Frequently Asked Questions About Antennas

The bandwidth of an antenna is a crucial parameter that refers to the range of frequencies over which the antenna meets a particular performance criterion. These performance parameters typically include gain, radiation pattern, and VSWR. Most often, VSWR is used to determine the bandwidth, known as the impedance bandwidth.

The bandwidth is defined by the lower and upper-frequency limits where the antenna maintains a VSWR below a specified value, commonly 2:1. This means the bandwidth is the range of frequencies over which the VSWR remains under 2, indicating acceptable performance.

For example, if an antenna meets the 2:1 VSWR specification from 2.38 GHz to 2.66 GHz, the absolute bandwidth is 2.66 GHz - 2.38 GHz = 0.28 GHz, or 280 MHz.

Another method to specify bandwidth is by using percentage bandwidth, which is more detailed in the theory section. This method involves determining the center frequency of operation.

Directivity and gain are two interrelated key performance metrics of an antenna.

Directivity is a numerical measure of how well an antenna focuses energy in a specific direction rather than dispersing it evenly in all directions. It indicates the antenna's ability to direct radio waves more effectively in a particular direction.

Gain, on the other hand, measures the antenna’s efficiency in converting input power into radio waves directed towards a specific direction, factoring in both the antenna's directivity and efficiency. While directivity is purely a measure of directional focus, gain also accounts for the energy losses within the antenna, such as those due to heat.

The antenna's radiation efficiency (er) ranges from 0 to 1 and defines the relationship between gain and directivity. Radiation efficiency quantifies the fraction of input power successfully radiated by the antenna.

Mathematically, the gain is the product of directivity and radiation efficiency:

Gain=Directivity×Radiation Efficiency

It is important to note that the losses addressed by radiation efficiency are intrinsic to the antenna, such as resistive losses and other internal inefficiencies. These do not include mismatch losses between the antenna and feed transmission line. Radiation efficiency solely pertains to the antenna's standalone performance.

Antennas can be broadly categorized into two types based on their radiation patterns: omnidirectional and directional antennas.

Omnidirectional antennas radiate and receive energy equally in all horizontal directions, providing 360-degree coverage. They are ideal for applications that require uniform coverage in all directions. A characteristic feature of these antennas is their pole-like radome, often seen in designs such as monopoles. Common types include the "Rubber Duck" and "Whip" antennas. Examples of omnidirectional antennas include Mobile Mark's ECO, DOD, OD, PSKN, MOD series, and base station antennas.

On the other hand, directional antennas focus the available energy in a specific direction rather than distributing it uniformly. These antennas are used in applications where coverage is needed in a particular area, or the signal must be concentrated in a specific direction. Examples of directional antennas include Mobile Mark's Yagi, PS, PND, and SCR series.

The choice between an omnidirectional and a directional antenna depends on the specific coverage requirements of the application. An omnidirectional antenna is suitable if comprehensive coverage in all directions is needed. A directional antenna is the better choice if targeted coverage in a particular direction is required.

Polarization of the field radiated by an antenna is a crucial specification that describes the trajectory of the tip of the electric field vector over time. There are three primary forms of polarization: linear, circular, and elliptical.

Occurs when the electric field vector traces out a straight line over time. This happens when there is only one electric field component or when two components have a phase difference of 0° or 180°.

Cable loss, also known as "insertion loss," refers to the reduction in power as it travels through a cable. This loss increases with the cable's length and the signal's frequency. Mobile Mark has tested their cables, measuring the loss at different frequencies. These results are plotted to help system designers determine the actual loss introduced by the cable in a specific setup.

The loss is typically expressed in decibels per 100 feet (dB/100 ft). To find the actual loss for a given cable length, one must adjust the value from the plot accordingly. It’s important to note that these measurements are taken with SMA connectors at the ends of the cable assembly, providing a practical indication of cable performance versus frequency.

Cables come in various types and materials, affecting their performance. Generally, thicker cables have lower losses, meaning they retain more energy. However, as the cable length increases, so does the loss. Therefore, it is crucial to consider the cable type and length when designing a system to ensure optimal performance.

• Antenna Gain: Higher gain antennas focus energy more effectively, increasing the range.
• Frequency of Operation: Different frequencies propagate differently, with lower frequencies generally traveling further.
• Distance Between Antennas: The greater the distance, the lower the received power, following the inverse-square law.
• Radiation Efficiency: The efficiency of the transmitting and receiving antennas impacts the amount of power radiated and received.
• Mismatch Loss: The loss that occurs due to impedance mismatches between antennas and their feed lines.
• Polarization Mismatch Loss: Losses occur when the transmitting and receiving antennas have different polarizations.

The Friis Transmission Formula is often used to estimate the received power in free space, considering the gains of the antennas, the frequency, and the distance between them. However, this formula assumes ideal conditions without any obstacles or interference.

• Obstacles and Reflections: Buildings, trees, and other structures can reflect and absorb signals, altering the effective range.
• Environmental Factors: Weather conditions, such as rain or fog, and the presence of objects that absorb or scatter signals can impact range.
• Medium Characteristics: The medium through which the signal travels can introduce additional losses.

Given these variables, accurately predicting an antenna's range is challenging. Theoretical models provide a starting point, but real-world testing is essential to determine performance in a specific environment. The range also depends on the minimum power required by the receiving antenna to function reliably, which can vary based on the application and sensitivity of the receiving equipment.

• Jack or Plug: Antennas are typically specified as either Jack or Plug to indicate the appropriate mating connector.
  
• Reverse Polarity Connectors: Some antennas use "Reverse" connectors, where the presence or absence of the center pin is the opposite of the standard Jack or Plug configuration. For example, a reverse polarity SMA Jack would have the outer housing of a typical Jack but without the usual center pin.

The main difference between dBd and dBi gain is the reference antenna used for comparison. dBd compares the antenna's gain to that of a theoretical dipole antenna, while dBi compares it to a theoretical isotropic antenna.

• The "i" in dBi stands for isotropic.
• dBi is the unit used to measure antenna gain relative to an isotropic radiator, which radiates energy uniformly in all directions.
• An isotropic antenna is theoretical and radiates equally in all directions, so its gain is 0dBi.
• A real antenna's gain is expressed in dBi by comparing its radiation pattern to an isotropic antenna.

For example, if an antenna has a gain of 5dBi, it radiates 5 decibels more power in a specific direction than an isotropic antenna.

• The "d" in dBd stands for dipole.
• dBd is sometimes used for omnidirectional antennas, where a theoretical dipole antenna is used as the reference instead of an isotropic antenna.
• The gain of a perfect dipole antenna at 0˚ elevation is 2.15dBi.
• To convert gain from dBi to dBd, you subtract 2.15 from the dBi value.

For example, if an antenna has a gain of 5dBi, its gain in dBd would be approximately 2.85dBd.