FM Transmitters

This is an image Schematic. No Description available.

Don't be fooled by the relative simplicity of this transmitter. With 4W of power, it's not a toy! Since it operates in the FM band (88-108MHz), a licence is required for broadcasting. With a good and properly mounted antenna this baby has a range of up to 50km. In mountainous terrain or in built-up areas, this figure is smaller, of course. The circuit is fairly small. The last two transistors dissipate quite a lot of heat. Make sure to protect them with sufficiently large heatsinks or else you'll have a nasty meltdown. The coils are not all that critical, but should still be winded carefully to ensure efficient operation. Use enamel-coated wire at least 1mm thick. As with all RF circuits, work slowly and pay attention to details.

This is an image Schematic. No Description available.

This FM radio-controlled anti- theft alarm can be used with any vehicle having 6- to 12-volt DC supply system. The mini VHF, FM transmitter is fitted in the vehicle at night when it is parked in the car porch or car park. The receiver unit with CXA1019, a single IC-based FM radio module, which is freely available in the market at reasonable rate, is kept inside. Receiver is tuned to the transmitter's frequency. When the transmitter is on and the signals are being received by FM radio receiver, no hissing noise is available at the output of receiver. Thus transistor T2 (BC548) does not conduct. This results in the relay driver transistor T3 getting its forward base bias via 10k resistor R5 and the relay gets energised. When an intruder tries to drive the car and takes it a few metres away from the car porch, the radio link between the car (transmitter) and alarm (receiver) is broken.

This FM transmitter (FM Tx) is about the simplest and most basic FM Tx it is possible to build and have a useful transmitting range. It is surprisingly powerful despite its small component count and 3V operating voltage. It will easily penetrate over three floors of an apartment building and go over 300 meters in the open air.

My FM Wireless microphone V5 has been very popular, both as a beginners project and as a kit. It is interesting that it has also been the biggest cause of lots of e-mail, the most common question being How can I make it crystal controlled?. The oscillator can be made crystal controlled; replace the 2p7 with a 5th harmonic crystal. The oscillator will then be very stable, so stable in fact, that it cannot even be modulated! This means that you could only use it to send CW (morse code). One solution to the problem would be to use a low-frequency crystal oscillator and modulate it. Since a crystal can be modulated by typically only 1KHz or 2KHz, then the multiplication factor would have to be around 72. This would require a 1.3888MHz crystal and several multiplier stages, each individually tuned. The first stages would also have to be double-tuned if the transmitter were to have low spruious signals; at total of eight tuned stages (plus output filter)! For example:

This project is a simple transmitter using only one crystal and will cover 145.00 to 146.00 MHz. The crystal is a 44.9333 MHz crystal for 145.500 receive, as used in the Trio (Kenwood) 2200, PYE, Motorolla, Tait equipment, to name but four. The frequency of the crystal is not critical as almost any other xtal for the 2-meter band will function. No provision has been made to tune the transmitter to different channels, as this transmitter was first used as a single channel "repeater box", leaving my main rig free to be used on other channels. The circuit is given above and simply mixes the output of a (more or less) conventional receiver multiplier (x3) with the output of a 10.7MHz VFO that is modulated with true FM.

So, without further ado, here is another FM Wireless Microphone with added microphone amplifier and power amplifier stage to increase the microphone sensitivity and range. Please note that this circuit is drawn from memory and I cannot guarantee that it will work well without some form of modification so please treat this project only as an idea. I had loads of success with this version of the circuit in the early 80s and had a range of about 2Km. I will build this version again later, after my present projects are complete, then present any possible changes to the circuit.

This project is a miniature, VHF FM (wideband) Wireless Microphone transmitter of the type that are commonly refered to as BUG's. Note that "BUGS" are illegal but "Wide-Band Frequency Modulation Wireless Microphones" (WBFMWMs) are not, as so many people have told me (including the RSGB!). Besides, the AF sensitivity of this transmitter prevents it from being an effective bug for eaves-dropping! I personally use one of these WBFMWMs plugged into my HF rig headphone socket so that I can "earwig" QSO's and nets when sitting on the toilet, washing the dishes, bringing in the coal, etc. I know from experience that this project can be used to stimulate interest in Radio in older children, and this was also one of the projects given to a group of scouts and girl- guides to construct. Surprisingly, I found the girls made a better job of soldering than the boys!

