What Is a Monopole Antenna?

A monopole antenna is a vertical antenna with a quarter wavelength. The antenna is mounted on a ground plane, which can be the actual ground or an artificial ground surface such as a vehicle-mounted vehicle body. The monopole antenna is fed using a coaxial cable at the lower end, and the ground conductor of the feeder is connected to the platform. In free space, the radiation pattern of the quarter-wave monopole antenna in the vertical plane is similar to that of the half-wave dipole antenna in the vertical plane, but there is no underground radiation. On a horizontal plane, a vertical monopole antenna is omnidirectional.

In order to meet the requirements of software radio and UWB (ultra-wideband) systems for antennas, some people want to design such an antennaan antenna that is broadband and can cover all the required frequency bands of wireless terminals. Broadband planar monopole antennas meet this condition

The shape of the planar monopole antenna can be square, circular, oval, or triangular. Considering the convenience and practicality of production, this article only studies square and square cut-angle antennas. The material of the antenna can be selected from a copper or iron plate with a thickness of about 0.5mm and cut into squares. The grounding plate can be a square copper-clad printed circuit board with a side length of 100-200mm. A 4mm round hole is drilled at the feed point, and an SMA socket is installed, and the core wire is exposed above the ground plate as the feed point of the antenna. The square planar antenna is welded vertically on the core wire of the SMA socket, and the feeding point is the midpoint of the bottom edge of the planar antenna, leaving a feed gap h during welding.

The square-shaped monopole antenna with no cut angle has a narrow impedance bandwidth, but its four variations can widen the antenna's impedance bandwidth to varying degrees. The first variant is to connect a grounding plate with a 1mm diameter conductor or a 2mm wide copper sheet at a corner of the bottom edge after making a square antenna. It is also called shorting post. Hereafter referred to as a-shaped antenna; the second variant is to cut a triangular notch on one side of the bottom edge, and the angle can range from 10 ° to 64 °, hereinafter referred to as b-shaped antenna; the third variant is to After making the b-shaped antenna, add a short-circuiting branch on the other side of the bottom, as shown in Figure 1 (a); the fourth variant is to build the b-shaped antenna on the other side of the bottom A triangular notch is cut symmetrically, hereinafter referred to as a d-shaped antenna, as shown in FIG. 1 (b). The reason why the square antenna is modified in four ways is to increase the impedance bandwidth of the antenna and reduce the size of the antenna.

According to a large number of experiments, the gap between the bottom edge of the plane and the ground plate will affect the maximum impedance bandwidth of the entire antenna. The symmetrical cut-angle antenna will leave a sharp corner after the cut, especially when the number of cut angles is large. Proper trimming of the left sharp corner can obviously improve the standing wave ratio and impedance bandwidth. Cut symmetrically in a 65mm x 65mm plane. The angle antenna is taken as an example, the number of cut angles is 40 °, and the height of the cut angle is 3mm, the impedance bandwidth is optimal. The 2: 1 VSWR bandwidth is 6.38GHz, the lower limit frequency is 1.99GHz, and the upper limit frequency is 8.37GHz. ^[2]

In order to obtain the relationship between the antenna side length and the cut angle and the impedance bandwidth, the impedance bandwidth of 7 copper a-shaped antennas with side lengths of 35-65mm was measured using a network analyzer to obtain the antenna side length and the 2: 1VWSR impedance The relationship of bandwidth. As the size of the antenna increases, the upper and lower frequencies of the impedance bandwidth of the antenna will decrease, and the impedance bandwidth will also decrease accordingly.

There are two kinds of materials used to make the antenna: 0.5mm thick copper plate and 0.2mm thick tinned steel plate. These two materials are made into the same shape antenna.

According to statistics, with the same shape of antenna, the impedance bandwidth of a 0.2mm tinned steel plate antenna is 5% -10% narrower than that of a 0.5mm copper plate antenna. Much taller. Low production cost is another great advantage of planar monopole antennas ^[3] .