PCB Antennas

PCB antenna structures can be monopole, dipole, slot, patch, and planar inverted F (PIFA).

Dipole and monopole antennas

Dipole antennas consist of two quarter-wavelength (N/4) long metal strips with a total electrical length required to be half a wavelength (N2) below the carrier frequency for maximum response.
Due to the widespread use of legacy protocols such as Bluetooth and Wi-Fi, much of the antenna literature focuses on antenna design in the 2.4 GHz ISM band. At 2.4 GHz, the length of the dipole antenna is about 6 cm, which may be too large for some applications. The omnidirectional radiation direction map has a concentrated electromagnetic energy perpendicular to its length (z-axis) with a zero point in the middle (along the z-axis).
This usually requires placement in the vertical direction to obtain maximum horizontal summer coverage one by one for designers intending to use dipole (or monopole) antennas in IoT devices, which is a cause for careful consideration.
The addition of a grounding element eliminates one arm of the dipole, resulting in a structure that includes a single quarter-wavelength conducting element with the grounding element acting as the other quarter-wavelength arm.
Similar to a mirror, the grounding element generates a virtual image of the same directional map (same direction and phase of current flow) as the conducting element.
In order for the grounding element to function optimally, it must be guaranteed to be much larger than half a wavelength. A finite grounding element leads to edge diffraction of the radiated wave, which causes the electromagnetic energy to move radially outward from the edge and leak behind the grounding element.
Ideally, for a perceptually infinite ground element, the gain of a quarter-wavelength monopole antenna should be twice that of a half-wavelength dipole antenna, because the ground element radiates only to the area above it.
This will lead to the formation of some degree of lift direction map. Again, create a zero point along the length of the conducting quarter-wavelength element (z-axis) to show the desired vertical orientation.

Bending monopole antennas

The bent monopole variants of L-shaped (or tilted whip and inverted L), planar inverted F (PIFA) and zigzag inverted F (MIFA) structures utilize the same grounding element and conducting arm approach, but with a different conducting arm pattern.
The bending of the alignment allows for more compact dimensions as well as a reduction in the zero point in the radiation direction diagram.
However, in these designs, the antenna alignment usually cannot be too close to the ground, which would cause the antenna to act as a transmission line without any radiation.
Among these variants, PIFA is the most common because of its smaller PCB area and wide bandwidth performance.
The PIFA has a fold on the main resonant line which introduces a capacitance that is offset by a short-circuit feed point at the end of the line.
Matching impedance to the antenna feed source is obtained by adjusting the lengths of A, B, and C, where A and C are sized to benefit the quarter-wavelength element, and B is adjusted for matching impedance. An external matching network can also be incorporated for additional adjustment.
The MIFA antenna increases the total length and resonates through different directions of alignment/line compression, thus resembling a spiral antenna. In MIFA, the horizontal and vertical lines form multiple turns.
As with PIFA, tuning is accomplished by adjusting the geometry within the zigzag lines. This effectively reduces the size of the antenna.
However, there are advantages and disadvantages, and other aspects may be affected, such as reduced gain and consequently reduced range, increased tuning dependence and consequently narrower bandwidth, and increased sensitivity to interference from external sources as well as local electronics on the board.

Dipole Antenna Variants

Printed dipole antennas mainly face a size disadvantage. This is especially relevant for low frequencies;
Here, the advantage of a monopole antenna element is its potential placement in a corner of the board, where the grounding element is used both as an RF ground plane/return path for the antenna and as a grounding element for the local electrical components of the crop networking module.
However, as the operating frequency increases, this disadvantage becomes less prominent.
In addition, different dipole patterns can be used to increase bandwidth and reduce the size as with bent monopole antenna structures.
Making the dipole arm gradually narrower is a known bandwidth enhancement technique.
For example, the plane tapered dipole antenna (also known as butterfly antenna) behaves as a uniform tapered transmission line. When the RF signal is applied to the feed source, the current will flow radially outward and form a closed magnetic field.
The current terminates abruptly at the edge of the butterfly, resulting in more reflections and narrower widths compared to its 3D biconical shape.
Nevertheless, the antenna can achieve relatively constant impedance and gain over a large frequency range relative to other dipole antennas.
Along the same line of reasoning found in the bent monopole quarter-wavelength antenna, a smaller form factor can be obtained by bending the conducting arm of the dipole element.
As mentioned earlier, this optimizes the Q but reduces the overall bandwidth and efficiency. Therefore, the use of self-resonant space-saving techniques (e.g., spiral or bent conductive arms) in applications must be weighed against the pros and cons.