Traditional antennas are designed according to the half-wavelength resonance theory of Hertz's theorem. When applied in the medium wave broadcasting frequency band, the height is generally about 60 to 150 m according to the frequency. In order to ensure the radiation effect and improve the radiation efficiency, a radial ground web having a diameter of about 0.5λ must be placed around the antenna mast or the tower body. This brings a series of problems: waste of limited land resources, investment in huge construction costs, increase in daily maintenance costs, low-cost uniform coverage of large and medium-sized urban areas, space and environmental pollution of strong electromagnetic waves, and elevated towers. The probability of lightning strikes and the safety of the tower itself.
Therefore, some people have been studying the miniaturization of medium wave antennas. However, because there is no breakthrough in the basic theory and deployment method of single-excitation resonant radiation, many of the ideal results have not been obtained. This is mainly manifested in four aspects: narrow provisioning bandwidth, small coverage, poor radiation efficiency and low carrier power. The root cause is that the short antenna's own radiation resistance Rr is too low, and the unnecessary reactance component is too large. Some research data show that when the antenna height is lower than 0.02λ, the bandwidth of SWR=2 will be less than 1%, which can not meet the requirement of all solid-state medium wave broadcast transmitter FO±9KHz, and the load SWR≤1.3.
Appropriately change the shape of the main radiator of the antenna (bold into a cylinder, load into a cap, called E cylinder), and insert a moderately sized, properly positioned secondary radiator between the main radiator and the ground plate (called the GP plate). (such as disc, called D disc), and simultaneously implement equal-power, non-resonant dual-excitation phase-shifting feeds on the two radiators. Antennas with a height lower than 0.01λ can be used at any frequency point of the medium wave broadcast. Solid-state medium wave broadcast transmitters achieve good matching and operate efficiently. We named this antenna: the Double Excita TIon Medium Wave Short Antenna.
DEMA has very obvious features:
1. Small size and small footprint.
DEMA, which operates in the frequency band of the China Broadcasting Center, has a GP board area of ​​36 m2, which is 2 17 of 17672 m2 for the traditional antenna ground network.
2. Low height and good safety
At this time, the height of the E cylinder is 3 m, which is about 0.01 λ, which is 4% of the height of the conventional antenna. At this level, the possibility of lightning strikes and collapses of the antenna is greatly reduced.
3. Frequency bandwidth, stable operation
The actual deployment data shows that such a low and short medium wave broadcast transmitting antenna has a SWR of less than 1.2 in the frequency range of 1000 kHz ± 11 kHz, which is advantageous for ensuring stable operation of the system.
4. Inductive weak, strong suppression
Although DEMA has a wide operating band, it is very sensitive to other antennas or other antennas to DEMA and different DEMAs in the same antenna launch field. This is mainly due to the short electrical length of the main radiator. The space electromagnetic field in the field area is small in size and low in height. It is necessary to implement equal power, non-resonant and double excitation phase shift feeds for effective operation and specific deployment network with band pass filter function. External energy is determined by strong suppression properties. This opens up the possibility of erecting multiple DEMAs in a relatively small area and simultaneously working to launch or gradually add DEMA to a traditional antenna launch site.
5. Low consumption and uniform coverage
Due to the above characteristics, DEMA can also effectively solve the problem of low-consumption uniform coverage and strong electromagnetic wave space and environmental pollution in large and medium-sized urban areas. The traditional antenna is limited by the height and the grounding network needs a large area of ​​land. It can only be set in the suburbs away from the center of the city. The radiated electromagnetic wave energy can only enter the urban area from a certain direction, in order to complete the farthest edge of the city. The coverage must increase the transmitter's transmit power, which will result in long-term investment in huge operating costs, while also creating unnecessary space and environmental pollution problems with strong electromagnetic waves. DEMA can be erected on the top of any floor in the city center and fed with a smaller power transmitter to achieve better uniform coverage. For large cities and cities with special shape distribution, multi-point low power can be used. Synchronous broadcast mode to achieve ideal coverage. If the implementation of the medium-wave broadcast medium and small power transmission is unattended, the operating costs will be further reduced.
