Most transmitter has several variable capacitors which are used to match impedance for transistors and antennas. I know people hate trimmers and so did I. The reason is that it is difficult to trim a system if you can't measure the performances.
To trim a transmitter you need to measure the output power.
Most transmitter are tuned with a dummy load of 50 ohm to substitute an antenna of 50 ohm.
Not everyone has a power meter, and how can you know that the antenna you connect is purely 50 ohm.
If not, the hole trimming is waste of time!
What you would like to do is to measure the radiated power out from the antenna you actually are going to use.
If you can measure the radiated energy field you can easy tune the system for max output field strength (max power).
So, how can we measure the radiated energy field?
The block diagram at right show you one easy way to measure the RF filed strength. To the left you find a dipole antenna.
The antenna should be cut to match the receiving frequency ...
Posted on Wednesday, June 22, 2011 • Category: FM Transmitters
This is 80W RF power amplifier that boosts FM Transmitter's power using 2SC2782 bipolar transistors in a tuned class C circuit. RF amplifier can be driven to full 80W power with less than 1 watt driving input power, so that a large gain margin results in this FM transmitter.
To obtain stability in this RF 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 feed trough capacitors for bypassing on the board, etc. It took some tweaking, but the amplifier ended up unconditionally stable.
Posted on Monday, June 20, 2011 • Category: TV Transmitters
This is a TV transmitter for transmitting video of various video sources such as video cameras, Satellite receivers, DVD players, game consoles, etc. TV transmitter's circuit is working on the 470-580 MHz frequency and can be received on UHF channels 21-34. TV Video transmitter can radiate as far as 300 meters by using a 10-20 cm wire antenna. TV transmitter requires voltage of 9-15 Volts. However, you can also use a 9v batteries. Oscillator is based around BF199 and BFR90 RF transistors. If needed the range of TV transmitter can be extended by replacing BFR90 with 2N3886 transistors.
Posted on Saturday, June 18, 2011 • Category: PIC
A bootloader enables download of hex-files directly into the flash-memory of a PIC or other microcontroller. The bootloader receives the user program via the PIC's UART and writes it directly to the program memory (self programming). This feature greatly speeds up the development process, because the chip remains in the target circuit and need not be moved between the target circuit and the programmer.
When no bootloader is installed, all memory in the PIC can be utilized for user programs. That is 4 K for the 16F873 (0x000 to 0xFFF). Installing a bootloader means, that some part of the memory is occupied by the bootloader. The user can download his program into the remaining memory space. The bootloader in figure 1 occupy 256 words (0xF00 to 0xFFF), that is 6 % of the memory in a 16F873. The disadvantage of loosing 6 % memory is little compared to the advantage of fast program download and more friendly development routines.
Posted on Friday, June 17, 2011 • Category: FM Transmitters
RF Field Strength Meter with Attenuator up to 200 MHz
The RF field meter unit is a great help to tune transmitters for best performance and output power. You can measure the radiated energy field and can easy tune the system for max output field strength maximum power.
This field strength meter comes with selectable attenuator. You can use it for measuring the antenna gain and pattern, compare different magnetic field strengths. See the following RF field strength meter schematic.
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