RF Data Modem

by Sanjay Mishra

RF Data Modems are available in a wide variety of operating frequency ranges, output power, sizes, power consumption, data interfaces and antenna connections.

Key considerations in choosing a RF Data Modem are

  • Operating Frequency Range
  • Is the operating frequency licensed or non licensed
  • Distance over which the RF data modem must operate
  • Desired Bandwidth of the RF Data Modem
  • Will the RF Data Modem have to operate around obstructions, is its operation a line of sight or non line of sight RF
  • Data processing requirements - Forward Error Correction, encryption, compression, RF protocol, networking

To have a custom RF Data Modem for your needs contact us. Our RF Data Modem designs are successfully used in both licensed and unlicensed frequency configurations. A custom RF data modem may result in better performance and lower cost. We can get your device FCC certified. We can also help configure and set up reliable RF communications with off the shelf equipment of your choice. We specialize in difficult RF communication system designs.

RF Data Modems operating over Unlicensed Frequency Bands

Part 15 Devices

No matter what the operating frequency, it is possible to construct unlicensed RF Data Modems by keeping the output power level to below the limit specified in FCC Part 15. The wireless devices are usually short range. For example, the RF device that hooks onto an Iphone or Ipod or other MP3 player and allows you to hear your songs on a FM radio station is such a device. We can design custom FCC Part 15 devices for you.

Interestingly amateur radio operators have been able to achieve long distance communications ranging to thousands of miles by using Low Frequency transmissions and high gain receiver antennas. Many of these devices are simple to construct too :-). Shows how much ingenuity and a love of wireless technology can achieve.

Low Power Local FM Radio Stations

There is a lot of interest in local FM radio station. These are usually run by schools, parks and such like. There is a little nice industry which supplies such needs.

Above HF (30 MHz) and below 800/900 MHz band>

Various bands are available below the 800/900 MHz. These bands are not globally standardized and the frequency available is not big. For example the 433-434 MHz band is available in Europe but is not available in US. We have RF Data Modem solutions that can be tuned to cover any band in this range.

800/900 MHz Band

This is a very popular band. The advantage of this band is that relatively high output power levels are allowed. The disadvantage is that there is no global standardization. The 902-928 MHz band is available in US while Europe uses the 800 MHz band.

2.4 - 2.5GHz Band

This is currently the most popular frequency band available. 802.11b, and g, 802.15.4 and Zigbee modules commonly run on this band besides a large assortment of standalone transmitters and receivers. A variety of factors

5.725 -5.875 GHz Band

This is the region where 802.11 a communications happen. This has not gained in popularity as much as the 2.4 GHz band. There are however a lot of cordless phones that are in this range. By using this range they avoid the over crowded 2.4 GHz range and achieve better quality.

61 to 61.5 GHz Band

This is a hot band for research. The key characteristic of this band that makes it popular is the relatively high bandwidth (500 MHz) available. If you want to implement USB over RF or Fiber over RF this is the band for you. At this frequency range RF does not propagate and is greatly attenuated by water vapour. This range is ideal for high bandwidth, short range communications. This range is also nearly line of sight so it might make sense to place a base station at the roof :-) The increasing speeds of cheap CMOS chips is sparking great interest in this frequency range.

Optical Band/Near Optical Band/Fiber Over Air

At this range instead of sending light over fiber you just beam it through the air. High data rates are possible. The downsides are obvious - susceptibility to rain, fog and smoke. Its advantages is its easy set up, high bandwidth and global acceptability.

RF Data Modems Operating Over Licensed Frequency Bands

Extra Low Frequency, Very Low Frequency

This is used for real long distance communications. It is also used for communication under sea. Sea water attenuates RF signal considerably and only very low frequency RF can get through sea water. Operating bandwidth is extremely low however there is no alternative for under water communications (think submarines).

A practical alternative for under water communications is to use a sound waves under water (sonar) and have a Radio Data Modem on a buoy on the surface of the water. Again, we can help you design such sonar radio communication systems.

Rant Alert: We feel neutrino emissions can successfully be used in such environments. If you have a few million dollars of funding available it would be fun to investigate this area. In reality we expect practical neutrino emission communication devices to be at least 30-60 years in the future unless some super secret lab already has it.


The good thing about HF is that it can be used for Non Line of Sight communications. Ionized layers in the atmosphere reflect radio waves. This fact was discovered early in the 20th century and widely used since. Global internet and satellite communication have to a large extent marginalized communications in this range. Since the ionized layers that reflect radio waves change with the time of day, with seasons and with solar spots, getting a HF modem to work reliably over long distances requires tweaking. The available bandwidth is very small.

However if you require cheap way to communicate short amounts of data over long distances this band is for you. It is great for sensor monitoring over long distances. Antenna design is critical for effective performance.

VHF/UHF Radio Data Modems

6.25 kHz, 12.5 kHz and 25 kHz channel bandwidths are available in different frequency bands. RF modems using a variety of modulation schemes (FSK, GMSK, 4FSK, AM) and output power are available. With 4FSK modulation it is possible to transmit about 9.6 kbps of raw data in a 12.5 kHz channel.

Wide band modems are used to transmit video and high bandwidth data.

We have both wide band and narrow band solutions in this range. You can also install custom paging solutions.

Microwave RF Data Modems

High bandwidth is available once we go to microwave ranges. These wide band, high bandwidth radio modems tend to be used for point to point links on microwave towers or as an essential component of fixed wireless internet access technologies.