To place an antenna for optimal performance, let’s quickly go through some guidelines to keep in mind when using cabled antennas:
· Place on a flat surface: Avoid bending of the antenna: slight bends may be ok, but sharp bends are to be avoided at all costs. Bending the antenna from straight configuration will impact performance, depending upon the antenna substrate material and design. In addition, metal surfaces can drastically impact the tuning or resonance of the antenna.
· Avoid placing antenna too close to a PCB: At least 20 mm of clearance should be maintained between the antenna and the PCB. If there is no way around this specification, you should consider placing the antenna perpendicular to the PCB. Clearance from the PCB removes the effect of the ground plane.
· Avoid the lead line too close to the antenna pattern: The lead line of the antenna should have a minimum of 5 mm clearance from the antenna pattern.
· Avoid placing the antenna under a display: The antenna should not be placed below LCD or any displays.
· Housing: Conductive metal enclosures must not be used with antennas. However, if only the metal housing is available, then a ferrite backed antenna should be used.
· Multiple Antennas: If the device features multiple antennas for diversity, they should be placed as far as possible from each other.
· MIMO: if the modem supports MIMO ( Multiple Input Multiple Output ) antennas, the same design rules apply. RF performance of a MIMO antenna will be better than a single antenna, but if you can place two separate antennas and maximize the distance and orientation, they will likely perform even better than a MIMO antenna. Typically, MIMO antenna is used instead of individual antenna when the device is limited in space.
Testing and Measurement
Once you finalize the placement of the antenna, it can be tested to measure the RF parameters such as: Efficiency, Frequency, Power Rating, Gain / directivity, Return Loss / VSWR, Bandwidth, Impedance and Polarization.
The test setup includes Network Analyzers like Vector Impedance Analyzers and power amplifiers.
Tuning Antenna with SWR (Standing Wave Ratio) Meter:
The most common piece of test antenna systems and test equipment used to tune is an SWR meter. SWR shows you how well your coaxial cable, ground plane, antenna mount, and antenna matches the output capability of your radio. Install an SWR meter in the feed line and adjust some parts of the antenna for the best SWR reading at your operating frequency.
Inferior or defective components, ineffective installations, and antennas not tuned to the specific location increase the SWR. An SWR value of 1:1 is perfect, whereas an SWR of 2:1 should be the maximum as a general rule. Modern radios start lowering transmitter output power automatically when SWR is above 2:1. When the SWR is over 3:1, it can damage the radio circuits.
Testing: Open Space or Anechoic Chamber:
The RF performance parameters for a particular antenna setup are obtained in an anechoic chamber or in open space . Keep in mind that the open space test does not factor in the loss of signal due to absorption by obstacles and interference from other EM waves around.
The anechoic chamber is a closed room capable of absorbing reflections inside the chamber. It isolates external signals and acts as a complete shield against internal and external interferences.
An anechoic chamber enables you to:
· Record directional radiation patterns.
· Measure gain in the swept frequency band.
· Set and measure differential phase and amplitude.
· Set and optimize the SWR.
With the help of radiation pattern, we can calculate antenna efficiency and see antenna gain values in different directions.