Antenna RF Basic Parameters Explained
Contents
What is an Antenna
An antenna converts electrical RF signals into electromagnetic waves and vice versa, acting as a transducer between guided and free-space propagation.
Key Antenna Parameters
I –Gain:
Gain measures power concentration relative to an isotropic radiator, expressed in dBi (An isotropic antenna is a theoretical antenna with a uniform three-dimensional radiation pattern).
Gain quantifies how effectively an antenna directs power compared to an ideal isotropic source, combining directivity and efficiency into a single dBi figure.
A higher dBi means more power concentrated in a narrower beam:
So, Gain is a measure of an increase in power. The higher the gain, the better the range:
Note: Directivity vs. Gain
Directivity is a purely geometric measure (ignoring losses), whereas gain accounts for real-world losses (dielectric, ohmic) in the antenna
Directivity measures how concentrated an antenna’s radiation pattern is in a specific direction compared to an ideal isotropic radiator, which emits equally in all directions.
Mathematically, directivity is the ratio of the maximum radiation intensity (U_max) to the average radiation intensity over all directions:
Radiation efficiency (η): quantifies the fraction of input power that an antenna successfully radiates as electromagnetic waves. It accounts for losses due to factors like resistance in the antenna’s materials and mismatches in the transmission line. Efficiency is calculated as:
Efficiency values range from 0 (no radiation) to 1 (perfect radiation), and are often expressed as percentages. For instance, an efficiency of 0.85 corresponds to 85% efficiency.
II- Radiation Pattern:
The radiation pattern captures how an antenna radiates energy into space as a function of angle, typically plotted in the azimuth (horizontal) and elevation (vertical) planes:
Following is an illustration for the radiation patten from “allaboutcircuits.com”:
Main Lobe, Sidelobes & Nulls:
- The main lobe points where radiation is strongest.
- Sidelobes are smaller undesired lobes.
- Nulls are angles of minimal radiation
Beamwidth:
The Half-Power Beamwidth (HPBW) is the angular width where radiation falls to –3 dB of the peak. Narrow beamwidth implies high directivity
III- Polarization:
Antenna polarization describes the orientation and time-variation of the electric field vector of a radiated wave. Linear polarization constrains the field to a single plane (horizontal, vertical, or slant), whereas circular polarization rotates the field vector.
In other words, Polarization specifies the trajectory traced by the electric field vector at a fixed point in space as a function of time.
Antenna polarisation determines how the receiving antenna must be oriented to capture maximum energym misalignment can incur severe losses, as orthogonal polarizations are “invisible” to each other.
Types of Polarization:
Linear Polarization
- Horizontal/Vertical: Field oscillates in a fixed, single planeM horizontal or vertical.
- Slant (±45°): A compromise orientation used in terrestrial MIMO systems to mitigate multipath fading; still a form of linear polarization but tilted relative to horizon.
Circular Polarization
- RHCP/LHCP: The E-field vector rotates at the carrier frequency, forming a helix in the propagation direction. Right-hand vs. left-hand is defined by the rotation sense when looking along the wave’s travel direction.
Polarization Loss Factor (PLF):
Δθ: is the angle between Tx and Rx polarization vectors. PLF ranges from 0 dB (when aligned) to ∞ dB (when orthogonal).
Notes
Perfect alignment yields zero loss. A 90° misalignment yields infinite theoretical loss, making the RX blind to the TX signal.
In practice, environmental reflections often introduce cross-polar components, so some energy may still be captured despite mis-alignment.
Practical tip: Use two antennas with orthogonal polarizations (e.g., ±45° or LHCP/RHCP) at Rx to mitigate fading and mismatch
IV – MIMO
MIMO (Multiple-Input Multiple-Output) exploits multiple transmit and receive antennas to increase capacity and reliability by transmitting independent data streams (spatial multiplexing) and combating fading (spatial diversity)
Spatial Diversity:
Send or receive redundant streams of same information signal in parallel through multiple spatial paths.
It allows the transmission of multiple copies of the same information (same radio signal) via multiple antennas over independent channels, therefore reduces the chance of information loss caused by fading.
so, it Increases reliability and range, also reduces probability of error.
How Spatial Diversity Improves Link Quality
- Multipath Fading Problem: As a signal propagates, obstacles (buildings, trees, terrain) scatter it into multiple paths; some may cancel or weaken the main signal at the receiver.
- Diversity Concepts:
- Frequency Diversity: Sends copies on different frequency channels.
- Time Diversity: Sends copies in different time slots.
- Spatial Diversity: Sends copies via different antennas (the type used in MIMO).
- Spatial Diversity in MIMO: Multiple transmit and/or receive antennas each send or capture a replica of the signal. The receiver combines these replicas to reconstruct the best estimate of the original signal, greatly reducing fading effects
Spatial Multiplexing:
It’s a fundamental MIMO technique:
- Different data streams are transmitted simultaneously over the same frequency via separate antennas.
- Send independent streams of the information in parallel along multiple spatial paths, allowing an Increase in rate (throughput).
How Spatial Multiplexing Improves Data Rates
- A user’s data payload is split into separate streams, these streams can traverse different spatial paths but occupy the same frequency and time slots.
- Can serve a single device with multiple streams or multiple devices simultaneously (multi‑user MIMO).
- In 5G NR, Massive MIMO extends this by packing tens or hundreds of elements in an antenna panel, allowing very high aggregate throughputs to many users
