Image Credit: RF Industries Inc.
Radio modems encode, transmit, receive and decode serial data using radio waves. They connect to serial ports on devices such as video cameras and data acquisition systems, and send signals to and receive signals from other radio modems. Although these modems do not require wires, their uses may be limited by uneven terrain, inadequate antenna heights, and antenna feeder cable loss. In these situations, a radio modem repeater may be required.
How Radio Modems Work
Radio modems transmit serial data as radio waves in a specific frequency according to a defined radio technique. This digital data is encoded prior to transmission, and then decoded prior to receipt. Once decoded, the serial data is provided to the connected device.
In the diagram below, two radio modems are used for communications between a touch panel and programmable logic controller (PLC). This is also an example of point-point communications.
Figure 1 - Industrial Grade Radio Modems - Image Credit: B&B Electronics
Frequency Bands and Radio Techniques
When selecting radio modems, buyers need to specify a frequency band.
- 900 MHz and 2.4 GHZ are unlicensed industrial, scientific and medical (ISM) bands that do not require FCC registration. For both, the maximum transmission power is 1 W.
- 5 GHz is part of the unlicensed national information infrastructure (UNII).
- 23 GHz is for microwave radio systems.
- UHF and VHF radio modems are also available.
To reduce the effect of localized frequency interference and noise, these modems use one of two spread spectrum techniques. Both methods use more bandwidth than the system requires.
- Direct sequence spreads the signal on a larger band by multiplexing it with a code or signature, a fast repetitive pattern. In the receiving radio modem, the original signal is recovered by receiving the whole spread channel and demodulating it with the same code. Direct sequence modems generally require faster circuits and a digital signal processor (DSP).
- Frequency hopping moves the signal through a set of narrow channel in sequence. The transmission frequency band is divided into a certain number of channels. Following a predetermined cyclic hopping pattern, the system periodically moves to a new channel. Frequency hopping avoids interference and noise by never staying in the channel for a long period of time.
Figure 2 - Frequency Hopping - Image Credit: HP / Jean Tourrilhes
There are three operating modes for radio modems.
- Point-to-point devices can transmit to only one radio modem at a time.
- Point-to-multipoint devices can transmit to multiple radio modems at a time.
- Repeater mode devices are designed to receive a radio signal and then retransmit it a higher level and or power.
When selecting radio modems, buyers should also consider range reducers and enhancers, advanced networking and security, low-cost and low-poer options, frequency usage, and certifications and approvals.
Related Products & Services
Access points are transmitter / receivers (transceivers) that connect to a network through an interface such as a bus or connector. They receive, store, and transmit data between the wireless LAN and the wired structure.
Network modems (modulators-demodulators) are devices or programs that allow computers to transmit data over telephone lines. They convert digital computer data to analog sound waves and then demodulate the carrier signals to decode the transmitted information.
Wireless modems transmit modulated data using electromagnetic waves.