The present invention refers to a television receiving loop antenna with UHF band.

Such an indoor UHF antenna is described in US Patent 6,429, 828 and is a loop having a diameter of about 20 cm.

It is an object of the invention to provide an antenna receiving UHF signals having a relative smaller size. It is a further object of the invention to improve the signal to noise ratio.

In accordance with the present invention a television receiving loop antenna with UHF band is tunable. Therefore the diameter of the antenna is small in respect of the wave length. The diameter is only about 5 cm. A broadcasted 470 MHz signal corresponds to a wave length of 64 cm. A broadcasted 870 MHz signal corresponds to a wave length of 34 cm. With the size of 5 cm a handheld antenna for Ultra High Frequency Television reception, UHF TV reception in short, is provided. The signal to noise ratio is proportional to the directivity of the antenna. For a portable TV receiver an omni directional system is wanted.

Ignoring the directivity of the antenna the smallest signal to noise ratio, SNR for short, is taken.

The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings.

Fig. 1 is a systematic diagram of a tuned loop antenna used in explaining the principles of the invention; Fig. 2 is a schematic diagram of the equivalent circuit for the antenna shown in Fig. 1;

Fig. 3 is a graph showing the noise factor versus the frequency for different loop diameters; Fig. 4A is an embodiment of an antenna with two loops and a capacitor plugged in parallel; Fig. 4B is an embodiment of an antenna tuned with a capacitor for a fine tuning and, with adding several loops in parallel for discrete steps; Fig. 4C is an embodiment of an antenna with a loop and a LC parallel circuit plugged in parallel; Fig. 4D is a schematic diagram of the equivalent circuit for the antenna shown in Fig. 4C; Fig. 4E is an embodiment of an antenna with two capacitors plugged in series; Fig. 4F is an embodiment of an antenna with several tunable capacitors; Fig. 4G is an embodiment of an antenna system with two independent antennas covering two different segments of one receiving band; Fig. 5 is a preferred embodiment of a UHF handheld antenna; Fig. 6A is a graph showing the noise figure versus the UHF band till 600 MHz ; Fig. 6B is a graph showing the noise figure versus the UHF band above 600 MHz ; Fig. 7A is a graph showing the bandwidth, derived from the Q, versus the frequency till 600 MHz and Fig. 7B is a graph showing the bandwidth, derived from the Q, versus the frequency above 600 MHz.

Figures 1 and 2 show a loop antenna 1 comprising a loop 2, a tune capacitor 3, Co for short, and a radiation resistance 4, Rr for short. The loop 2 has a diameter 5. An amplifier 6 amplifies a signal at the output 7 of the loop antenna 1. The radiation resistance 4, a coil 8, Lo for short, a loss resistance 9, Rloss skin for short, and a voltage source 10 substitute loop 2. An electromagnetic field 11, Hi or E/H field for short, induces a voltage 12, Vi for short. A signal to noise ratio at the input, SNRin for short, is considering the noise of the radiation resistance 4, and the signal that is coming from the antenna. A signal to noise ratio at an output 13, SNRout for short, is defined by: SNRin-NFarchiìecture. NFarchitecture is the noise figure of the tune capacitor 3 and the amplifier 6. The capacitor 3 is plugged in parallel with

the loop 2 to cancel out the capacitance for the noise and power matching to the amplifier 6.

The resonance of this system with the loop 2 is used to select the channel. For a fixed loop 2, if the series resistance 4,9 is constant versus a frequency, the bandwidth remains constant. In that case, the tunable loop antenna 1 acts as an antenna and a tracking filter. In series with the input impedance 4 of the antenna, the loss resistance 9 due to the skin effect is present. This loss resistance 9 makes the antenna noisy. The noise figure of the antenna 1 is then: NF=1. 10 R+R°ssskin R, Loss resistance 9 in series with the radiation resistance 4 of the loop 2 or in series with the tune capacitor 3 should be minimized, as it will increase the total noise figure.

The loop antenna 1 with the coil 8, the radiation resistance 4 and the loss resistance 9 is tuned thanks to the capacitor 3. The quality factor Q of this resonance circuit is then: Q= 1 L Rr + Rloss skin-\1 C° The equivalent input voltage is: Vi = 2. 7r. uo. So. Hi. f, where Hi is the H field 11 received, So is the loop area,, uO is the free space permeability 4#.10-7 H/m.

For the resonance, the output voltage available between 7 and ground is: Vo = Q. Vi In figure 3 graphs 21,22, 23 show the noise figure versus the frequency for loop diameters 5 of 3,4 and 5 cm. In a section 24 the value of the capacitor 3 becomes too small compared to the parasite capacitance of the schematic, C becomes less than lpF.

The noise figure, NF for short, improves, if the diameter 5 is increased.

Unfortunately this will also increase the inductance value of the loop 2, what can make tuning to the highest frequency impossible because it would require an unrealistic small capacitor 3, which is always limited by the loop parasite capacitance. Fortunately Rioss skin and Rr are frequency dependent in such a way that NF improves for higher frequencies. These effects are illustrated in this figure 3.

