Otis Singleton
Electrically small antennas have a shorter length than the wavelength they are intended for, which results in a roughly uniform voltage distribution. This leads to a high impedance, causing a small amount of power to be transmitted to the antenna and thus, a lower gain. Electrically small antennas also have a low radiation resistance, which is a key factor in antenna performance, as it is responsible for converting electrical energy into electromagnetic waves. This results in a lower efficiency, as a significant portion of the power is lost instead of being radiated.
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What You'll Learn
Electrically small antennae have a low radiation resistance
Electrically small antennas have a length much shorter than the wavelength at which they operate. This results in a roughly uniform voltage distribution across the antenna, leading to a low radiation resistance. Radiation resistance is the part of an antenna's feedpoint electrical resistance caused by the emission of radio waves. In simple terms, it is the resistance opposing the current due to the recoil force experienced by electrons when they emit radio wave photons.
The low radiation resistance of electrically small antennas is a result of their small size relative to the wavelength. This size disparity leads to a high impedance, which in turn results in a small resistive component, or radiation resistance. The radiation resistance of electrically small antennas is typically much smaller than the Ohmic loss on the radiating elements, suppressing radiation efficiency.
The radiation efficiency of an antenna is influenced by its radiation resistance and Ohmic loss. Ohmic loss refers to the resistive loss on individual components of the antenna, which contributes to the overall loss resistance. The loss resistance of an antenna increases when it is mounted near the ground, reducing its radiation efficiency.
Electrically small antennas have a relatively low radiation resistance, which makes them inefficient at transmitting radio waves. This is because the power transmitted to the antenna is small due to its high impedance. As a result, electrically small antennas radiate an insignificant fraction of the power they receive as radio waves.
To compensate for the low radiation resistance, electrically small antennas rely on associated inductors or capacitors to achieve a reasonable impedance match. By matching the impedance, these antennas can operate over a small bandwidth without requiring frequent re-tuning. However, their small size also contributes to their narrow bandwidth, as a shorter antenna length results in a lower capacitance value in the radiation environment.
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Small antennae have a high capacitative component
Electrically small antennas have a length that is much shorter than the intended wavelength. This results in a roughly uniform voltage distribution on the antenna, with no voltage fluctuation, and hence no magnetic field or radiation. The impedance of such antennas is large, leading to small power transmission.
The high impedance in electrically small antennas is due to their high capacitive component. The capacitance of an antenna element, such as a metal wire or rod, is important because it forms a capacitive voltage divider with the input capacitance of the connected amplifier. The relationship between the charge, voltage, and capacitance in a capacitor is given by the equation Q = C × U, where a smaller capacitance requires a higher voltage for the same charge.
In the case of electrically small antennas, the capacitance is low, leading to a high voltage requirement. This results in a very high voltage across the capacitor, even with low transmitter power. The high voltage can lead to RF burn and shock issues, which are more serious than with inductive loading of short antennas.
To counteract the high capacitance in short antennas, an inductor can be added. This counteracts the inherent increase in capacitive reactance, leaving only the resistive component. However, this resistive component is very small, leading to low radiation resistance in electrically small antennas.
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Voltage distribution is roughly uniform, so there is no magnetic field
Electrically small antennas, or antennas with a length much shorter than the intended wavelength, do not radiate efficiently. This is due to the roughly uniform voltage distribution on the antenna at any given moment, resulting from the antenna's length being much shorter than the wavelength. This uniform voltage distribution leads to a lack of voltage fluctuation, which in turn means there is no magnetic field and, consequently, no radiation.
The absence of radiation in electrically small antennas can be attributed to the high impedance, which results in a minimal power transmission to the antenna. Additionally, electrically small antennas exhibit a very high capacitative component to their impedance, with a low resistive component, specifically a low radiation resistance of 1 or 2 Ohms. This low radiation resistance is caused by the emission of radio waves from the antenna, where a radio transmitter applies a radio frequency alternating current, resulting in the radiation of the current as radio waves.
