10 variables affecting DIY FM transmitter range

For electronics novices and amateurs, we supply a variety of FM transmitters. Some are designed for short-range transmission and others for long-range transmission. Customers occasionally claim that they did not receive the specified quality or transmission range. In this post, I’ll go over ten parameters that have an impact on the transmission range and sound quality of DIY FM transmitters.

  1. The FM transmitter’s design: Some hobby FM transistors have three transistors and three inductors, while others have only one transistor and one inductor. All 3-transistor FM transmitters feature an audio amplifier circuit, but single-transistor FM transmitters simply have an oscillator circuit and no further amplification for frequency-modulated signals. As a result, the transmitters with three transistors outperform the others. The value of the resistors and capacitors in the circuit determines how far the signal may travel. In most cases, the transmitter’s designers have a rough range in mind. They pick the components or build the PCB in such a manner that their transmitter transmits the signal up to a particular range with a certain quality, based on their goal. Most FM transmitters, for example, use S9018 (AM/FM Amplifier, Local Oscillator of FM/VHF Tuner) and S9014 (pre-amplifier) transistors. You’ll see why they’re used if you look at their datasheets.

2. Antenna length: I’ve discovered that extending the antenna length also increases the broadcast range. I originally tried the long-range 3 transistor FM transmitters with a 20cm long antenna, which could only broadcast up to 100 metres. However, when I raised the length to 60cm, the signal could easily travel up to 500 metres. Any FM transmitter with a longer antenna receives better reception and travels further.

3. Digital versus analogue FM receivers: Digital FM receivers outperform analogue receivers when it comes to testing transmitters.The first benefit is that we know the precise frequency at which the transmitter transmits to the receiver. On a digital FM receiver, we may search for a free frequency faster than on an analogue one. If we can find one, we can use a free frequency to send our signal without interfering with commercial FM broadcast transmissions. Interrupting commercial FM broadcasters is unlawful in various countries.

The tuning of analogue FM radios is not exact. Two commercial FM broadcasts also interfere with each other’s transmissions. When the transmission signal is powerful enough, it overlays commercial broadcast transmissions, resulting in clear audio on the receiver. The signal goes away as we get further away from the transmitter (outside the transmission range), and the commercial broadcast resumes. It is advised that we utilise a digital FM receiver in order to avoid signal overlapping and distortion.

4. FM receiver antenna: The majority of commercial FM receivers come with a telescopic antenna. Their reception is fantastic. You should use a receiver with a telescopic antenna if possible. When we utilise a mobile phone’s FM function, the reception isn’t as excellent as commercial FM broadcasts. You may, however, test the transmitter with your phone. However, there is no assurance that the phone’s FM receiver will be able to receive the signal at its full range of transmission.

5. Power: If a transmitter’s power source ranges from 3 to 9 volts, the transmission is more powerful at 9 volts than it is at 3 volts. You won’t obtain the same signal strength with a 3V cell as you would with a 9V battery.

6. Voice or audio transmission: The transmitted audio signal is louder than the voice-activated signal when the FM signal is connected to an audio source, such as a mobile phone or an MP3 player. The electret microphone converts speech into an electrical signal, which is oscillated and transmitted as an electromagnetic wave, although it will never be as powerful as a direct audio transmission. The audio signal is an electrically active signal that may be regulated and amplified according to our demands using a volume controller. As a result, audio transmissions supplied directly from an audio source travel further than conventional voice signals.

7. Volume controller/audio booster: We have a volume controller on two of our FM transmitters. I’ve discovered that the volume controller influences the broadcast signal’s intensity, which in turn influences how far the signal travels. A volume controller is available on the following transmitters:

A. Basic FM transmitter with three transistors (with no tuning).

b. A sophisticated FM transmitter with three transistors (with tuning)

8. Transmitter location: What is the location of your transmitter? Your FM transmitter will produce superior results in an open environment rather than in an urban region with many buildings or obstructions. If at all feasible, emit the signal from a higher altitude so that it can go further. I tested all of my transmitters from a height of 7–10 metres, which is normally the second or third storey of a structure, as well as from the ground floor, and I discovered that the transmission quality and range varied greatly. Commercial FM stations’ antennas are kept in hills or on top of buildings since testing the transmitter from a height yields the best results.

9. The frequency of transmission or reception: To avoid signal overlapping, the frequency we are broadcasting on should be free. When we use an FM transmitter that does not include a changeable capacitor or inductor (for example, an FM transmitter with three transistors), the transmitter emits its signal at a random frequency that we cannot adjust. A variable capacitor or inductor, for example, is not present in the FM microphone or the FM transmitter with inclosure. This prevents the transmitters from altering their frequency, which would cause the commercial FM broadcast to overlap. It’s critical that the transmitter broadcasts on an unlicensed frequency that isn’t used by commercial radio stations. There is a substantial risk of hearing distortion if we transmit to an occupied or nearby frequency. As we get further away from the transmitter, commercial FM takes over the original frequency, and our FM signals decrease.

There is no variable capacitor or inductor in these transmitters.

10. Weather: The capacitance and inductance of the circuit are affected by the weather. Even little changes in the transmitter’s electric characteristics might cause the frequency to wander. The transmission is also affected by the wind, air pressure, temperature, and humidity. Please read the following three articles:


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