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Many friends asked me how to realize multi-frequency antenna, indeed for beginners friends of antenna design, multi-frequency antenna is a big difficulty, but it's had to overcome and master key, today I will give you in a detailed analysis on the multi-frequency antenna implementation plan total main method,
A, resonance branch law
Branch method is used the most, is also one of the most easy to understand multifrequency realization method of the traditional GSM/DCS/PCS and dual-band WIFI antenna is widely used in the design of, its the easiest way to understand is a branch of the antenna, corresponding to a working frequency and between each branch is independent of each other, advantages of this method in the frequency tuning is very obvious, because of the relatively independent branch, so in regulating at a frequency will have great influence on the other band, now we can look at a few simple set of pictures, as follows:
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Figure 1 branch method of dual-band WIFi antenna
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Figure 2 branch method is used to GSM/DCS antenna
From figure 1 and figure 2, the two are branch of multi-frequency antenna using the method, in figure 1, the frequency for dipole antenna, the shorter branches corresponding to 5 g wifi frequencies, and long branches corresponding to the wifi frequency of 2.45 GHz in figure 2 to 2 g mobile phone unipolar substructure form, through proper feeding position can be adjusted its effective length of long branch and short branch, and adjust the working frequency of high frequency and low frequency, now I'm using HFSS software design of a simple dual-band wifi antenna, to study the effect of each branch of antenna total set frequency,
Model and the parameter variables defined as shown in figure 3 and figure 4, the dipole day thread rotate 360 degrees is symmetrical, so only need to consider the design size can, another arm to the rotation of the simple copy can only,
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Figure 3 model variables defined
Initial size of the model calculation (PCB antenna based on FR4 medium) : determine the working frequency band of the antenna, the antenna to achieve 2.45 GHz and 5.5 GHz two frequencies, the length of each branch is between 0.25 ~ 0.25 medium wavelengths corresponding frequency between the free space wavelength, using long branch achieve 2.45 GHz resonance, the long branch length is 15 mm & lt; L1 + H<30 mm, the length of short branches 6 mm & lt; L2 & lt; 15 mm, so the initial values can be classified as L1=18 mm, H=5 mm, L2=7 mm, the L1 + H 2.45 GHz resonance, L2 resonance, achieve 5.5 GHz model as shown in figure 4:
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Figure 4 HFSS established three-dimensional model
Finally after HFSS simulation software to optimize the parameter value determined as follows: L1=18.5 mm, H=3 mm, L2=7 mm, the S11 parameter solution as shown in figure 5, can be seen from the diagram, the antenna on the 2.45 GHz and 5.5 GHz band has very good impedance and bandwidth characteristics, meet the job requirements of dual-band WIFI,
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Figure 5 S11 simulation figure
Now to analyze different branch changes affect antenna working band.
L1=16.5 mm, 18.5 mm and 20.5 mm, see the antenna, the change of the working frequency band, as shown in figure 6:
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Figure 6 L1 influence on S11
Can see from figure, when L1 increases gradually, the antenna working in low frequency band gradually decreases, and the overall work frequency band and high frequency are almost not affected, so you can come to a conclusion that L1 band of low-frequency work has a decisive influence, and the influence on high frequency almost negligible,
Also, keep unchanged L1 and L2=5 mm, 7 mm and 9 mm, S11 simulation results are shown in figure 7:
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Figure 7 L2 affect S11
From the figure, the L2 changes, high frequency part of the antenna resonance appears very large fluctuations, and low frequency part is almost not affected, so can conclude that branching structure has the relative independence between the multi-frequency antenna branches,
Note: the branching structure is usually used in two frequency effect is more ideal, when the band than three frequency bands or branch of the different length of more than three side, between the branches of entangled get bigger, troubles caused antenna performance of each frequency band, so much branched structure suggest no more than three branches, when the design
Second, design of frequency doubling
Frequency doubling method a lot of friends all don't know, frequency doubling the simplest explanation is a branch implement multiple frequencies, the use of the principle of harmonic, the requirement of working frequency f as a fundamental, depending on the basic theory of electromagnetism, can know the signal except on the fundamental wave frequency f produce resonance, would trigger a second harmonic, harmonic, etc., three times in 単 branch of antenna design, may, by way of some structure reasonable using the harmonic characteristic to realize 単 branch structure antenna multifrequency resonance,
For a standard structure of monopole and dipole antenna (as shown in figure 8), if the antenna base wave resonance frequency f0, the antenna at the same time will be in 3 f0, 5 f0, 7 f0... , (2 n + 1) f0 produce resonance, namely the antenna will be an odd number of times of the fundamental frequency resonance, even times because of the harmonic on the offset effect and does not produce resonance, as shown in figure 9,
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The structure of the figure 8 standard dipole antenna
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Figure 9 antenna frequency doubling resonance
Shown in figure 8 antenna for work at 900 MHZ standard PCB dipole antenna structure, its an arm length is 75 mm, 0.25 medium between fundamental wave wavelength between 0.25 free space wavelength,
Figure 9 can see very clearly that the antenna at 900 MHZ, 2700 MHZ, 4500 MHZ produce resonance, but S11 value the more backward, the poor, so can be concluded that the multi-frequency harmonic can be used for antenna design,
May have many friends, of course, such a question, multi-frequency 単 branches are in 3 times of fundamental wave, while the actual antenna few multi-frequency antenna in the design of high frequency resonance point just appeared on the odd times of fundamental wave, the frequency multiplication principle still can use?
