Study on the characteristics of electromagnetic-magnetic vibrator combined ultra-wideband UWB antenna

This paper starts with the physical structure analysis of the electromagnetic-magnetic combined antenna, combines experiments with numerical simulations, and uses a combination of frequency domain and time domain measurements to study the characteristics of the electromagnetic-magnetic combined antenna.

1 Theoretical Analysis

1.1 Antenna Structure

Figure 1 is a schematic diagram of the structure of an electro-magnetic dipole combined ultra-wideband antenna [2]. In the figure: ① is the antenna coaxial feeding area; ② is the outer conductor plate; ③ is the current loop regulator; ④ is the upper plate of the TEM horn; ⑤ is the lower plate of the TEM horn.

Figure 1 Schematic diagram of the structure of the electromagnetic-magnetic vibrator combined UWB antenna

1.2 Antenna physical structure and characteristics analysis

The physical structure of the antenna is closely related to the antenna performance. The mismatch between the input impedance of the radiating antenna and the characteristic impedance of the ultra-wide spectrum pulse source causes reflections at the antenna feed to varying degrees. From the antenna structure shown in Figure 1, after the excitation pulse (as shown in Figure 2) enters the coaxial feeding area, a pulse current with broadband characteristics (as shown in Figure 3) is fed into the antenna. Part of the current radiates to the free space through the current loop (or magnetic vibrator) formed by ①, ③, and ②, and at the same time generates reflected waves and heat loss (due to the low upper frequency limit of the excitation pulse, the heat loss of the antenna can generally be ignored); the other part of the current radiates to the free space through the impedance gradient TEM horn (mainly manifested as an electric vibrator radiator) formed by ①, ④, and ⑤, and also generates reflected waves.

Fig.2 Excitation pulse waveformFig.3 Excitation pulse spectrum

According to the above analysis, the equivalent circuit of the antenna can be obtained, as shown in Figure 4. Rring and Rtrumpet are the radiation resistances of the magnetic oscillator (current loop) and electric oscillator (TEM horn) of the antenna, respectively, and both have a nonlinear relationship with the frequency f of the excitation signal.

Figure 4 Equivalent circuit diagram of electro-magnetic dipole antenna [page]

The current loop in the antenna is a parallel resonant circuit. As the frequency increases, the current loop gradually changes from a low-frequency short-circuit load to a radiator based on magnetic oscillators, and the radiation resistance Rring of the magnetic oscillators also increases accordingly. For low frequencies, the current loop is a small loop radiator, equivalent to a magnetic basic oscillator, and its radiation characteristics are equivalent to those of a magnetic basic oscillator; for high frequencies, it is equivalent to a large current loop radiator, and the large current loop radiation theory can be used to analyze its input characteristics and radiation characteristics.

In the electromagnetic-magnetic dipole combined ultra-wideband antenna, the TEM horn is equivalent to a series resonant circuit. As the frequency increases, the TEM horn gradually changes from a low-frequency open-circuit load to a radiator based on the electromagnetic dipole, and the radiation resistance Rtrumpet of the electromagnetic dipole also changes accordingly. For low frequencies, the most basic physical model of the TEM horn is a dipole antenna, and its radiation field is the vector superposition of several dipole fields. Its time domain radiation field expression [4] is:

Where: f(g) is the ratio of the characteristic impedance of the TEM horn to the impedance of free space, δ(a)(t) is the impulse function, h is the height of the horn mouth, l is the length of the horn, and V0 is the amplitude of the step voltage fed into the antenna.

As the frequency increases further, the TEM horn turns into a high-frequency short-circuit load. When the frequency f is very high, the electric oscillator of the TEM horn and the magnetic oscillator of the current ring are seriously detuned and are in a short-circuit and open-circuit state respectively, resulting in a large reflection at the feed source.

In the combined electric-magnetic dipole ultra-wideband antenna, the far-field radiation fields of the electric dipole and the magnetic dipole are both vertically polarized waves. By adjusting the parameters Lring, Ctrumpet and phase center distance of the magnetic dipole and the electric dipole, the two antenna dipoles can form complementary electric-magnetic dipole radiation [3], thereby reducing the dependence of the antenna's radiation resistance on the signal frequency, expanding the antenna's operating frequency band, reducing the reflection caused by the mismatch of the antenna load, and making the spatial transient radiation fields of the two radiators superimpose on each other, thereby maximizing the antenna's radiation efficiency.

2 Simulation calculation

Based on the above analysis, numerical simulation software was used to simulate the 50x50x50cm3 electro-magnetic vibrator combined ultra-wideband antenna. Figures 5 and 6 are the numerical simulation results.

(a) Antenna standing wave curve (b) Antenna input impedance diagram

Figure 5 Simulation results of 50cm electric-magnetic dipole combined antenna

Figure 6 Directivity diagram of 50cm electromagnetic-magnetic dipole UWB antenna at different frequencies in the θ=900 and φ=900 planes [page]

3 Experimental Results

Through simulation and analysis, an electro-magnetic dipole combined UWB antenna with a length, width and height of 50 cm was optimized and designed (as shown in Figure 7), and the antenna was tested using frequency domain and time domain measurement methods.

Figure 8 shows the antenna standing wave curve, impedance circle diagram and time domain reflection measurement curve measured by Anritsu's vector network analyzer MS4623B. The frequency domain measurement results show that the antenna's standing wave coefficient is less than 3 within the 10-fold frequency range of 100MHz~1GHz, which is basically consistent with the results of numerical simulation. From the time domain measurement curve Figure 8(c), it can be seen that the antenna's largest reflection point is at the output end of the feed source.

Figure 8 MS4623B network loss measurement results[page]

Figure 9 shows the waveforms of the incident pulse, reflected pulse and radiation field pulse measured by Tek TDS684 when the antenna is fed with an ultra-wideband pulse signal. Among them, the bottom width of the excitation pulse signal is about 2ns, the front edge is 350ps, and the peak voltage is 150V. The incident signal and the reflected signal use a non-inductive capacitor voltage divider (voltage divider ratio is 100:1), and the radiation field measurement uses a TEM horn measurement antenna with a bandwidth of 80MHz~2GHz and an effective height of 6.1cm. The measurement point is located 10m away from the mouth in the maximum radiation direction of the antenna. The maximum positive peak of the reflected wave is 29.8V, the maximum negative peak is -35.6V, and the electric field strength at the measurement point is 24.6V/m. The radiation efficiency of the antenna is obtained as Kw=Wr/Wg=60% (Wr=Wg-Wref is the radiation energy of the antenna, Wg is the excitation pulse energy, and Wref is the energy of the antenna reflected pulse).

Figure 9 Time domain measurement waveform

4 Conclusion and existing problems

Through the above analysis and research, it is concluded that the electro-magnetic dipole combination antenna uses the complementary state of the current loop and the TEM horn to achieve bandwidth expansion. If the antenna structure parameters are adjusted appropriately, the miniaturized UWB antenna can achieve wide bandwidth and high radiation efficiency. Although the antenna size is small, the coaxial transition feeding structure can solve the problem of high-power UWB pulse feeding. The antenna structure and transient pulse radiation problems are relatively complex, and it is not suitable to directly and completely use the time domain method. It is relatively easy to use the frequency domain method to study the structure and characteristics of the antenna, which creates conditions for the study of the time domain characteristics of the antenna.

The existing problems are: due to the complexity of the antenna structure, it is quite difficult to solve the current distribution of each part of the antenna. In addition, when performing simulation calculations on the electromagnetic-magnetic dipole combination antenna, due to the limitation of computing resources and the large difference between the established model and the actual antenna, errors are caused between the calculation and measurement results.