This small transmitter uses a hartley type oscillator. Normally the capacitor in the tank circuit would connect at the base of the transistor, but at VHF the base emitter capacitance of the transistor acts as a short circuit, so in effect, it still is. The coil is four turns of 18swg wire wound around a quarter inch former. The aerial tap is about one and a half turns from the supply end. Audio sensitivity is very good when used with an ECM type microphone insert.

L1 is 0.112uH (this tunes to the middle of the FM band, 98 MHz, with VC1 at its centre value of 33pF). L1 is 5 turns of 22 swg enamelled copper wire close-wound on a 5mm (3/16") diameter former. Alternatively, you can have a fixed 33pF cap instead of VC1 and have L1 as an adjustable molded coil (eg UF64U from Maplin). VC1 will give you a tuning range of 85 - 125 MHz, and a possible choice is the Philips type polypropylene film trimmer (Maplin code WL72P). Two sets of oscillator bias resistors are given, the ones in the brackets give about 20% more RF power. Mike is our favourite Omnidirectional sub-mini electret (Maplin code FS43W). Ant is a (lambda / 4) whip monopole (eg 76 cms of 22 swg copper wire). Q1 is configured as a Clapp oscillator. Frequency modulation results from the audio voltage changing the transistor's base-emitter capacitance.

This is an image Schematic. No Description available.

This is an image Schematic. No Description available.

My FM Wireless Microphone has been a very popular project with beginners and experienced constructors alike. It has been used inside guitars and as the basis of a remote control system. I do however, receive many requests for a higher powered circuit and better microphone sensitivity. Now I can introduce the new FM Wireless Microphone (v5), which also has a better frequency stability, over 1Km range (under ideal conditions) and is good on microphone sensitivity. This has been achieved by adding an RF amplifier buffer (with 10dB gain) and an AF preamplifier to boost the modulation a little.

The transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres. The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.

The circuit of the transmitter is shown in Figure 1, and as you can see it is quite simple. 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). Because the trimmer cap is very sensitive, make the final frequency adjustment on the receiver. When assembling the 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. The frequency stability is improved considerably by adding a capacitor from the base of Q1 to ground. This ensures that the transistor operates in true common base at RF.

Here are some utility circuits for use with the Ramsey FM10a, and other small FM stereo transmitter kits. This information may be helpful for setting up a micro powered FM radio station. The FM10a and similar kits tend to put out a low quality, broad band output signal. This is not a big problem with a 5-10 milliwatt signal, but amplifying the output of such kits beyond 50mw is a bad idea, as the amplified signal can cause interference with other radio services, drawing unwanted attention to your hobby station.

This miniature transmitter is easy to construct and it's transmissions can be picked up on any standard FM radio. It has a range of up to 1/4-mile (400 meters) or more, depending on the line-of-sight, obstructions by large buildings, etc. It is great for room monitoring, baby listening, nature research, etc. L1 is 8 to 10 turns of 22 gauge hookup wire close wound around a non-conductive 1/4-inch diamter form, such as a pencil. C4 is a small, screw-adjustable, trimmer capacitor. Set your FM radio for a clear, black space in the lower end of the band (88MHz). Then, with a non-metallic/ non-conductive trimmer tool, adjust this capacitor for the clearest reception. A little experimenting and patience may be in order. Most of the parts values are not critical, so you can try adjusting them to see what happens.

This is another easy-to-build miniature transmitter that uses a minimum of parts. Construction is straight forward and non-critical. Although this design uses a 3-volt power source (such as a lithium coin or button cell), a 9-volt battery can be used, instead, by increasing the value of R1 to 15K and R2 to 1K. C4 is an optional RF bypass capacitor that may help improve performance and increase the range a bit. Experiment to find best results. L1 was made by stripping 22 gauge hookup wire of it's insulation, then wrapping it in the grooves of the screw threads of a 1/4 diameter bolt, and then back-screwing the bolt out of the resulting coil. 8 turns were made around the bolt. By wrapping the turns in the threads, a uniform seperation was made between the coil windings.

This is an image Schematic. No Description available.