Hitting the target band with minimal power loss
Two Key Performance Targets
The best antennas will be efficient ones. This means that they will both resonate at the correct frequencies, and that they will do this with minimal transmit and receive losses in the antenna, cables and matching circuit (if required). Badly-designed antennas that are not efficient will suffer from short battery life and may not work at all when installed in the final enclosure.
Resonance frequencies
The first design goal is to achieve one or more resonance frequencies in the centre of the target bands for the radio standard your device is designed to use.
Minimal power loss
The second goal is to radiate power as efficiently as possible in the target band(s), eliminating power losses. Power is lost in varous ways. These include:
- Impedance mismatches inside the PCB that cause reflections
- Induction creating heat variously between the transmission lines, connecting wires and the PCB dielectric and the enclosure.
VSWR
VSWR (Voltage Standing Wave Ratio) is a measurement use to show the efficiency of transfer of power to an antenna. This transfer is adversely affected by any impedance mismatch. The coefficient of the power reflected back from the antenna due to this mismatch is called the Return Loss and is expressed in -dB (the higher the negative value the better). A VSWR less than 3 is considered satisfactory and is the accepted minimum design target for a cellular radio. A VWSR of 2 or better means that there is little to gain from addig an impedance matching circuit. As VSWR increases, the transmit power of the radio module decreases and data errors can occur. Potentially more worrying for the designer, reflected power can damage the radio.
- VSWR: 2 = Return Loss of -9.5 dB approximated to -10dB
- VSWR: 3 = Return Loss of -6 dB
Resonance Frequency Bandwidth
The frequency gap between two – 6 dB or – 9.5 dB points in the curve on the resonance frequency is called the -6 dB or -10 dB bandwidth. A -10 dB bandwidth (equal to a VSWR of 2) bracketing the target resonance frequency is therefore the goal for an antenna design.
Putting it together
In the example on the right the green graph shows that the -10 dB Return Loss bandwidth is from an estimated 1000 MHz up to 1250 MHz whereas across the target resonance frequency shaded in grey it is below the -10 dB target, and thus the antenna must be adjusted. With the adjustment the red graph shows that the antenna now performs more or less satisfactorily at both target bands (in this case GSM 900 and 1800).
Trust the experts. How Crout can help
The Crout team has extensive experience in the tuning of antennas in real-life application situations, typically delivery several dBm of improved performance that leads to lower in-field costs through greater transmission range and longer battery life.
The team offer a range of services to help you get the best possible performance from an antenna, contact us today find out how we can help.
