How does the ground influence the antenna radiation pattern
What is Antenna Radiation Pattern?
The radiation pattern of an antenna visually shows how it sends energy out into space by depicting the strength of the signal and its direction and polarization in a three-space.
Ground’s Effects on Radiation Patterns
The impact of the ground on the radiation pattern of your antenna varies based on the polarization of the radio waves and the frequency of the signal.
Influence on the Effective Isotropic Radiated Power (EIRP)
The EIRP represents the power emitted by an antenna in all directions. The ground’s reflective characteristics could boost the EIRP by redirecting power towards the horizon in angles; however, it might also cause energy losses in certain scenarios. The EIRP can be represented as:
\[EIRP\left(dBm\right)=P_t\left(dBm\right)-L\left(dB\right)+G(dBi)\]
Where
- \(P_t\) is Transmitter input power in dBi
- \(L\) are the Losses in dB caused by the interaction with the ground
- \(G\) represents Antenna gain in dBi
The impact of the ground on EIRPs varies depending on the ground type (such as soil or concrete), its conductivity level, and the antenna’s height above it. A conductive ground tends to reflect signals efficiently and can enhance antenna gain in certain directions. On the other hand, a less conductive or absorptive ground may lead to a decrease in radiated power output, resulting in reduced EIRPs.
Impedance Matching and Return Loss
To ensure power transfer in an antenna setup, it’s essential to match the antenna correctly with the transmission line. The proximity of the ground can influence the impedance of the antenna, resulting in a mismatch between the antenna and the transmission line. This discrepancy can result in reflections that are measured by return loss.
\[{Return\ Loss\left(dB\right)=-20log}_{10}{\left(\left|\mathrm{\Gamma}\right|\right)}\]
Where
\[\mathrm{\Gamma}=Mismatch\ coefficient\]
When the antenna connects with the ground surface it’s on, the impedance that the antenna deals with changes well. If the ground affects the antenna’s impedance, the return loss will go up, showcasing a mismatch in impedance and more power reflection. Ideally, a lower return loss signals better impedance matching, leading to improved power transfer to the antenna.
Voltage Standing Wave Ratio (VSWR)
The Voltage Standing Wave Ratio (also known as VSWR or SWR for short) is a metric used to gauge how well power is being exchanged between the antenna and the transmission line system in use. When the VSWF value hovers close to a 0 for both transmitter and receiver signals as they travel through space or along cables, this tells us that most of what’s being sent out by our equipment reaches its intended destination without wasting much energy along the way. However, when we start seeing higher numbers pop up on our meter, that’s usually a sign that things aren’t quite in sync. There might be some mismatch happening that’s causing power loss or distortion during transmission.VSWR can be calculated as:
\[VSWR=\frac{Maximum\ Voltage}{Minimum\ Voltage}\]
Ground reflections can change the impedance of the antenna and the transmission line, causing voltage standing waves. If the antenna is too close to the ground or if the ground has varying dielectric properties, the VSWR may increase, leading to less efficient power transfer. A higher VSWR indicates more reflected power and less efficient radiation.
Antenna Efficiency
The effectiveness of an antenna is determined by the proportion of power it emits compared to the power it receives. The ground interaction can lead to types of losses, like conductive losses, dielectric losses, and reflection losses, which can diminish the overall efficiency. Antenna efficiency can be expressed as:
\[Efficiency=\frac{Power\ Radiated}{Power\ Supplied}\]
Reflections from the ground can impact how well an antenna works by changing phases and affecting power distribution patterns. For example, having an antenna or a not-so-great ground could cause big losses and result in less power being sent out in the right direction. The efficiency also relies on what the ground’s made of. Metal surfaces might reflect better, but sand or dirt may absorb energy and lower efficiency levels.
Bandwidth and Ground Effects
The antenna’s bandwidth is determined by the frequencies it can work well with; the ground can impact this by affecting how the antenna resonates at frequencies. A reflective ground might make the antenna work better at a frequency but reduce its overall range; on the other hand, a non-reflective ground could widen this range instead.
Influence on Antenna Quality Factor (Q)
The quality factor (or Q factor for short) of an antenna refers to how its bandwidth compares to its center frequency. A higher Q value means a bandwidth, and a lower Q value indicates a wider bandwidth range. The interaction with the ground can influence the antenna’s Q value by adjusting its resonance frequency defined as.
\[Q=\frac{f_0}{f_2-f_1}\]
Where \(f_0=Center\ frequency\) and \(f_1\,f_2=Band\ edges\).
When an antenna is close to the ground, its ability to work effectively may decrease due to the frequency range it can operate within, especially if it’s placed near a conductive surface, like the ground.
Fractional Bandwidth (FBW) and Ground Influence
The Fractional Bandwidth (FBW), a metric representing the antenna’s range in percentage terms, is influenced by its interaction with the ground; it can either expand or contract depending on whether the ground reflects or absorbs some of the emitted energy.
\[FBW=\frac{f_2-f_1}{f_0}\times100\%\]
If the terrain boosts the antenna’s signal in certain directions, it might expand the frequency band where the antenna works effectively by broadening its frequency range; however, ground losses or significant reflections could potentially reduce this bandwidth.
Front to Back Ratio (F/B) and Ground Reflection
The front-to-back ratio (referred to as F/B for short) evaluates the proportion of emitted power directed toward the front versus the back of an antenna system. When positioning an antenna, the ground surface reflections from the ground have the potential to influence this ratio by deflecting power in a different direction.
\[\frac{F}{B\left(dB\right)}=Boresight\ Gain\left(dB\right)-Reverse\ Gain(dB)\]
A ground surface, depending on its type, can cause the antenna to radiate more power backward, decreasing the F/B ratio. Antennas designed to radiate primarily in one direction (directional antennas) are particularly affected by ground reflection, as the reflected signal can interfere with the direct radiation pattern.
Axial Ratio (AR) and Ground Impact
Circularly polarized antennas use the Axial Ratio (AR), which indicates the effectiveness of polarization quality in the presence of ground distortion that can alter the antenna signals’ polarization and diminish circular polarization quality.
\[AR=\left|\frac{E_{right}}{E_{left}}\right|\]
Interactions with the ground that impact polarization may result in an increased ratio, which suggests that the antenna is no longer perfectly circularly polarized. This could potentially hinder communication quality for systems that depend on polarization.
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Log period Antennas: Widely recognized for their frequency range capabilities and consistent radiation distributions, which make them ideal for a variety of communication purposes.
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Biconical Antennas: These broadband dipole antennas have an omnidirectional radiation pattern in the H-plane, suitable for various applications.
Testing Antennas: These antennas are commonly utilized for assessing the performance of antennas.
Conclusion
By considering these factors, you can gain a comprehension of how the earth’s surface impacts the radiation pattern of your antenna and enhance the efficiency of your antenna setups to achieve better results in terms of performance enhancement and operational effectiveness. Whether you are engaged in managing gadgets or overseeing large-scale communication networks, having a clear grasp of the dynamics between your antenna and the ground is pivotal for ensuring stable and dependable wireless connectivity.
References:
[1] JEM Engineering. 2024. 10 Factors that Affect Antenna Performance. https://jemengineering.com/blog-10-factors-that-affect-antenna-performance/
[2]Jack Portley. 2023. Ground Plane Considerations for Optimising Antenna Performance in Compact IoT Devices. https://knowhow.distrelec.com/internet-of-things/ground-plane-considerations-for-optimising-antenna-performance-in-compact-iot-devices/
[3] Dinobell Communication Co.,Ltd. 2019. Effect of Ground. https://www.dinobell.com/effect-of-ground.html
