10 factors that affect the transmission range of DIY FM Transmitters

BuildCircuit sells several types of FM transmitters for electronics beginners and hobbyists. Some are for short-range and some for long-range transmission. Sometimes, customers complain that they didn’t get the mentioned quality or the required range of transmission. In this article, I am explaining 10 factors that affect the transmission range as well as the transmitted sound quality of DIY FM transmitters.

  1. Design of the FM transmitter: Some hobby FM transmitters are designed with 3 transistors and 3 inductors and some are designed with only one transistor and one inductor. All the FM transmitters with 3 transistors have an audio amplifier circuit whereas the single transistor FM transmitters have only the oscillator circuit, there is no extra amplification for frequency modulated signal. That’s why the transmitters with 3 transistors perform better than the other one. The resistors and the capacitors placed in the circuit have a huge role in determining how far the signal can go. Usually, the designers of the transmitter have a tentative range in their minds. Based on their objective, they choose the components or design the PCB in such a way that their transmitter would transmit the signal up to a certain range with a certain quality. For example, we usually see S9018(AM/FM Amplifier, Local Oscillator of FM/VHF Tuner) and S9014(for pre-amplifier) transistors in most of the FM transmitters. If you see their datasheets, you will know why they are used.

2. Antenna length: I have noticed that increasing the length of the antenna also extends the range of transmission. I tested the long-range 3 transistor FM transmitters with a 20cm long antenna first and it could transmit up to 100 meters, but when I increased the length to 60cm, the signal was transmitted up to 500 meters easily. If you a longer antenna in any FM transmitters, you get better reception and they travel farther.

3. Type of FM receiver- digital or analog: Digital FM receivers are far better than analog receivers to test the transmitters. The first advantage, we know the exact frequency the transmitter is transmitting to the receiver.  We can search for a free frequency on a digital FM receiver than on an analog one. If we can find a free frequency, we can use that for transmitting our signal without interrupting the commercial FM broadcast signals. It is illegal to interrupt the commercial FM stations in several countries.

In analog FM receivers, the tuning is not precise. The transmission from two commercial FM broadcasts also overlaps with each other.  When the transmission signal is strong, it overlaps the commercial broadcast signals and the audio is heard clearly on the receiver. Once we go far away from the transmitter(beyond the transmission range), the signal fades away and the commercial broadcast is heard again. In order to eliminate the overlapping of signals and distortion, it is recommended that we use a digital FM receiver.

4. Antenna of the FM receiver: Most of the commercial FM receivers come with a telescopic antenna. Their reception is excellent. It is recommended that you use a receiver with a telescopic antenna. When we use the FM feature of a mobile phone, the reception is not as good as the commercial FM broadcasts. However, you can use your phone to test the transmitter. But, there is no guarantee that the phone’s FM receiver can receive the signal to the maximum transmission range.

5. Power: If a transmitter has a power supply rating from 3V to 9V, the transmission is stronger at 9V than at 3V. At 3V, you will not get the same signal strength as you would get with a 9V battery.

6. Transmission method- voice or audio: If you transmit the FM signal connecting it to an audio source, for example, a mobile phone or an mp3 player, the transmitted audio signal is stronger than the voice-activated signal. A voice is converted to an electrical signal by the electret microphone, that electrical signal is oscillated and transmitted as an electromagnetic wave, it would never be as strong as the direct audio signal. The audio signal is an electrically activated signal which can be controlled by a volume controller also and amplified according to our needs. Therefore, audio signals directly fed from an audio source are transmitted farther than the normal voice signal.

7. Volume controller- audio booster: Two of our FM transmitters have a volume controller. I have noticed that the volume controller contributes to the strength of the transmitted signal which eventually determines how far the signal would travel. The following transmitters have a volume controller:

a. FM transmitter with 3 transistors- Basic (with no tuning)

b. FM transmitter with 3 transistors- Advanced (with tuning)

8. Transmitter location: Where are you transmitting from? Your FM transmitter will give better results in an open space rather than in an urban area with a lot of buildings or obstacles. Make sure that you transmit the signal from a height (if possible) so that the signal can travel farther. I tested all of my transmitter from a height (7-10m), usually the second or third floor of a building, and also tested them from the ground floor, I found the transmission quality and range vary drastically. Testing the transmitter from a height gives the best result, that’s why commercial FM stations’ antennas are kept in hills or on top of buildings.

9. The transmitting or receiving frequency: The frequency we are transmitting to should be free in order to avoid the overlapping of signals. Usually, when we use an FM transmitter without a variable capacitor or variable inductor(for example, FM transmitter with 3 transistors- Basic), the transmitter transmits its signal at a random frequency, which we cannot change. For example, the FM microphone and FM transmitter with enclosure do not have a variable capacitor or inductor.  That completely restricts the transmitters from changing their frequency which ends up overlapping the commercial FM signal. It is very important the transmitter transmits to a free frequency that is not used by commercial stations. If we transmit to an occupied or a closer frequency, there is a high chance of hearing distortion. As we go far away from the transmitter, the commercial FM overtakes its original frequency and our FM signals fade away.

