Application of ISM radio moduls for radio frequency measurements

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Dr. Nagy Lajos
Department of Broadband Infocommunications and Electromagnetic Theory

I investigated applicability of measurement of the ISM band radio modules in two totally different areas. The applied module is the RFM22 which based on the Si4432 transceiver chip of Silicon labs. This device has been developed to communicate in two directions thus it is capable of being a receiver and a transmitter as well. It is constantly tunable in its frequency range that is from 240 to 930 [MHz]. It has a -118 [dBm] average sensitivity in receiver mode, while in transmit mode the output power is configurable from 8 [dBm] to 17dBm with 3[dBm] resolution. The device has Digital RSSI (Received Signal Strength Indicator), which has a +/- 0.5 dB resolution from -100 [dBm] to -20 [dBm].

The paper demonstrates two practical applications of the modules after the theoretical analyses.

The first scope is the indoor localization that based on field strength distribution. Three unite broadcast unmodulated sinusoidal signals in three different frequencies in the 868 [MHz] ISM band during the measurement. The fourth device scans the channels one by one and transmits the measured RSSI values to the PC. The each result is stored in a matrix, where the elements of the matrix indicate the point of a map database. The transmitters are in known positions while we are on the move whit the receiver in the indoor environment. So we obtain a radio map that we can use to position directly or to test and also to tune indoor wave propagation models. I investigated the Motley-Keenan model that I tuned on the measured values. The average error of the localizations was approximately 3 meters in the examined environment.

The other investigated area related to application of modules is measurement of material parameters. We developed a measurement scheme based on a coaxial designed open resonator cavity. If we induce it with a sinusoidally variable field between 700 [MHz] and 900 [MHz], you can create a resonance curve. The induction is performed by a transmitter radio module, while the performance values are measured by a receiver module. If we put sample material to the cavity, the resonance peak will be shifted and the Q-factor will be modified as well. The scale of these changes is in analytically expressible connection with dielectric constant of material. Electromagnetic field simulation were performed with HFSS software to define parameters of the probe and to check the correctness of theory.


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