Figure 4 shows several architectures, which are then possible to increase the frequency without increasing the noise figure.

Figure 4A shows an antenna 31 comprising two loops 32,33 and a tune capacitor 34 plugged in parallel. Loops 32 and 33 comprise coils LI and L2. The capacitor 34, C2 for short, tunes the antenna 31. If the two loops 32,33 are identical to that of the Fig. 1 antenna 1, then Lo=LI=L2. To compute the radiation resistance a current source is plugged on the antenna 31. The E/H field emitted is identical to that of the Fig. 1 case when the same current flows in the Fig. 1 and Fig. 4A antennas 1 and 31. That means these two antennas 1 and 31 of Fig. 1 and Fig. 4a have the same radiation resistance. Due to that configuration the equivalent inductance is L/2, and R, oss skin is reduced. To achieve the same frequency as in the Fig. 1 case, C2=2 : Co. The equivalent input voltage is unchanged. The mutual coupling is not considered. As the capacitor 34 is bigger, it is easier to increase the frequency, but the area has doubled. Rloss skin Series = R, + 2 Q 2 Q-1 Lo Rr+R/2-V4C, When LiL2, for the Fig. 4A configuration, the formulas become: where So, SI, S2 is the surface of the reference loop 2, and the two loops in parallel 31 and 32.

Rr is always between Rrl and Rr2. If one loop 31 is very big compared to the second 32 then the circuit is equivalent to the smallest loop 32.

Figure 4B illustrates an antenna 41, comprising a capacitor 42 and loops 43- 45. The capacitor 42 and the loops 43-45 are plugged in parallel. The loops 43-45 comprise coils Ll, L2 and L3. The antenna 41 is tuned both with the tune capacitor 42 for a fine tuning and, with adding several loops 43-45 in parallel for discrete steps. This must be made without adding losses elements with respect to the radiation resistance. It can be done mechanically with a screw, with an electromechanical relay, or electrically with switches in a micro electro mechanical system, MEMs for short. See therefore in the internet :

EMR and MEMs : litt ://www. simplemetworks. com/eng/index. html http ://www. memsrus. com/CIMSprod. html Micro Machined Relay for High frequency Application: Y. Komura, M. Sakata, T. Seki, K.

Kobayashi, K. Sano, S. Horiike, K. Ozawa-Omron Corporation http://www. omron. co. jp/r-d/doc/mmr-for-hf pdf The mercury microswitch-R. Timothy Edwards C. -J. Kim http://klabs.org/richcontent/MAPLDCon01/Presentations/B/B2A_ Edwards_S. pdf The Superior RF Switch Technology - C. Whecler - MeroLab Inc. http ://www. microlab. net/ Figures 4C and 4D show a loop antenna 51 with a loop 52 plugged in parallel with a LC parallel circuit 53. The LC parallel circuit 53 comprises a tunable capacitor 54, C3 for short, and a coil 55, L4 for short. L4 is aimed at reducing the value of C3. It can be viewed as a negative capacitance which decreases with f. The antenna 51 parameters are the same as in the Fig. 1 case. 3 = 3-1 2 Q=-I Lo L4. 92 Q 1 7 To achieve the same frequency as in the other cases, with L4=Ll=Lo, C3 is twice the value that gives C3'=Co. The advantage to the Fig. 4A case is an area reduction. The disadvantage is the Q has double compared to the Fig. 4A case. Besides considering some resistive losses in the capacitors 53 or 54, these ones are amplified with the negative capacitance 55 done with L4 making the NF worse. That was not the case with the Fig. 4A configuration.

Therefore, to achieve the same frequency as in the Fig. 1 and Fig. 4A case, the NF increases a lot.

Figure 4E illustrates an antenna configuration 60 with an antenna 61 and an amplifier 62. The antenna 61 comprises a loop 63 and three tune capacitors 64,65 and 66, Ca , Cb and Cc for short. The loop 63 comprises a coil Li. The loop 63 is in series with the capacitor 65. A parallel circuit 68 composed of capacitors 64 and 65 and loop 63 is in series with the capacitor 66, which is connected to the amplifier 62. A tap circuit 67 composed of Cb and Cc adapts the antenna 61 to the amplifier 62. The two tune capacitors 64 and 65 are plugged in series so as to increase the frequency. The capacitors 64 and 65, Ca and Cb for

short, have a small series resistance because with a bigger series resistance this architecture 64,65 of capacitors 64,65 is not free of noise.

Figure 4F illustrates an antenna 71 and an amplifier 72. The antenna 71 comprises a loop 73 and six tune capacitors 74-79. The loop 73 comprises a coil, Ll for short. Four tunable capacitors 74-77, twice Ca and twice Cb for short, are plugged in series.

The loop 73 is in series with the capacitors 74 and 77. A first tap circuit 80, composed of capacitors 74 and 75 adapts the antenna 71 via the tune capacitor 78 and a second tap circuit 81 composed of capacitors 76 and 77 via the tune capacitor 79 to the amplifier 72. This is a way to get the signal differentially.