These transmitters do not  have a variable capacitor or inductor

10. Weather: Weather has impacts on the capacitance and inductance of the circuit. Even negligible change in its electric parameters can drift the frequency of the transmitter. The wind, atmospheric pressure, temperature and humidity also have effects on the transmission. Please read these three interesting articles:

We are selling these FM transmitters

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BC-56145

Type 1- Long range DIY FM transmitter with 3 transistors and 3 inductors

US $11.95
  • Long-range FM transmitter: 100m-500m. Tested several times
BC-4614345

Type 2- Long range DIY FM transmitter with 3 transistors and 3 inductors

US $8.38
  • Long-range FM transmitter: 100m-300m. Tested several times

DIY KIT 44 – FM Transmitter DIY kit using 3 transistors

This is yet another FM transmitter DIY kit for beginners and hobbyists. It is slightly different from our previous FM transmitters. It has 3 NPN transistors and the transmission range is around 50-100 meters.

It is an enhanced wireless microphone with excellent transmission quality. The voice transmitted by the microphone can be heard using an ordinary FM radio. You can also transmit audio signals.

Frequency range: Around 83Mhz
Working voltage: 1.5V -9V
Range: More than 100 meters at 4.5V. For the general radio to receive the FM signal, users need to solder a 50cm wire to the kit.

The following is the schematic and working principle:

The electret microphone converts the voice to an audio signal, the C2 works as a coupling capacitor sending the signals to the base of Q1. The signal received at Q1 is supposed to be modulated. When there is a sound signal, the transistor junction capacitance changes and it eventually changes the oscillation frequency which we can call ‘frequency modulation’ (FM).

FM signal is then fed to the high-frequency tuning amplifier via C6. The tuning amplifier amplifies the signal and then, C12 and L3, as well as the wire antenna transmits the signals as high-frequency electromagnetic waves.

  • R1 is a bias resistance to electret microphone (MIC), generally selected in the range 2K-5.6K.
  • R4 is the collector resistance.
  • R5 is the base resistor, providing a bias current to Q1.
  • R7, R8, C5, L1, C8, and C7 together form a high-frequency oscillation circuit.
  • R7 to the Q2 base to provide bias resistance.
  • C5 and L1 make the oscillation circuit.
  • Q3, R9, R10, L2, C10, C11 constitute the high-frequency power amplifier circuit.
  • C8 is the feedback capacitor to the transistor Q2.
  • R8 works for the stability of the Q2 DC operating point.
  • C7 is the capacitor for grounding distorted AC signals.
  • R9 to the power transistor Q3 to provide the base current.
  • C10 and L2 together work as a tuning circuit and the oscillation circuit C5 and L1  also tune at the same frequency to get the maximum output power so that the transmitter can transmit the electromagnetic signals as far as possible.

Check out the following images for assembly tips. You can see all the images on Flickr also. 

Check out the other images of the kit

One mistake: Step 1: There are 3 x 2.2K and 2 x 22K resistors. Not 5 x 2.2K resistors. Sorry for this mistake

 

The simplest and the cheapest FM transmitter- Do-it-yourself(DIY) kit for amateurs

Here’s one of the easiest, simplest and the most popular FM transmitter for amateurs and electronics beginners. With this DIY kit, you can transmit your voice or audio over an ordinary FM radio within the FM broadcast band. It is a DIY kit designed by Sagar Sapkota. You can buy this kit from BuildCircuit Store. The best thing about this transmitter is that you don’t have to make your own inductor for this kit. Making an inductor is bit hard for amateurs. Besides, this kit is not only capable of transmitting voice using microphone but also transmitting music from your music player. (more…)

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Gallery- one transistor FM transmitter

GO TO THE ARTICLE PAGE

step11- another angle StripBoard with columns The back side of the strip board connection of wires to stereo jack step3- fix 10pF (C5)capacitor Step 15- Solder the antenna Step 14- Fix 9V connector step 13- Solder the stereo speaker jack Step 13- Another angle Step 13- a close view Step 12- Solder inductor (2) Step 12- solder inductor Step 13- a close view (2) Step 11- Fix the electret microphone step 11- microphone step 11- solder 30PF(C6) step 11- another angle1 Step 10- Fix R¤ and R3- 33Ohm resistors step 10- another angle step 8- top view step 9- another angle step 9- Solder C8(0.1uF) Step 7- Solder 0.1uF(C2) step 8- another angle Step 8- Solder R1(2.2K) Step 6- Fix 680pF(C3) and a jumper wire connecting to GND Step 5- Fix 30PF(C4 ) and a jumper wire connecting to GND step 4- Fix 30pF(C7) step 1- solder S9018(Q1) transistor step 2- fix 22k(R2) and 2.2K(R3)

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How to make one transistor FM transmitter on a stripboard- page 2/2

This is PAGE 2/2 of the article titled “How to make one transistor FM transmitter on a stripboard”. Go to previous page.

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BC-0012

1 Transistor FM Transmitter DIY Learning Kit

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Continued….

Now, solder C8 (0.1uF- code 104) capacitor. One terminal of the capacitor is connected to the +ve and the other to the -ve terminal of battery. After connecting C8, it is not necessary to connect C1 because both have the same purpose in the circuit. So, you can simply ignore C1.

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Assembly tutorial- FM transmitter DIY kit

If you have to come to this page directly from a search site, please go to this article first. This is the second article that gives  assembly instructions for FM transmitter kit. Please click here for the first article.

In this article, we will see how to assemble the FM transmitter DIY kit.

First of all, look at the schematic and PCB image given below.

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