BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a basic matching circuit of a mono-band loop antenna of the prior art.
FIG. 2 is a circuit similar to FIG. 1 but slightly modified to provide better performance.
FIG. 3 is an example of a first preferred embodiment of the present invention of a matching circuit for a dual-band loop antenna.
FIG. 4 is an example of a preferred embodiment of the present invention of a matching circuit for a tri-band loop antenna.
FIG. 5 is a Network Analyzer plot showing the Return Loss RL (S11) of the dual-band loop antenna of FIG. 3 at frequencies of 27.1 and 49.875 MHz.
FIG. 6 is a Smith Chart graph of the complex impedance of the dual-band loop antenna of FIG. 3 at frequencies of 27.1 and 49.875 MHz.
FIG. 7 is a Network Analyzer plot showing the Return Loss RL (S11) of the tri-band loop antenna of FIG. 4 at frequencies of 27.075, 40.7, and 49.85 MHz
FIG. 8 is a Smith Chart graph of the complex impedance of the tri-band loop antenna of FIG. 4 at frequencies of 27.075, 40.7, and 49.85 MHz.
FIG. 9 is a plot showing the Standing Wave Ratio (SWR) of the tri-band loop antenna of FIG. 4 at frequencies of 27.075, 40.7, and 49.85 MHz.
Use of the same reference number in different figures indicates similar or like elements.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention describes the design of a small multi-band loop antenna and the method of tuning it. The matching circuit contains additional reactance elements, such as capacitors and inductors. They transform a mono-band matching circuit into a multi-resonance matching structure and provide impedance matching for two or more frequency bands. These multi-band loop antennas can be used for frequencies extending from a few megahertz to several hundred megahertz. The number of components used, such as capacitors and inductors, depends on the number of resonant frequencies and their values depend on the size of the loop and required resonant frequencies and impedance. The more frequency bands that have to be covered the more complex becomes the matching circuit. The terms capacitive and inductive means may imply capacitors and inductors or any other devices capable of providing the function of a capacitor or inductor. Transistors or transistor circuits in integrated circuits (IC) also provide this function; they are cited by way of illustration and not of limitation, as applied to either capacitive or inductive means.
We now describe a typical example of a first preferred embodiment of the dual-band antenna as illustrated in FIG. 3. The dual-band antenna FIG. 3 is a further development of FIG. 2. Inductor L3 and capacitor C3 form an additional series resonant circuit, creating a dual-band antenna. Coupled between Port1 and the junction of capacitor C1 and L, is the series resonant circuit 31 comprising capacitor C3 and inductor L3. The remaining circuit components were already described in FIG. 2. In this example L, R, C1 and C2 are first tuned to the middle frequency 38.487 MHz between the two required frequencies 27.1 MHz and 49.875 MHz (27.1+49.875)/2=38.487 MHz. Then C3=22 pF with a Q of 300 and L3=1000 nH with a Q of 39 is added and a dual resonant circuit is created. Now the lower resonant frequency is 27.1 MHz and the higher resonant frequency is 49.875 MHz. Circuit 31 (C3, L3) is series resonant at middle frequency (38.875 MHz), thus the circuit is capacitive at 27.1 MHz and inductive at 49.875 MHz. Therefore it is recommended to tune it to the higher frequency (49.875 MHz) by changing the L3 value and the lower frequency (27.1 MHz) by changing the C3 value.
Still referring to FIG. 3, we describe the method of tuning the dual-band antenna. The antenna first has to be tuned to the middle frequency (38.487 MHz) between the two desired frequencies without circuit 31 (L3 and C3) connected for approximately twice as big an impedance as required. C1 will tune for resonant frequency (38.487 MHz) and C2 for impedance matching. Next circuit 31 has to be connected. It has to be series resonant in the middle (38.487 MHz) between the two required frequencies. C3 has to be tuned precisely to the lower operating frequency (27.1 MHz) and the L3 inductor value will adjust the upper operating frequency (49.875 MHz). The impedance matching can be adjusted by changing the capacitor C2 value. Note that the middle frequency (38.487 MHz) is the parallel resonant frequency of L, R, C1 and C2 without C3, L3 and the series resonant frequency of C3, L3.
FIG. 5 is a Network Analyzer plot (Curve 1) showing the Return Loss RL of the dual-band loop antenna of FIG. 3 at frequencies of 27.1 (Point A of Curve 1) and 49.875 MHz (Marker 1 of Curve 1). The RL at 27.1 MHz is about −30 dB and at 49.875 MHz is −19.6 dB. The y-axis is marked from −45 dB to +5 dB, the x-axis from 22 MHz to 52 MHz. FIG. 6 is a Smith Chart graph (Curve 2) and shows the complex impedance of the above described dual-band antenna of FIG. 3. Curve 2 illustrates a first Antenna impedance of about 49 Ohms at 27.1 MHz and at Marker 1 a second Antenna impedance of 55.783 Ohms−j9.835 Ohms at 49.875 MHz.
We now describe a typical example of the preferred embodiment of the tri-band antenna as illustrated in FIG. 4. The inventive circuit extends the above described dual-band antenna of FIG. 3 to a tri-band operation. A tri-band antenna can be created by inserting between L3 and the junction of C1 and L the parallel resonant circuit 41 comprising capacitor C4 and inductor L4. The remaining circuit components were already described in FIGS. 2 and 3. For the third frequency of 40.68 MHz a value for C4 is 56 pF with a Q of 300 and for L4 is 100 nH with a Q of 39. For those skilled in the art, it is understood that all values used in the above dual and tri-band antenna example are valid only for this particular case. They will have to be different if the antenna inductance L and/or antenna resistance R are different. They also have to be different for different frequencies.
