Frequency Modulation (FM) was invented in 1933 and everyone is familiar with its use in Radio Broadcast. You would think that with Bluetooth, Wi-Fi and all the other wireless communication systems and technologies out there, FM would be displaced. But that is definitely not the case. In fact the popularity of FM has increased over the years with many new devices and applications.
Practical FM Transmitters
FM transmitters for instance are used for church parking lot services with devices like the Basewish FM Transmitter. This small, powerful device allows you to broadcast at any frequency in the FM band. People who want to listen can tune in to this frequency using their car radio or FM radio receiver.
Every car stereo has an FM radio receiver but not all cars have Bluetooth connectivity. Specially not old cars. FM transmitters make it possible to transmit music from your phone to your car stereo.
A similar device can be used to broadcast Christmas music to people who drive up to your home. Visitors can enjoy the lights while listening to the music on the FM radio in their cars.
You can also use an FM transmitter to broadcast music throughout your house. SCOSCHE FMT4R makes a really cheap FM transmitter that can be used to broadcast audio to FM radios all over your house. This can be used with an old iPod or your phone. Just plug the cable into the audio port of your handheld device, set the frequency and start transmitting. Now you have a radio station within the confines of your own home!
Frequently asked questions
Will FM broadcast ever be switched off?
Yes, there is a plan to switch broadcast FM off and transition to digital broadcast. In fact in 2017 Norway started to turn FM broadcast off in parts of the country. In Europe and other parts of the world Digital Audio Broadcasting (DAB) is going to take its place.
What is the range of an FM transmitter?
Range depends on the transmitter power among other factors such as antennas, propagation environment and quality of the transmitter. When most people refer to FM they refer to the FM broadcast band – 88-108 MHz. Here are some typical ranges that people have achieved in the real world:
| Transmit Power (Watt) | Range (Miles) |
| 0.5 | 0.2 |
| 1 | 0.6 |
| 10 | 4 |
| 15 | 6 |
| 50 | 20 |
How far can you transmit FM legally?
Only 200 ft. or 61 meters. Some of the FM transmitters sold on Amazon can reach about 1000 meters. Clearly this exceeds the FCC limit and are not recommended for use in the USA or Canada, for instance.
What is the best frequency for your FM transmitter?
The best frequency is 87.9 MHz as it is seldom used for radio broadcasts. Hence the potential for interference to and from your FM transmitter is minimum. In general it is always worthwhile to do a check of the FM band in your region. There are various online tools that allow you to do this. And if you want to get more elegant with it, you can use an RTL-SDR software-defined radio with SDR# running on your laptop to accomplish this.
How do I eliminate strong FM signals?
FM signals are everywhere and most Software-defined Radios (SDRs) are unable to deal with strong FM signals. In fact FM signals prevent SDRs from receiving other signals of interest. We have written an article about this.
What are some of the precautions to be taken when operating FM transmitters?
- Make sure your FM transmitter is certified. In most geographies of the world there is a requirement for Radio Frequency equipment to be certified. In the USA, the FCC provides information on maximum limits for RF emissions. For an FM transmitter to be certified, it has to fall below these limits and then an FCC ID is granted. Take the example of the APROTII FM transmitter. It has an FCC ID: 2ASVO7C-73. If you go to the FCC data base you will be able to pull the test report associated with this transmitter and see that it passed the FCC test.
- Make sure that your antenna is connected prior to transmitting signals. If this is not done, the transmit signal could reflect back into the device and destroy the transmitter circuit.
- Use a good power supply. With the longer range FM transmitters like the Signstek, customers have reported an annoying humming sound. This is sometimes due to improper or a lack of grounding in the power supply. To eliminate the hum, replace the supply with a 3 prong 12 VDC power supply.
- Don’t amplify the output signal with external amplifiers. While it may be tempting to connect an external amplifier to increase the range of your FM transmitter…. DON’T. Not only do you risk damage to the electronics but you will also most likely exceed FCC limits and cause interference to other devices.
Why is FM radio better than AM?
FM is less susceptible than AM to radio interference and other propagation-based effects such as signal fading. In FM signals the information is contained in the frequency of the modulating signal. Amplitude variations don’t impact this. In AM on the other hand the information is contained in the amplitude of the modulating signal. So if there is a spike in the amplitude for instance, it will impact the quality of the received signal.
Does AM travel further than FM?
AM and FM are modulation techniques and this has no impact on distance. However AM signals in the broadcast band vary in frequency from 540 to 1700 kilohertz while FM signals vary from 88 to 108 MHz. The lower frequency AM signals can travel further. In fact at night AM signals travel hundreds of miles by reflection from the ionosphere. This is also called “skywave” propagation.
What is the highest transmit power of an AM and FM station?
The highest power of an AM station is 50 kW while the highest power of an FM station is 100 kW.
Why is the AM reception worse than FM reception?
There are two main reasons for this:
- AM is more susceptible to the effects of interference than FM
- AM antennas are poorly designed and this results in lower quality of reception
Summary
FM technology is here to stay. It’s robust and has many use cases as we have described in the article. A good way to experiment with FM is to get yourself an SDR like the HackRF and use it with GNU Radio.
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