The answer is, of course, can use, I in the above examples demonstrate the dipole antenna is for standard, also is not do any bending dipole methods such as processing standards, so no processed monopole dipole or the high frequency resonance are close to an odd number of times the fundamental wave frequency points, that's the theory, but when to bend structure after processing, such as antenna of high frequency resonance frequency points will slowly become low, as demonstrated above antenna, if dual very reasonable bending process, the high frequency resonant frequency can be adjusted to 1700 MHZ, 2400 MHZ, and the fundamental wave frequency low frequency won't change, so through the process of the structure of the free adjustment of the antenna times point, now on the market of the common spring multi-frequency antenna is the use of such a design principle, by adjusting the spring coil radius, pitch, parameters such as frequency and bandwidth, can get the ideal job
Three, and parasitic branch to add
Parasitic branch by adding a short circuit is the main principle of coupling perks to produce high-frequency resonance or develop a way of high frequency bandwidth, parasitic branch directly after joining, can produce coupling effect, and the primary branches antenna is equivalent to the primary branches and added a capacitor between parasitic branch, native directly through the coupling way to parasitic branch feeder, parasitic branch will be produced and its structure size corresponding to the point of resonance frequency, so as to expand the working bandwidth of the whole antenna,
As shown in figure 10 is a by adding a parasitic branch to expand examples of high frequency bandwidth, the antenna is a work in the GSM/DCS/PCS spectrum phone PIFA antenna, as follows:
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Figure 10 add spurious branches of
2 g mobile phone
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figure 11 after add spurious branches S11 Figure 12, antenna primary branches through multiple M1 and M2 two working frequency points, increase the ground after the parasitic branches, antenna broaden out the M3 frequency point, because of M2 and M3 near the comparison of two frequency point, so after the superposition of M2 and M3 to greatly expand the antenna bandwidth in the high frequency part of the work,
Parasitic branch to add, of course, there are multiple and varied ways, can be added to the feed end to figure 10, can also be added to the end of the antenna, directly through the coupling to produce new work frequency point (as shown in figure 12), or can expand the bandwidth by coupling feed way (as shown in figure 13),
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Figure 12 end add spurious branches
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Figure 13 coupling feed
See the above content, maybe some friends will ask, I speak in the above method seems to be only for only 2-3 working band design, then in the presence of additional spectrum and should be how to design, such as whole netcom's LTE antenna, the requirements of the antenna working band can cover the current operator support exactly ten five frequency, five modules work directly 17 frequency or frequency to describe is able to cover 790-960 MHZ and 1600-2700 MHZ, so like this antenna should be how to design?
Actually this kind of antenna design is not difficult, just need to me in the concentration of foregoing multifrequency method for the combined application can be simple, that is to say, in a pair of antenna may also use frequency doubling, much branched, add parasitic branch and so on a variety of ways, as long as reasonable, adjust the structure and size can be designed to meet the requirements of LTE antenna, of course, the design of the antenna processing simulation also needs a lot of debugging and testing to complete, this process will take some time,
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