1. Field of the Invention
The invention relates to a coaxial conductor structure for fault-free transmission of a TEM basic mode of a RF signal wave.
2. Description of the Prior Art
The transmission quality of coaxial conductors for the TEM basic mode of RF signal waves decreases for increasing signal frequencies, given the fact that undesirable higher-order modes are able to propagate for higher frequencies, for example TE11, TE21 modes etc., which by way of mode conversion processes may be excited at interference locations and then come to overlay the TEM basic mode.
In particular with regard to future expansions of, or changes to, existing transmission ranges to incorporate higher frequencies for RF signals, which have been specified in the frequency usage plan for the Federal German Republic, it is important to look for ways of permitting an essentially fault-free high-frequency signal transmission of the TEM basic mode of RF signals via coaxial lines of a maximum possible diameter, so as to enable maximum possible transmission output for a minimum of losses.
In a contribution by Konoplev, I. V. et al; “Wave Interference and Band Gap Control in Multi-Conductor One-Dimensional Bragg Structures”, Journal of Applied Physics, vol. 97, Nr. 7, p. 073101-073101-7, April 2005, DOI: 10.1063/1, 1863425, a one-dimensional coaxial Bragg structure has been described, which is intended to selectively influence the propagation behavior of electro-magnetic waves by way of constructive and destructive interferences. To this end the coaxial waveguide structure is provided with a periodical structure of groove-like depressions on its inner and outer conductor walls, the geometric design of which impacts in different ways upon the reflection behavior of RF waves which pass through the corrugated coaxial conductor structure.
The coaxial conductor structure according to the invention is based on the knowledge that the transmission behavior of coaxial conductors for RF signal waves changes significantly if electrically conducting ring-shaped structures, “ring structures” for short, are fitted between the inner and outer conductor at respectively equidistant distances, which structures provide a completely surrounding current path, that is a current path closed in the ring circumferential direction. The ring-shaped structures are designed as separate structures and are disposed each so as to be radially spaced apart to both the inner and the outer conductor.
When observing the propagation behavior of the TEM basic mode along a conventional coaxial line, that is outer and inner conductors are electrically insulated by an intermediate dielectric. In terms of a dispersion diagram, there is a linear correlation which exists between the frequency f or circular frequency ω, and the propagation constant β of the RF signal wave of the form ej(ax-βx), that is ω=c(β). This linear correlation is represented in a dispersion diagram ω(β). See
If on the other hand, according to the representations in
When looking more closely at the dispersion diagram, it can be recognized that according to the invention two propagation channels form along the coaxial line shaped according to the invention for the respective propagation modes. These channels are an inner propagation channel (ic=inner core) between the inner conductor IL and the rings R, and an outer propagation channel (oc=outer core) between the rings R and the outer conductor (AL). For a suitable geometry choice for the coaxial conductor containing the ring structures, a frequency band window Δf forms between the TE11,ic mode propagating along the inner propagation channel and the TE21,oc and TE11,oc modes propagating along the outer propagation channel. The result is that on the one hand, the TE11 mode for lower frequencies propagates in the outer propagation channel, that is representing a TE11,oc mode, and for higher frequencies flattens, and that on the other hand, for higher frequencies, a propagatable TE11,ic mode and a propagatable TE21,oc mode form both along the inner propagation channel and along the outer propagation channel.
This flattening of the TE11,oc mode causes the frequency band window Δf to form, which towards higher frequencies is capped by the lower of the two lower cut-off frequencies fco,lower of the TE21,oc mode or the TE11,ic mode, and in which the TEM mode is able to propagate without interference, that is without being adversely affected by interfering higher modes.
Using the measure according to the invention, and given a suitable design for the ring parameters and coaxial parameters, a frequency band window may be created and utilized for example between approx. 6.8 GHz and 10.6 GHz for an interference-free propagation of the TEM mode. This knowledge can be derived by performing theoretical tests on an elementary cell which comprises a ring disposed between the inner and outer conductor and repeats with the periodicity p in longitudinal direction of the coaxial conductor structure on the basis of the Bloch Floquet theorem in conjunction with periodic marginal conditions. As such the upper and lower cut-off frequencies can be determined as a function of geometrical sizes by which the coaxial conductor structure can be characterized.
The upper cut-off frequency fco,lower of the frequency window can be determined approximately by the two lower cut-off frequencies fco,TE21,oc of the TE21,oc mode or the TE11,ic mode fco,TE11,ic, depending on which of the two modes has a smaller lower cut-off frequency, using the following equation:
wherein:
c:=speed of light
d1:=diameter of inner conductor
d2:=inner diameter of ring
d3:=outer diameter of ring
d4:=diameter of outer conductor
with d1<d2<d3<d4
The lower frequency fco,upper of the frequency window can, however, be characterized by the ring resonance frequency fco,TE11ring in the following manner:
Further a closer look at the dispersion diagram shown in
fco,TEM≈c/2p
In the above approximation c is the speed of light and p is the axial length of an elementary cell, see also
On the basis of this knowledge according to the invention a plurality of tests has been carried out in order to check the robustness of the above-discussed effect. That is the targeted creation of band gaps in which an interference-free propagation of the TEM mode becomes possible. The following embodiments show possibilities, where an interference-free propagation of the TEM mode can be observed within a frequency window Δf forming due to the measure according to the invention and where in addition a targeted influence can be exerted upon the propagation behavior of the modes involved.
