF Power Meter -  UHF Power Amplifier

Read more ...

30W VHF FM Amplifier BLF245

This FM amplifier has been designed to take place on a heatsink microprocessor PC equipped with its fans, the advantage of this method of cooling has been selected for the fact that it is not very common and expensive. The Power out of the VHF FM amplifier can be achieved by 30 Watt.

The size of the printed circuit of the amplifier will adapt quite easily to the type of heatsink as you have available, if possible, because in many cases, those of recovery, the fans have already lived and the price of a model remains very affordable. The dimensions of the circuit must be respected at best because of the presence of lines granted strip-line. These dimensions are 73 mm wide and 63 mm in height.

 

  

 

The method of amplification is Class AB. Below is a description of mounting image. The layouts are made of carnations, but it is also possible to achieve with simple crossings "legs of resistance", for example .. The printed circuit will be achieved with double-sided glass epoxy, 0.8 mm thick.


Source: 30W VHF FM Amplifier BLF245

See more : Power Amplifier - RF Meter - UHF Amplifier

Read more ...

Designing a Combiner for VHF/UHF

Here's is one way of combining several antennas on VHF/UHF is by using a power combiner (or power splitter if you prefer to look at it from that angle). Using the principle of impedance transformation in a quater wavelength transmission line, a proper matching can be made between antennas and the downlead cable.

To build a RF combiner you need to design a coaxial quaterwave line, having a characteristic impedance Zo, determined from Zo^2 = Z1*Z2, where Z1 and Z2 are the impedances being matched. This is normally done by using tubing - round or square - with air as dielectricum.

Simple impedance calculation
The characteristic impedance for the two types of lines are:

a = the diameter of the inner round conductor.
b = the inner diameter - or side lenght - of the outer conductor.


Combiners for 144, 432 and 1296 MHz
2-way combiner for 144 MHz.
4-way combiner for 432 MHz.
2-way combiner for 1296 MHz.

Exact impedance calculation

See more : 2.4 GHz Power Amplifier - RF Power Meter -  UHF Power Amplifier

Read more ...

VHF Video Transmitter 60-200 MHz

Here's a simple video transmitter for VHF TV channel will accept baseband video input, hence it can be driven by most CCD cameras and VCR video outputs. It ouputs roughly 80mW and when used with a 40cm telescopic antenna over 100 metres range is possible.

The transistor of the video transmitter can be a BC108, BC546, BC337 or a 2N2222. L1 is wound on a 10 mm air former. Use 6 turns 24 SWG for frequency 60-80 MHz, 4 turns for 150-180 MHz, and 2 turns for 180-200 MHz

You can use this with a monochrome or color video signal. To transmit sound just build the wide band FM transmitter and tune it to the audio channel.

See more : Audio Video RF Modulator - PLL FM Transmitter - Circular FM Antenna

Read more ...