Still referring to FIG. 4, we describe the method of tuning the tri-band antenna. First all steps for dual-band antenna tuning must be performed. Then the parallel resonant circuit 41 has to be added in series with the existing series resonant circuit 31. This will create the third resonant frequency, which has to be situated between the two other frequencies set before. As already mentioned above, C4 and L4 is the parallel resonant circuit 41. It will create the third resonant frequency in the antenna circuit. There is an infinite number of C4 and L4 values that will place the third resonance at the right frequency (40.68 MHz in the above example). Putting C4 and L4 into the circuit will change the other two frequencies and impedance, so it is necessary to find such a set of C4, L4 values which will make the smallest possible change. It is not easy to calculate by hand the values of C4 and L4, because of great number of reactive components. In practice it is recommended to use one of the computer simulation programs such as ADS, Eagleware or Ansoft to find the required values.
To start one needs to known the antenna inductance L, antenna resistance R and the required three operating frequencies. L and R can be measured with a Network Analyzer at the middle frequency, which is 38.487 MHz in the present example. Once one knows the L and R values (antenna model), they are put into the Simulation Program and one finds the C1 and C2 values that will give the required impedance at the selected middle frequency. Then one adds C3 and L3 and finds the right values to obtain the right impedance at the lower and upper operating frequency (27.1 and 49.875 MHz). One has to adjust the existing C1 and C2 values as well. Now the right values for the dual band antenna have been obtained. Once done, one adds C4 and L4 to the circuit and finds the right values to get the third resonant frequency (40.68 MHz in the present example). One also has to adjust C1 and C2 for the required impedance. Now one can put the simulated values into the real circuit and measure impedance and Return Loss RL (S11). Some small adjustment may be necessary due to component tolerance and antenna measurement accuracy.
FIG. 7 is a Network Analyzer plot (Curve 3) showing the Return Loss RL of the tri-band loop antenna of FIG. 4 at frequencies of 27.075 (Marker 1 of Curve 3), 40.7 (Marker 2 of Curve 3), and 49.85 MHz (Marker 3 of Curve 3). The RL at 27.075 MHz is −21.405 dB, at 40.7 MHz is −22.763 dB, and at 49.85 MHz is −16.813 dB. The y-axis is marked from −45 dB dB to +5, the x-axis from 25 MHz to 55 MHz. FIG. 8 is a Smith Chart graph (Curve 4) and shows the complex impedance of the above described tri-band antenna of FIG. 4. Curve 4 describes the antenna impedance which is 42.553 Ohms+j 1.939 Ohms at 27.075 MHz (Marker 1 of Curve 4), 42.695 Ohms+j 1.138 Ohms at 40.7 MHz (Marker 2 of Curve 4), and 66.054 Ohms−j3.553 Ohms at 49.85 MHz (Marker 3 of Curve 4). FIG. 9 is a plot (Curve 5) showing the Standing Wave Ratio (SWR) of the tri-band loop antenna of FIG. 4 at frequencies of 27.075 MHz and 1.192 (Marker 1 of Curve 5), 40.7 MHz and 1.178 (Marker 2 of Curve 5), and 49.85 MHz and 1.338 (Marker 3 of Curve 5). The y-axis is marked from 1 dB, the reference point, to 11 dB. The x-axis is marked from 25 MHz to 55 MHz.
It is understood by those skilled in the art that it is possible to arrange dual and tri-band antennas in different topologies without differing from the scope of the invention. Described next are some possible variations of these topologies, by way of illustration and not of limitation, as applied to those topologies.
In a second preferred embodiment of the present invention of a dual band antenna, in a modification from FIG. 3, the series circuit 31 C3, L3 connection parallel to L, R is changed into a parallel C3, L3 resonant circuit connected in series between C1 and L.
In a second preferred embodiment of the present invention of a tri-band antenna, in a modification from FIG. 4, the original series combination of the L3, C3 series resonant circuit 31 in series with the L4, C4 parallel resonant circuit 41 is changed into a parallel connection of the L3, C3 series resonant circuit with the parallel L4, C4 resonant circuit, i.e., both circuits 31 and 41 are coupled in parallel between Port1 and the junction of C1 and L.
In a third preferred embodiment of the present invention of a tri-band antenna, in a modification from FIG. 4, the L3, C3 series resonant circuit 31 is converted into a parallel resonant circuit and L4, C4 is changed into a series resonant circuit, i.e., both circuits are coupled in series between Port1 and the junction of C1 and L.
In a fourth preferred embodiment of the present invention of a tri-band antenna, in a modification from FIG. 4, the L3, C3 series resonant circuit 31 is converted into a parallel resonant circuit and L4, C4 is changed into a series resonant circuit, i.e., both circuits are coupled in parallel between Port1 and the junction of C1 and L.
Regardless of the topology, the antenna tuning procedure remains the same, because each added circuit has a particular function. The tuning still has to be done step by step because of the significant number of LC components.
Elements previously discussed are indicated by like numerals and need not be described further.
In the illustrated embodiments, the process of the invention is shown, by way of illustration and not of limitation, as applied either to the selection of the frequencies, the component values or the topologies.
While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.
Patent Application
20080055173
