Antenna Performance Simulation Research Based on Miniaturization Design of Shortwave Wideband Antenna

The sector cone antenna is a short-wave broadband antenna with superior performance, which is widely used in communication fields such as broadcasting and maritime transportation. But when its frequency of use is low, the area is larger. Therefore, it is necessary for the miniaturization of its low-frequency use. Through effective loading means, it can not only meet the impedance matching characteristics, but also meet the needs of communication performance.

An ideal biconical antenna has arbitrary frequency band characteristics. The highest operating frequency of the operating frequency band of the biconical antenna depends on the geometric size of the excitation area, and the lowest operating frequency depends on the length of the antenna arm. When the total length of the antenna is half the wavelength of the lowest operating frequency, it generally has good impedance frequency characteristics in the range of 10 times the frequency. This objectively determines that an antenna with a lower working frequency limit has a larger span and floor space.

The characteristic impedance of the biconical antenna is:

among them,

Is the apex angle of the cone. For example, when Z0=300Ω,

=18.77º, in this case, a bipolar feed antenna with an impedance of 300Ω is applicable. In the short-wave band, due to the low frequency of use and the larger size of the cone antenna, it is often simplified to a wire grid cone structure. When erected horizontally, the antenna is simplified into a fan-cone structure, as shown in Figure 1.

Choosing reasonable fan angle, opening angle, number of wires, size of excitation area and other parameters of the fan cone can obtain a wider bandwidth.

Biconical antennas can be analyzed by analytical methods, and many documents have also made detailed introductions. It is too complicated to strictly derive and calculate the sector cone antenna, and the method of moments can be used to calculate. This paper uses FEKO electromagnetic simulation software to verify the numerical calculation results. Design a fan-cone antenna with a frequency range of 2MHz-30MHz and a supporting tower distance of 76 meters. The antenna is installed on the actual ground, and the ground parameters are set to εr=15 and δ=0.01.

The number of sector cone antenna wires is 11, the sector cone angle is 25 degrees, and the antenna opening angle is 100 degrees. 3.1 Calculation results of the directivity of the sector cone antenna

The calculation results of the FEKO software of the antenna directivity are basically consistent with the numerical calculation results provided in the literature. The horizontal plane has good omnidirectional performance. The maximum directional gain on the vertical plane is greater than 5dB. 3.2 Matching situation of antenna and 300Ω parallel double wire (voltage standing wave ratio)

Antenna and 300Ω matching voltage standing wave ratio

The above is a basic introduction to the performance of a full-size fan-cone antenna. As described in the introduction and antenna form and performance analysis, the span of the antenna is determined by the wavelength of the lowest operating frequency. The lower the frequency and the longer the wavelength, the larger the corresponding footprint. This is relatively difficult to achieve under the situation of increasingly tight land resources. Therefore, there is a need for a method that can not only ensure the performance of the original antenna, but also significantly reduce the size of the antenna to solve this problem.

Design a fan-cone antenna with a frequency of 2MHz-30MHz and a support tower distance of 53 meters. The antenna is installed on the actual ground, and the ground parameters are set to εr=15 and δ=0.01. The number of sector cone antenna wires is 11, the sector cone angle is 25 degrees, and the antenna opening angle is 100 degrees.

4.1 Calculate data when the antenna is not loaded

4.1.1 Antenna voltage standing wave ratio

Unloaded antenna and 300Ω matching voltage standing wave ratio

4.1.2 Antenna impedance data

Unloaded antenna impedance curve

4.2 Calculate the data after the antenna is loaded

It can be seen from the above data that when the antenna span is shortened, the real part of the antenna's low-frequency impedance is significantly reduced, and the imaginary part is more inductive. Must pass effective loading, make the low frequency band of the aerial reach the matched state.

We load a section of dipole wire at the end of the dipole on the top of the antenna, as shown in Figure 2. By continuously optimizing the length of the loading wire, we can adjust the real part of the low-frequency impedance of the antenna. By loading a capacitive device at the feed end, the imaginary part of the antenna impedance is effectively adjusted. Finally, the low frequency band will meet the conditions of impedance matching.

4.2.1 Antenna directivity calculation results

2MHz vertical plane pattern

2MHz horizontal plane pattern (Δ=maximum direction)

16MHz vertical plane pattern

16MHz horizontal plane pattern (Δ=maximum direction)

30MHz vertical plane pattern

30MHz horizontal plane pattern (Δ=maximum direction)

Compared with the original size design performance, the directional results after loading have slight changes in out-of-roundness, but the horizontal omnidirectional performance is still good. The maximum directional gain on the vertical plane is greater than 5dB.

4.2.2 Antenna voltage standing wave ratio

Loaded antenna and 300Ω matching standing wave ratio curve

4.2.3 Antenna impedance data

Loaded antenna impedance curve antenna original size design and performance comparison after miniaturization design

The performance comparison of the two antennas is shown in Table 1 below.

Table 1 Two kinds of antenna performance comparison table

Antenna form frequency range span voltage standing wave ratio maximum directional gain

Original size design 2MHz-30MHz 76 meters maximum 2.34 greater than 5dB

Load design 2MHz-30MHz 53m maximum 2.17 is greater than 5dB

It can be seen from the data in the above table that the performance indicators of the two antennas are equivalent, but the load design is 23 meters shorter than the original size design.

This article introduces the basic performance and form of the sector cone antenna, and uses the FEKO electromagnetic simulation software to carry out the simulation calculation of the antenna performance. Finally, through the effective loading design of the antenna, the span of the fan cone antenna from 2MHz is reduced from 76 meters to 53 meters. The calculated antenna performance index is equivalent to the original design. This is of great significance for future sector cone antenna projects under limited site conditions.

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