How to Select and Place an Antenna for Optimal RF Performance

rscasny — Fri, 08 Oct 2021 20:35:52 GMT

Current Revision posted to Documents by rscasny on 10/8/2021 8:35:52 PM

Introduction

Wireless technology has emerged as the primary medium of communication for connected devices. To enable such widespread wireless communication, a wireless transceiver circuit with an appropriately selected and placed antenna is necessary. In this spotlight, we will focus on the various aspects of locating and mounting an antenna to ensure the RF circuit is functioning at maximum efficiency.

The focus of this paper is on cabled antennas.

Terminology

Before we discuss the aspects of antenna placement, let’s discuss some basic antenna terms:

Antenna Efficiency: Is the best way to assess the performance of an antenna and to compare performance between multiple antennas. Antenna efficiency measures how well an antenna converts RF power at its terminals into radiated RF power. The closer to 100%, the better the antenna
Radiation Pattern: The radiation pattern is a graphical representation of the radiation from an antenna as a function of space. It describes how the antenna radiates energy into space or how it receives energy.
Isotropic Radiator: An isotropic radiator is a hypothetical lossless antenna that radiates its energy equally in all directions. This imaginary antenna would have a spherical radiation pattern.
Antenna Gain: It is the ratio between radiation intensity in a given direction and the radiation intensity of an isotropic antenna.
Attenuation / Path Loss: It is the reduction of the power density of a propagating wave as it travels in space.
Characteristics Impedance: It is the ratio of magnitude of voltage and current waves in an infinite transmission line at a zero reflection wave condition.
VSWR: The voltage standing wave ratio (VSWR) is a measure of how much power is delivered to a device as opposed to the amount of power reflected from it (which is known as Return Loss). It is the ratio of the maximum voltage to the minimum voltage in a standing wave pattern developed when power is reflected from a load.

Antenna Selection

Learning the wireless standard or protocol used by the transceiver is important in selecting the matching antenna. Each wireless standard uses different operational radio frequencies for communication. Antenna sizes vary proportionally to the wavelength (λ).

Some popular short and medium range wireless protocols and their respective frequencies are:

Other protocols include LTE (cellular), GNSS (GPS), and various sub 1GHz ISM protocols such as LoRa.

You can verify the tuning-frequency accuracy by using a Vector Impedance Analyzer. Along with the resonant frequency, this analyzer can help in determining other antenna parameters such as impedance, reactance, and SWR.

Antennas range from highly directional (i.e., radiates in a narrow cone) to highly omnidirectional, which radiates in all directions to provide broader coverage. Real world antennas fall somewhere in between these two as there are no perfect omnidirectional or directional antennas. A majority of antennas in devices (and in the protocols mentioned in the table above) are essentially omnidirectional, so we shall limit our discussion to the more commonly used omnidirectional antennas.

There are many antenna choices – in terms of form factors, materials, and antenna families. The two super classes of antennas are PCB (also sometimes called Chip, or SMT) – where the antenna is directly soldered to a Printed Circuit Board and the other class is a ‘Cabled’ antenna, where the antenna is connected to the PCB via some type of coaxial cable (often a micro-coaxial cable).

Antennas may also be categorized by their construction / design such as -- PCB, Whip, Helical, etc. They must be correctly selected based on the application, gain parameters, cost and placement requirements of the system. One antenna may be favored due to factors such as cost, while antenna gain is selected based on the intended distance of coverage. Whip and PCB antennas provide high bandwidth and uniform radiation pattern. They are reliable and cannot be detuned, but they are expensive. Helical antennas have low bandwidth, can be detuned by interference, but are comparatively less expensive. These are just a sampling of types of antennas that a system designer may select, but not a complete list.

Connector Selection

For Cabled Antennas, RF transceivers / modems are coupled to the antenna by using the appropriate connector. In some instances, if your RF transceiver / modem is provided as a full assembly (e.g., daughter board), the connector on the PCB will already be pre-selected. However, if you are using a chipset and need to place the connector, then you need to be careful to avoid signal losses, which can result when there is a mechanical mismatch or a loose-fitting antenna, making the choice of a connector very important. A list of commonly available connectors is given below.

Press-fit connectors come with a limited mating cycle, hence, you have to avoid repeated detachment and attachment of these connectors. Note, the orthomode transducer (OMT) common connectors for many small device manufacaturers is the MCX – which looks a lot like the MMCX or other connectors. Be very careful to know what connector is specified and what type is provided on the other end, so you have the correctly mating conectors.

Finally, in applications were RF performance is key and there is concern of an impedance mismatch, it may be necessary to include or optimize the RF matching network on the PCB, to match the characteristic impedance of the connectors of the device and the antenna. Any mismatch in impedance can result in a reflection loss. Basic details on the matching network parameters should be available on your Antenna Applications Specification, or if not or under special circumstances, you may need to turn to an RF expert to help you design an optimally matched network.

Antenna Cable Selection

After deciding upon the antenna and the connector, you have to select a cable of suitable length. The cable size and characteristics impedance must be verified as well. You can experiment by opting for multiple cables of different lengths before selecting the optimal one.

You have to tightly mate the connector on the device to that of the antenna. In the case of micro connectors, they have to be pressed straight down on the receptacles. A tilted press can result in a damaged connector.

After placing the antenna, the performance of the antenna has to be measured. To achieve optimal performance, you have to conduct multiple iterations of parametric measurement at multiple locations. It is always beneficial to finalize multiple options of mounting locations. Choose the shortest cable length to avoid excess cable in order to limit losses and poor efficiency.

Best Practices for Cabled Antenna Placement

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 a straight configuration can impact performance, depending upon the antenna design and substrate material. 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 a lead line too close to an antenna pattern: The lead line of the antenna should have a minimum of 5mm clearance from the antenna pattern.
Avoid placing an antenna under a display: An antenna should not be placed below LCD or any displays.
Housing: Conductive metal enclosures must not be used with antennas. However, if only a 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 your modem supports MIMO (Multiple Input Multiple Output) antennas, the same design rules apply. A MIMO antenna RF performance 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 antennas are used instead of individual antennas 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 equipment used to tune and test antenna systems is an SWR meter. SWR shows you how well your coaxial cable, antenna mount, ground plane, and antenna matches the output capability of your radio. Install an SWR meter in the feed line and adjust some part of the antenna for the best SWR reading at your operating frequency.

Defective or inferior 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 will 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 open space or in an anechoic chamber. 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 the radiation pattern, we can see antenna gain values in different directions and calculate antenna efficiency.

Tags: mimo, 5g, molex, radiation, optimization, gps, lte, vswr, isotropic radiator, efficiency, gnss, antenna, radio frequency