Using the design of a coaxial conductor according to the invention and given a suitable design and geometry choice of the ring-shaped structures fitted between the inner and outer conductor of the coaxial line, a low-pass filter function for RF signals can be realized in that the ring-shaped structures are respectively connected with the outer conductor via at least one electrical connecting web, preferably via two, three or more electrical connecting webs, wherein the electrically conducting connecting webs, where providing two or more connecting webs, are evenly distributed in the circumferential direction along the ring-shaped structures between these and the outer conductor. The connecting webs form local electrical connections between the ring structures and the outer conductor and represent local inductivities, so-called shunt inductivities. Again, this results in varying propagation behaviors for the inner and outer propagation channels, as described above, but now for the propagation behavior of the TEM mode. By way of theoretical tests on an elementary cell which comprises a ring arranged between the inner and outer conductor, which is connected with the outer conductor via at least one electrically conducting connecting, web called a “spoke” in the following, and which cell repeats with the periodicity p in longitudinal direction of the coaxial conductor structure, a band gap can be ascertained on the basis of the Bloch-Floquet theorem in conjunction with periodic marginal conditions, in which the TEM mode is not able to propagate. This band gap is limited by an upper fo and a lower fu cut-off frequency, which can be determined as a function of geometric sizes of the coaxial conductor structure in the following manner:
assuming only one spoke between ring and outer conductor; for two or more spokes similar empirical formulae can be developed;
Typically the upper cut-off frequency fo of the band gap can be determined approximately by three lower cut-off frequencies, depending upon which of the three cut-off frequencies has the smallest value, that is fTEM,oc for the TEM mode capable of propagating in longitudinal direction of the outer propagation channel, fTE11,ic for the TE11,ic mode capable of propagating in longitudinal direction of the inner propagation channel, and fTEM,mix for the TEM mode capable of propagating in both propagation channels with respectively anti-parallel E field orientations.
A further preferred embodiment of the coaxial conductor structure provides for the use of ring structures between inner and outer conductor which can be divided into two groups as regards their shape and/or size, wherein structurally identical ring structures are contained in each group.
The arrangement of the ring structures along the coaxial conductor is chosen such that the group affiliation of the ring structures alternates bi-periodically with axial sequence between inner and outer conductor. Due to this measure the transmission quality of RF signals along the coaxial conductor structure can be significantly improved.
The invention will now be described, without limiting the general inventive idea in any way, by way of exemplary embodiments with reference to the drawings, in which:
A first embodiment of the invention provides for the periodic arrangement of n, which is greater three individual rings R along the coaxial conductors. See
As a variation from classically designed rings R, the effect according to the invention can be observed also in coaxial conductor structures which comprise an inner conductor IL′ and an outer conductor AL′ which in turn deviate from the classical circular coaxial geometry. An arrangement of this kind is schematically shown in
A further embodiment is based on the ring arrangement according to the embodiment illustrated in
In a further embodiment shown in
Another way of influencing the dispersion properties of the coaxial conductor structure designed according to the invention with regard to the progression or propagation behavior of the TEM modes is via the capacitive coupling of two adjacently arranged ring structures. Tests in this respect have shown that the higher the capacity is between two adjacent ring structures, the more advantageous are the effects formed with regard to a substantially interference-free propagation at least with respect to the TEM basic mode.
In order to choose a maximum coupling capacity CL,
The type of impact upon the propagation behavior of the TEM mode is revealed in the dispersion diagram shown in
It is evident that the TEM mode, in contrast to the speed-of-light straight, as is the case in
The consequence of this splitting is that it leads to a band gap BL within which none of the TEM mode portions TEMic, TEMoc and TEMmix is propagatable. As such the band gap BL in the depicted case is limited by an upper and a lower cut-off frequency fo and fu, to which the following relationships apply:
The occurrence of such a band gap BL represents a kind of blocking area for the propagation behavior of the TEM mode, which is caused by the electrically conducting spokes EV between ring R and outer conductor AL, that can be utilized as a low-pass filter arrangement. It is of course possible to adapt the spectral position of the band gap and also its spectral width to the respective technical requirements by a suitable choice regarding number, arrangement, form and size of the spokes EV and also of the ring arrangement between the inner and outer conductor in an optimizing way.
Number | Date | Country | Kind |
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10 2010 027 251 | Jul 2010 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2011/003469 | 7/11/2011 | WO | 00 | 1/14/2013 |
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WO2012/007148 | 1/19/2012 | WO | A |
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Konoplev, I.V. et al: “Wave Interference and Band Gap Control in Multiconductor One-Dimensional Bragg Structures,” Journal of Applied Physics, vol. 97, No. 7, S. 073101-073101-7, Apr. 2005, DOI 10.1063/1.1863425, 7 pgs. |
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Number | Date | Country | |
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20130112477 A1 | May 2013 | US |