Fig. 4G illustrates an antenna systems 91 with a first antenna configuration 60 and a second antenna configuration 92. The second antenna configuration 92 comprises an antenna 93 and an amplifier 94. The antenna 93 comprises two parallel loops 95 and 96 and a tap circuit 97 composed of tune capacitors 98 and 99 to adapt the loops 95 and 96 to the amplifier 94 via a tune capacitor 100. A total tuning range is achieved by switching the signal at outputs 103 and 104 of the configurations 60 and 92. Both antenna configurations 60 and 92 are separated by a switch 101 to give their signal to a TV receiver 102. Compared to the solution shown in Fig. 4B the switch does not need to be ideal. It will less alter the resonance frequency, the Q and the NF. But this solution implies at least two times the similar tap circuits, and more loops.

Figure 5 shows UHF hand held antenna 111 and an amplifier 112. The antenna comprises two loops 113,114, a switch 115 and seven capacitors 116-121. Each loop comprises a coil, L1 for short. The second loop 114 is added to the first loop 113 by the switch 115. The capacitors 117 and 118 are variable capacitance diodes, Cvar, varicaps or varactors for short. A series circuit 123 composed of the loop 113, the capacitor 119 and the varactor 118 is plugged in parallel to the varactor 117 and the capacitor 116. A tap circuit 124 composed of the four capacitors 116-119 adapts the antenna 111 to the amplifier 112 via the tune capacitor 122.

The loop is a 5cm diameter. Ll=89nH. Rr=l. lQ at 575MHz. R) ossskin==0. 18Q at 575MHz.

Rr varies with f4, and Rloss with Vf. CvARa and CvARb are two varicaps with the following parameter: Cvar varies between: 2 pF to 20pF.

Rseries varies between: 0. 3Q to 0. 692.

The capacitor 120, Cb for short, is added to reduce the series resistance of the varicap CVAR.

Cb=0.4pF.

The capacitor 121, Ca for short, is added to reduce the capacitance CVAR. Ca=4pF.

The series resistance for the R, L, C series resonator varies between 1. 16Q (Cmin) to 0. 47Q (Cmax). The capacitance varies between 0.75pF to 1.67pF.

The switch 115 is added to achieve the highest frequencies range.

When the switch 115 is off, the frequency range is 410 MHz to 615MHz.

When the switch is on, the frequency range is 580 MHz to 870MHz.

Figures 6A and 6B show a noise figure, deduced for the above mentioned two frequency ranges. Therefore for the UHF band, the NF varies between 4. 5dB to ldB.

Figures 7A and 7B show a bandwidth, derived from the Q.

Therefore, for the UHF band, the bandwidth varies between 3MHz to 22MHz.

To improve the lower interval, a higher Rr is needed, that means a loop diameter bigger. For the second interval, the Rr makes the bandwidth increase. To decrease it, the Rr needs to be reduced that will unfortunately increase the NF.

If the parasite capacitance of the loop Cp=0.4pF is taken into account, the tuning range of the system described in Fig. 5 is 370MHz to 606MHz. Two others loops with the same dimension needs to be plugged in parallel to go up to 700 MHz. The NF and bandwidth curves remain the same, as the radiation resistance is unchanged. Another solution is to take a smaller diameter, and then increase the fmax but that makes the NF higher.

REFERENCE LIST: 1 loop antenna 2 Loop 3 capacitor (Co) 4 radiation resistance (Rr) 5 diameter 6 amplifier 7 Output 8 coil (Lo) 9 loss resistance (Rloss skin) 10 Current source 11 electromagnetic field (Hi) 12 Voltage (Vi) 13 Output 14 15 16 17 18 19 20 21 graph 3 cm 22 graph 4 cm 23 graph 5 cm 24 Section 25 26 27 28 29 41 antenna 42 capacitor 43 loop with Li 44 loop with L2 45 loop with L3 46 47 51 antenna 52 loop with Lo 53 LC-circuit 54 capacitor, (C3) 55 coil L4 56 57 60 antenna configuration 61 antenna 62 amplifier 63 loop 64 tune capacitor (Ca) 65 tune capacitor (Cb) 66 tune capacitor (Cc) 67 parallel circuit 68 tap circuit 71 antenna 72 amplifier 73 loop 74 tune capacitor (Ca) 30 31 Antenna 32 loop with Ls 33 loop with L2 34 capacitor (C2) 35 36 37 91 antenna system 92 antenna configuration (second) 93 antenna 94 amplifier 95 loop (parallel) 96 loop (parallel) 97 tap circuit 98 tune capacitor 99 tune capacitor 100 tune capacitor 75 tune capacitor (Cb) 76 tune capacitor (Cb) 77 tune capacitor (Ca) 78 tune capacitor (Cc) 79 tune capacitor (Cc) 80 tap circuit 81 tap circuit 111 UHF hand held antenna 112 amplifier 113 loop with Li 114 loop with Li 115 switch 116 capacitor 117 varactor 118 varactor 119 capacitor 120 capacitor 121 capacitor 122 capacitor 123 series circuit 124 tap circuit