Radiation resistance is observed in transmitting antennas, where radio waves are produced by time-varying electric currents. Electrons accelerate as they move back and forth in the metal antenna due to the electric field created by the oscillating voltage supplied by the radio transmitter. This acceleration of electrons generates electromagnetic waves, which carry momentum away from the emitting electron.
The radiation reaction, or the recoil force experienced by an electron when it emits a radio wave photon, causes a reduction in the electron's momentum. This recoil force acts in the opposite direction of the electric field, reducing the average velocity of the electrons for a given driving voltage. Consequently, this force acts as a resistance opposing the current, contributing to the overall radiation resistance at the antenna terminals.
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Small antennae have a high impedance, so power transmitted is small
Electrically small antennas have a high impedance, which means that the power transmitted to the antenna is small. This is because, in the case of electrically small antennas, the length of the antenna is much shorter than the wavelength, resulting in a roughly uniform voltage distribution. This leads to a high capacitative component to the impedance, which, in turn, results in a very small resistive component, or radiation resistance.
Radiation resistance is the part of an antenna's feedpoint electrical resistance caused by the emission of radio waves from the antenna. A radio transmitter applies a radio-frequency alternating current to an antenna, which radiates the energy of the current as radio waves. The radiation resistance depends on the proximity of the antenna to conducting and insulating objects, particularly the height of the antenna from the ground. The shorter length of electrically small antennas means that they have a lower radiation resistance, which results in a smaller amount of power being radiated.
The high impedance of electrically small antennas can be counteracted through impedance matching, where the overall system of the antenna and transmission line is designed so that the impedance is as close as possible, reducing power losses. This can be achieved through an antenna tuner or impedance matching network between the transmitter and antenna. However, as the length of the antenna decreases, the antenna losses become a larger part of the net resistance projected by the antenna to the driving circuit, resulting in lower efficiency.
The low radiation resistance of electrically small antennas also means that they have a narrow bandwidth. This is because a higher stored energy-to-energy lost ratio results in a higher Q, which corresponds to a lower bandwidth.
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Small antennae have a low value of capacitance
Electrically small antennas have a low value of capacitance. This is because the length of an electrically small antenna is much shorter than the wavelength, resulting in a roughly uniform voltage distribution with little to no voltage fluctuation. Due to this, small antennas exhibit high impedance and low radiation resistance, making them inefficient at transmitting radio waves.
The low capacitance of small antennas leads to a high series reactance, which can be addressed by increasing the thickness of the wire or by adding radial wires or plates to resemble a parallel-plate capacitor. However, these solutions have limitations and may not always be feasible.
To compensate for the low capacitance and improve performance, techniques such as electrical lengthening can be employed. This involves adding inductance to increase the electrical length, making it resonant at the desired frequency. This is commonly achieved by introducing a loading coil or inductor in series with the antenna, which cancels out the antenna's capacitance. While this improves power transmission, electrically short antennas still radiate less power and have lower gain compared to full-sized antennas.
Additionally, the radiation resistance of small antennas is generally low, often below a tenth of an ohm. This low radiation resistance contributes to their inefficient transmission of radio waves. To address this issue, parallel resonance can be created using a capacitor across the loop terminals, although the improvements in radiation resistance and efficiency are limited.
In summary, electrically small antennas have a low value of capacitance, which results in high impedance, low radiation resistance, and inefficient radiation. Techniques such as electrical lengthening and the use of capacitors can be employed to improve performance, but there are limitations, and small antennas still radiate less power compared to larger ones.
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Frequently asked questions
Electrically small antennas have a low radiation resistance, which is caused by the emission of radio waves from the antenna. This results in a drop in efficiency and a lower gain than a full-sized antenna.
Radiation resistance is the part of an antenna's feedpoint electrical resistance caused by the emission of radio waves from the antenna. The radio waves are generated by time-varying electric currents, consisting of electrons accelerating as they flow back and forth in the metal antenna.
Electrically small antennas have a high impedance, which means that the power transmitted to the antenna is small. This creates bottlenecks, preventing all the power from reaching the load.
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