FM TRANSMISSION

FM Transmission is one of the most interesting branches of electronics. It’s an ideal place to start as the circuits are very simple and produce absolutely amazing results. With as few as 20 components, a circuit can be designed to transmit an impressive distance and produce an audio quality that sounds almost indistinguishable from reality.

The transmitters we will be describing in this section are also extremely sensitive. You can hear a pin drop on a wooden floor at 6 feet. Some of the circuits are stable enough to be carried as a hand-held microphone, while others have been designed for very long range.
All the transmitters have been tested in a built-up area to coincide with the conditions you will experience. Obviously, an open-field will achieve a far greater range.
Many of the specifications presented in other magazines are totally false, as the circuits are poorly designed. You will see what we mean after you have studied the projects. You will even be able to improve other designs and adapt them to your own requirements.
We will also describe test-equipment projects, to test the performance and range of our FM transmitters. This allows you to detect improvements in your design without having to travel miles for a range-test.
All the designs we are presenting are available READY BUILT for those who do not want to assemble a kit.
Most of the built-up versions use a more-powerful circuit than the design in the kit, so you cannot compare pricing.
We have sold over 10,000 fully-built devices and more than 40,000 kits. This has been one of our specialty areas for more than 15 years and we have developed a full range of models.
Our two most-popular made-up FM transmitters are 900ft and 2400ft:

SB-900F
900ft FM transmitter

SB-2400F
2400ft FM transmitter

As an example of our neat designs, here is our WASP project:

It needs a 3v supply (two AAA cells) plus a switch and antenna. The range is approximately 200-400 yards. You can see the compactness of the board. This is one of the most important considerations with a circuit containing a high-frequency oscillator. In this design, the components follow the circuit diagram and this makes it very easy to follow when you are referencing the circuit against the project.
Many design-engineers consider it “childish” to layout a PC board to follow the circuit diagram, but this line-of-thinking makes no sense.
The author has experienced this with a Philips TV set where one component was on the other side of the set, on another PC board!
It took an hour to track it down.
When components are laid out in a logical way, you can visualize the circuit and this makes servicing so much easier.
With high frequency circuits, layout is very important. Compactness makes them much more stable and increases their output. This is due to the higher “Q” of the circuit called the “oscillator circuit or “tank circuit.”
These are just some of the things you will learn when you read the projects in this section.
This is not an isolated topic. We will be combining FM transmitting with microcontrollers to produce “remote data gathering projects.”

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Our FM TRANSMITTER projects:

BEETLE MkIII - connects your guitar to the airwaves $12.50
VOYAGER MkII - 2400ft transmitter $7.50
WASP - 600ft transmitter $7.40
Field Strength Meter - measures the output power of our bugs $6.40 LED Power Meter - measures the output power 70¢