Literally speaking, an omnidirectional antenna is an antenna that radiates uniformly in all directions. But in reality, the term 'omnidirectional' needs to be understood in conjunction with communication application scenarios. For example, for antennas in free space (such as those on low Earth orbit satellites), people may want them to radiate uniformly in all directions in three-dimensional space, while for ground high-frequency stations, people may want them to radiate uniformly in all directions (360 °) around the azimuth angle (horizontal plane).
In another scenario of ground station antennas used in low Earth orbit satellites, people hope that omnidirectional antennas can not only cover azimuth angles of 360 °, but also cover all elevation angles above the ground, thereby generating a hemispherical dome radiation pattern over the Earth.
The classification of omnidirectional antennas is not based on any specific antenna design structure, but on the radiation patterns they generate. Based on the above introduction, people may expect the gain size in the desired direction to be completely consistent, but not all types of omnidirectional antennas can achieve this. There may be some deviation, so depending on different applications, radiation patterns that are very close to omnidirectional can also be accepted. This leads to the term gain ripple. This term sets acceptable limits for gain variations in different directions, and theoretically, what people expect is perfect uniformity. Normally, a gain ripple of 2-3 dB is acceptable, but this limitation may be slightly smaller or larger depending on the communication application.
Let's study and introduce several typical omnidirectional antennas commonly used in various wireless communication applications.
Marconi 1/4 λ (monopole) antenna
The Marconi monopole antenna is also commonly known as a vertical plane antenna. The length of the radiating element is usually 1/4 λ and oriented vertically. When the transmitter drives the radiating element, it utilizes the ground as the return path for the electric field generated on the counter plate and radiating element. The intermediate frequency (MF) radio broadcasting station uses an example of this practical antenna. As medium range local radio stations, they need to provide omnidirectional service coverage along the azimuth plane. In addition, another advantage of a vertical monopole antenna is that it can provide better surface wave (ground wave) coverage compared to any horizontally polarized antenna due to its ability to generate vertically polarized radiation patterns.
For this type of antenna, it is important that the ground on which the antenna is installed has good soil properties (conductivity and dielectric constant). But this is not always possible. Therefore, most single polar planar antennas use artificial enhancement techniques to improve efficiency and performance. Usually, almost all installations involve laying a set of metal wires or mesh structures around the antenna. These antennas can be laid on the ground or buried a little under the soil. This serves as an artificial ground plane, typically with a radius of 1/4 λ around the monopole base.
Amateur radio enthusiasts often use vertical ground plane antennas very effectively in high-frequency bands and very VHF/UHF (high frequency/ultra-high frequency) omnidirectional ground radio communication. This antenna structure is simple, easy to manufacture, and does not require an antenna rotator, making it popular among radio enthusiasts. On these frequency bands, this antenna can effectively utilize an artificial ground plane composed of radial conductors without the need for absolute grounding. Therefore, it does not need to be installed near the actual ground.
As long as the monopole antenna is integrated with the artificial ground radial system, it can be installed smoothly anywhere or at any height above the ground. High frequency antennas can be installed on the roof of buildings and radiation lines can be laid on the roof, while very high frequency/ultra-high frequency antennas can be installed on masts and connected to radiation poles on bases near vertical elements. The radiation pattern of this antenna is truly omnidirectional in azimuth, but provides extensive elevation coverage in elevation, with a deep invalid point at a 90 ° elevation angle.
There are multiple variants of this antenna, some of which have radiation element lengths different from the 1/4 λ version. One very popular version is a vertical monopole with a 5/8 λ length radiator. For ordinary high-frequency amateur radio enthusiasts, this structure may be a bit too high, but it can fully withstand frequencies of 6 meters, 2 meters, 70 centimeters, and beyond. The advantage of a 5/8 λ vertical antenna is its high gain, omnidirectional azimuth mode, and greater compression in the elevation plane, making it more suitable for DX operations with low takeoff angle radiation. Both of these antenna variants are resonant single frequency standing wave antennas. Of course, they can also be transformed into multi band antennas through the appropriate use of notch filters.
Omnidirectional antenna for ground communication
Below is a brief introduction to other omnidirectional antennas used for ground communication, which have omnidirectional azimuth modes with varying degrees of gain and elevation lobe compression.