Second, the working principleThe antenna structure is composed of a main radiation tube E, a sub-radiation disk D, and a ground plate GP.
When the E-cylinder is separately subjected to resonant feeding, a horizontal annular magnetic field HE is generated between the E-cylinder and the GP plate while generating a vertical curved electric field EE. The HE at this time lags behind EE 90°, which is the phase characteristic of the near-inductive field of a typical electric fundamental oscillator. As the electromagnetic energy diffuses to the surroundings, due to the difference of the transmission path and the change of the electric field strength complex vector and the magnetic field strength vector correlation component, the phases of EE and HE will gradually approach until they reach the same phase at a certain distance, and are synthesized. The PoynTIng vector forms radiation (partial electromagnetic energy exits the power source to the space at infinity), which is called the interacTIon zone.
In fact, the induced electromagnetic field decays very rapidly with increasing distance, and the energy that can reach the interaction zone to synthesize the Baining vector to form radiation is very small, especially for low and short antennas. At the same time, the path of the magnetic field decays faster than the electric field. This is the root cause of the low radiation efficiency and narrow bandwidth of such antennas.
If the D-disk and the E-tube are simultaneously applied with equal-power, non-resonant and double-excitation phase-shifting, the superposition of the magnetic field HE and HD causes the distance of the interaction zone to approach the antenna body, thereby achieving a relatively high energy state. The glassytin vector synthesis (the synthetic electric field EC is in phase with the synthetic magnetic field HC) and forms radiation. At this time, due to the common radiation of the D disk and the E cylinder, the two radiation resistors RD and RE are simultaneously enlarged, and effective radiation is realized while ensuring the working bandwidth, so that the broadband effective radiation having a height lower than 0.01λDEMA becomes a reality.
In addition, the tapered cap loaded on the E-cylinder also has the effect of compressing the main lobe in the vertical direction to improve the radiation performance of the ground wave.
The vertical field type is compared with the traditional λ/4 oscillator antenna as follows:
The horizontal field type is the same as the traditional λ/4 oscillator antenna, which is a non-directional concentric circle.
Since the radiation formed by the interaction between the D disk and the E cylinder brings the interaction zone closer to the antenna body, the field strength distribution characteristics of the antenna and the conventional λ/4 oscillator antenna are as follows:
Theoretically speaking, DEMA belongs to the category of non-resonant antennas. It also has the characteristics of low voltage of the disk and the cylinder, and its working bandwidth has nothing to do with the size and height of the antenna. However, the high-power wideband phase shifter is difficult to manufacture after all. The operating bandwidth at the low end of the mid-wave band will be lower than its high end, and the radiation efficiency will be lower. Therefore, in order to achieve better working results at the lower end of the mid-wave band, the size of the DEMA is still larger, which is of course beneficial for increasing the capacity of the feed power.
Third, the main indicators1, interface impedance 50Ω or 75Ω (working frequency)
2. Interface form IF45 (≤5Kw/3m, ≤10Kw/6m)
IF70 (≤50Kw/9m)
3, impedance matching f = fo, S ≤ 1.1;
Δf ≥ 18KHz, S≤1.3;
Δf ≥ 20KHz, S ≤ 2.0
4, the polarization form is vertical
5, antenna gain 0--3dB
6, antenna efficiency ≥ 80%
7, the installation location can be flat and roof
8, antenna spacing transmit power traditional antenna DEMA
1Kw 15m 8m
5Kw 24m 12m
10Kw 30m 15m
30Kw 40m 20m
50Kw 50m 25m
9, wind resistance can withstand 11
10, the service life ≥ 30 years
Fourth, the installation formFlat installation
The connection method for the flat installation is as shown below:
The specific installation form is as follows:
The height of the grounding plate to the ground is 2~3m. When the working frequency is low or the input power is large, this height is higher.
Roof installation
The connection method for the roof installation is as shown below:
The specific installation form is as follows:
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