Even the best equipment is useless without a good antenna – BH7CNC.OP (that's me)
This article does not involve calculations; it is purely conceptual learning and will not result in misalignments due to mathematical formulas. It aims to be as clear and easy to understand as possible.
We welcome feedback, corrections, or additional information from experienced users.
There are many ways to classify antennas; here, I will use an example to illustrate and then assign it a category.
Common sense
High-frequency antennas are typically smaller because their wavelengths are shorter. Conversely, low-frequency antennas (such as shortwave) can be mounted on various supports, such as PCBs or directly on the enclosure, and they can also be made from various materials, including gold, silver, copper, iron, tin, etc., and in various shapes, such as bending the DP to the left and right to change its shape, which will affect its radiation characteristics.
First, let's get an understanding of the key performance indicators (KPIs).
First, it's important to introduce a key antenna performance metric: the radiation pattern (which also includes gain). This can be represented in various ways, with polar plots being common. However, the most intuitive representation is a 3D plot.
Here, I will use a simulation of a square loop antenna to explain it:
This is a polar plot, where concentric circles represent gains. Because of the coordinate labels, it's also easy to see that the XY plane represents an aerial view, while XZ represents a horizontal view. Furthermore, by imagining this, we can visualize a distorted apple shape with wave-like patterns, similar to a 3D wave pattern:
The arrangement of the blue square-ring antenna, meaning that the vertical line passing through the center of the ring is the direction of maximum radiation (receiving), with a maximum gain of 3.3 dBi.
Loop antenna
A resonant antenna, also known as a standing wave antenna, exhibits current distribution characteristics with standing wave properties.
Horizontal dipole:
Verified:
Inverted V:
Horizontal dipoles, V-shaped antennas (both positive and negative), and other configurations slightly alter the wave pattern. However, all of these types of antennas are essentially the same type: a linear antenna. So why is it called a linear antenna? Because the diameter of the antenna's resonant element is extremely small compared to the wavelength. Therefore, it is referred to as a linear antenna, and it also functions as a standing wave antenna (typically narrowband), because the antenna has an open structure with no current loop, resulting in standing waves on the antenna itself. This corresponds to a traveling wave antenna, which is classified based on the current distribution on the antenna. Another deformation based on the same principle is:
Unbiased antenna:
This type of transmission can also be considered a special case of transmission, but it's quite extreme. The key difference is that the transmission point isn't at the center, and there are different input impedances and initial currents, which also results in slight changes to the wave pattern.
GP:
This antenna uses the ground as another oscillator, so a ground network is needed to reduce grounding resistance and make the current more balanced. Generally, a dipole antenna looks like an upright sphere placed on the ground, making it omnidirectional. However, because it's upright, it has vertical polarization. The handheld rubber-ducted antenna is also a GP, but it treats you holding the handheld device as the other half of the oscillator. Another variation of the GP involves increasing the effective diameter of the upright oscillator to weaken its "wireline" characteristics and create broadband characteristics:
Cobwebb spider web antenna:
The DP bends for multiple frequency bands were formed into square frames, which didn't fundamentally change anything.
The first type of Yagi antenna:
The resonator is a standing wave antenna; overall, it is a traveling wave antenna.
It is a directional antenna, which also includes a horizontal dipole with an added reflector and a focusing element. The higher the number of elements, the higher the gain, and the larger the front-to-back ratio (the ratio of the radiation gain at 180 degrees).
Parabolic antenna:
This isn't a "wire antenna" anymore; the bandwidth is much wider. This is mainly because the coil is thicker, and I'm using a thin wire to approximate it. The radiation pattern is still similar to that of a DP antenna, but with a wider bandwidth. Other variations, such as sector antennas, dual cone antennas, batwing antennas, etc., all involve making the coil equivalent larger:
The list above is not exhaustive, but it should contain some basic concepts related to loop antennas and provide a general understanding of "wire antennas."
Traveling wave antenna
Both TEM (Transverse Electromagnetic) waves and field-line waves can be considered as wave types, corresponding to the previous section. Unlike traditional waves, TEM waves do not reflect back; therefore, the current distribution in the antenna is different. However, these antennas typically form a closed loop (although there are exceptions where they can also function as standing-wave antennas), resulting in a wider bandwidth (with exceptions like small loops). Let's first look at some common examples:
Circular antenna:
In reality, this shape can be a square, triangle, or hexagon, as long as it's circular. However, rings also come in different sizes, and the current distribution between small and large rings is quite different, leading to different radiation directions. For example, the initially simulated square ring was a large ring, while the smaller ring had a radiation direction that was perpendicular to the large ring! Furthermore, the radiation direction of a ring has limitations. By using multiple spatial orientations that are perpendicular to each other, it's possible to create an omnidirectional antenna. Examples include half-rings, double rings, and triple rings, all of which have altered their beam patterns:
Three-ring antenna:
Half-wave antenna:
Logarithmic Period Antenna:
It is a broadband directional antenna, but in the non-radiated segment it exhibits wave propagation, and in the radiated segment it exhibits standing wave characteristics.
There are also some that are used for shortwave communication, but they tend to be quite large.
Equivalent oscillator:
The "folded dipole" antenna, literally meaning a dipole that is folded, results in a T2FD (folded twice) or T3FD (folded three times - also known as a tri-band antenna). It can be folded indefinitely, but due to the terminal resistor, it functions as a wave guide antenna with a relatively wide bandwidth.
T2FD:
T3FD (Three-wire antenna):
Second type of Yagi antenna:
In this case, the "BAM" (Yagi) driven oscillator is a lumped oscillator, so it combines directivity with a certain bandwidth.
Helical antenna:
Please note that some of you might be confusing the spiral antenna with the GP antenna, which has a coil. The entire spiral antenna is spiral-shaped and used to rotate current within the coil, making it circularly polarized. This type of antenna typically includes a reflector (a circular metal plate).
Rhomboid antenna:
It is also a variant of T2FD.
Waveguide V-Antenna:
It is half of a rhombus-shaped antenna.
Various waveguide antennas:
What speaker, pot-like devices, etc., are also wave antennas. These antennas typically use very high frequencies, and the radio waves already exhibit a light-like property (microwave), but the feed of the pot can be any of the above forms, and the speaker can be square, round, or other unusual shapes...
A brief closing remark.
Antennas can be designed in various unusual shapes and also incorporate multiple buffering effects.
For example, a set of four-arm Yagi antennas can be designed for dual-band operation with vertical and horizontal polarization. Alternatively, if the four arms are connected at a 90-degree angle, it creates a Yagi antenna with 45-degree polarization.Do they look alike?
For example, short-wave signals from multiple frequency bands and different radiation directions can be configured as a positive V shape in the A band, a horizontal line in the B band, an inverted V shape in the C band, and then connected with insulated wires to hang.Doesn't it resemble a sector antenna?
Therefore, to understand antennas properly, it's important to carefully examine their principles. Once you have a general understanding and the ability to imagine how they work, understanding antennas becomes much easier.
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