Vertical dipole
Horizontal dipole may be one of the most common types of antennas. Its center is a 1/2 λ long wire. It is an efficient resonant antenna that can generate horizontally polarized bidirectional radiation modes in a horizontal configuration. However, dipoles can also be oriented vertically. This will generate an omnidirectional azimuth mode, which is very suitable for ground communication. Although vertical dipoles may be relatively high in structure and not suitable for practical use by radio enthusiasts in most high-frequency bands, their physical size can be fully utilized in the very high frequency/ultra-high frequency bands. The polarization of a vertical dipole is vertical.
Normal mode spiral antenna
Spiral antennas are often used in various VHF/UHF (high frequency/ultra-high frequency) applications. According to the physical dimensions of the spiral antenna, it can radiate along the axis or orthogonally (with a wide edge) to the axis. These two different modes (depending on the design) are called axial mode and normal mode, respectively. The axis of the normal mode spiral is located in the vertical direction and can be used as an omnidirectional antenna. The advantage of this antenna is that, with the same gain, its physical length is smaller than that of a monopole antenna, or the length of the helix can be increased by increasing the number of turns to achieve higher gain.
J-Pole antenna
This is basically a half wave dipole antenna, driven vertically at the bottom instead of driving in the middle like a regular dipole. The problem with driving dipoles at the end is that the impedance of the feeding point at the end is extremely high, making it difficult to achieve impedance matching for effective power transmission. The J-Pole antenna employs a clever technique to avoid this issue. It has added another 1/4 λ section at the lower end of the dipole and a parallel conductor near the first section. This lower part is now like a 1/4 λ transmission line. The bottom ends of the lower part are short circuited together. This results in a perfect high matching impedance between the 1/4 λ bottom portion and the dipole top portion. Therefore, the total vertical length of J-Pole is 3/4 λ. The feeder of the transmitter is connected at a suitable position, slightly above the bottom of the structure, where good impedance matching can be achieved. This antenna has high gain and small takeoff angle, making it suitable for DX.
The above figure A is an example of a J-Pole antenna, and B is a J-Pole antenna made of copper wire or copper tube with a diameter of 3-10mm (suitable for high power).
Slim Jim antenna
Slim Jim is a variant of J-Pole. The length is the same, and the gain and omnidirectional radiation mode are also the same. Slim Jim can be regarded as a folded dipole (rather than a simple dipole), and its high impedance end is driven using the same method as J-Pole (1/4 λ short circuited TL part). The physical structure of Slim Jim allows for the use of either a visible transmission line or a 300 Ω or 450 Ω transmission line to construct it, making it absolutely lightweight and portable.
The above figure A is an example of a J-Pole antenna, and B is a J-Pole antenna made of copper wire or copper tube with a diameter of 3-10mm (suitable for high power).
Slim Jim antenna
Slim Jim is a variant of J-Pole. The length is the same, and the gain and omnidirectional radiation mode are also the same. Slim Jim can be regarded as a folded dipole (rather than a simple dipole), and its high impedance end is driven using the same method as J-Pole (1/4 λ short circuited TL part). The physical structure of Slim Jim allows for the use of either a visible transmission line or a 300 Ω or 450 Ω transmission line to construct it, making it absolutely lightweight and portable.
Vertical collinear array antenna
Technically speaking, this is a stacked antenna array. It is like cascading one or more vertical dipoles of 1/2 λ length above a regular 1/4 λ vertical monopole antenna. The upper end of a single dipole drives the lower end of the next upper dipole. However, a phase shift of 180 ° needs to be introduced between the junction of the monopole and the upper dipole. This can be achieved through various methods, but the simplest one is to use a 1/4 λ long transmission line as a parallel stub and short-circuit it at the far end. This is the required phase shifter. Even if a dipole portion is used above the monopole, it is still an excellent antenna. It has excellent omnidirectional mode in azimuth, high gain, small takeoff angle, and excellent DX effect. Adding more than one dipole at the top of the first dipole section can generate high gain, but it may not be practical for high frequencies. However, multi section vertical collinear antennas are commonly used in very high frequencies, especially in practical ultra-high frequencies.
