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Antenna considerations – a key component of your M2M solution
No solution is stronger than its weakest link! Even if you have developed a perfect product and solution, bad choice of antenna might lead to hassle or unpredicted cost during installation and deployment. A poor built-in antenna might cause needs to use extensive amounts of external antennas, resulting in higher product- and installation cost.
Electromagnetic waves travel by the speed of light (c = 299 792 458 meters per second), the wavelength (λ) defines frequency, shorter wavelength means higher frequency. The ideal length of an antenna would be exactly one wavelength. The formula is:
In other words, a M2M product using a 2G (GSM) module operating in the 900MHz band would need an antenna with a length of approx 0.33 meters. In most cases such antennas would be too long and bulky to be embedded into products. A normal compromise is to use a “quarter wavelength” antenna, being exactly ¼ of the wavelength.
Another consideration to make is what sort of RF cable to use, connecting the antenna to the 2G (GSM) or 3G (UMTS) module. RF cables differs in terms of power loss (attenuation), cable length is always the same, the longer cable the more loss of signal power. Loss or attenuation is normally measured in dB (Decibel), 3dB loss equals ½ the output power or ½ the received signal strength.
Antennas comes in many forms (Dipole, Yagi, Parabolic, Strip-line etc.), normally the choice will be a compromise between functionality, size, price and design. A thorough test of antennas is quite complicated, since it requires an anechoic chamber, accurate (expensive) RF test equipment and so on.
However, a comparison test of different antennas is much easier and probably enough to make the right choice. Take a number of antennas that you might consider to use, connect them to your product (assuming you have some indication of received signal strength) and see which one provides the best result. Normally you would perform such a test in a place where the received signal is rather weak. Once the choice is made, it’s advisable to test a couple of the same antennas to verify that quality (received signal strength) is consistent.
Do you need help ? Telenor Connexion have a testlab, where these (among other) tests can be done. Just contact us…