Question statement
For these years came, in the most mobile phones used the wireless frequency modulation (FM) the signal to receive the module. Because wave long (λ~3m), occupies the frequency modulation frequency band to be big (band width is FM frequency modulation frequency band 76~108MHz 35%), exempts raises earphone’s ties often to treat as the antenna use, but (perhaps telephone’s ledger wall also has user) to treat as the grounding, therefore when listening to needs to connect exempts raises the earphone. This procedure’s shortcoming cannot use the blue tooth to exempt raises the earphone, also cannot use the audio frequency speaker. Recently, transmitted the tonic train signaling in the short distance the technology to start to emerge, namely transmitted the tonic train signaling from the portable MP3 player to the domestic receiver or on the car radio, the mobile phone very quick will also have this function. Therefore, the people and exempt antenna’s function raise earphone’s ties to separate, should better put the antenna to the mobile phone inside.
In conformity aspect question
When antenna integration telephone inside will have two problems. First, because the modern terminal device’s size is very small, the space is limited, may receive the wave length to be restricted. In addition, the printed wiring board has one (earth surface) is at least generally completely the metallization, most (is equal greatly highly with the place between in the 4~8mm scope to not to 0.003λ). Therefore, any single pole antenna’s length must reduce massively, with the aim of having the resonance, the result has the very big loss in the band width and antenna’s receive efficiency aspect. The second question is transmitting antenna’s isolation. Inside the launch electric wire and telephone’s frequency modulation antenna is places, in the GSM850/900 wave band, the launcher can have the serious disturbance particularly together.
Driven antenna’s design concept
Many companies proposed the corresponding design proposal. Take the Laird science and technology share Limited company as the example, they have developed one kind of built-in antenna (ActivvTM), may use in receiving the FM frequency-modulated signal. Its performance and the use active design concept’s earphone ties are similar, this is the radiating element and the Pre-amplifier designs together, and conformity in the same place. Compares with the traditional passive antenna, this method has the following merit.
* does not request antenna’s resistance to be 50Ω, therefore has avoided the loss, also does not need to limit the band width the matching unit.
* Pre-amplifier’s gain can reduce the noise which the FM frequency modulation receiver produces, generally is about 6dB. This was equal in the use gain has increased the 6dB passive antenna.
* is big as a result of driven antenna’s gain, delivers the FM frequency modulation receiver’s signal level to be more appropriate. This is because in standard frequency modulation receiver’s automatic gain control (AGC) the dynamirange is limited. Must pay attention a spot is, increases the gain not to be able to improve the radio frequency signal-to-noise ratio SNR (, because noise and signal similarly enlarged), but when transforms the tonic train signaling, it indeed massively enhanced the signal-to-noise ratio.
* the amplifier does not need the unconditional stability, regarding the transistor which assigns (its isolation equivalent darameter S12 is not equal to zero), can only use the ohmic load to be stable it, but the ohmic load seriously will reduce the gain and will increase antenna’s noise.
Compares with the passive antenna, driven antenna’s performance is much better, but these merits have the price. The design and the determination characteristic complex degree will increase (will talk about this in next), most importantly, must in not have in the ohmic load situation to achieve stably, the linearity was friends with. In addition, the active element needs the static electricity protection, but cannot reduce the sensitivity.
Driven antenna’s characteristic - G/T loss
The driven antenna does not need the gain, the efficiency, the returns to lose these ordinary parameters to describe its characteristic. For example, so long as increases Pre-amplifier’s gain, or connects the first-level amplifier again in the out-port, may increase its gain at will. Therefore, the gain itself does not have the significance. On the contrary, driven antenna’s main performance index is after the total output noise temperature regularization overall gain (antenna and amplifier’s gain), namely G/T loses (G/T degradation).
In the practical application, except receives the antenna characteristic influence, two external factors will affect the value which G/T will lose: Receiver’s noise figure and ambient noise temperature Ta. Receiver’s noise figure can increase antenna’s output noise Tout (, thus reduces G/T), the ambient noise temperature will also increase the output noise.
The noise level elevated the noise effect which the expression active device and the resistor produced to reduce. Only if is the built-in antenna, its radiating element’s gain is very low, in the ordinary circumstances, antenna’s actual temperature is controlling the noise temperature. Moreover, the noise background strong expression, in the low noise’s ideal situation, may reduce to radiating element’s request, will not reduce G/T obviously. This may qualitative understand like this: The efficiency high antenna receives the signal is bigger than the signal which the efficiency low antenna receives, but it receives the noise is also quite big. Therefore, in antenna out-port’s signal-to-noise ratio not good somewhat. The second spot was receiver’s noise figure NFrec also increased the antenna efficiency noise, so long as but amplifier’s gain was quite big (i.e. Gamp>NFrec), its influence was very small, compared with the passive antenna, this enhanced system’s performance (to say from noise background aspect). Must pay attention a spot is, these two aspects (are also background noise and NFrec) the function cannot separate generally. When for example, the background noise temperature is high, receiver’s noise figure plays the role is small, in turn is also this.
Generally speaking, we did not know that radiating element’s efficiency and amplifier’s gain respectively is big (cannot obtain at least through survey, but may obtains these data through simulation or model analysis). On the contrary, the G/T loss is through surveys antenna’s output noise total output to obtain directly, by now the antenna placed in a special environment (e.g. in dead room, Ta=T0), the gain is obtains with the gain method of substitution with the aid of a gain known reference antenna through the survey. But must be careful. Must the noise which produces the measurement equipment remove through the demarcation, when survey does not have the metal object (coaxial cable line or power line) and it is connected. As a result of this reason, the Laird science and technology company developed one kind to replace the electric cable line the system, it has used optical fiber’s principle, together with one with battery power supply’s Pre-amplifier, these decide the small antenna’s electrical specification together in might the surveying (to see Figure properly 1). It is estimated that the coaxial cable line introduction measuring error’s establishment as shown in Figure 2. In Figure 2, in the monopole antenna in the length different situation which extends from the foundation plate, has surveyed their gain with the coaxial cable line and the optical fiber system. May see, when the length is about 10mm (this is built-in antenna’s virtual length), the measuring error surpasses 20dB.
Figure 1 the use optical fiber system, has not used the electric cable line the metering equipment
Figure 2 when surveys the small antenna, metal electric cable line introduction measuring error
Finally, must pay attention to the human body to the small antenna in the FM frequency modulation frequency band gain influence is frontage, particularly user’s body and the antenna or the ledger wall contact in situation. This is because, about 100MHz, the human body is the efficiency quite good antenna, the half wave length is about 1.5m. Figure 3 has given when the human body contact antenna and has not contacted the antenna, receiving antenna’s output frequency spectrum. Very obvious, when with hand contact antenna, G/T enhanced 10~15dB. Because majority users when listens to the telephone and the mobile phone are depend very much near, therefore may suppose reasonably, the FM frequency modulation antenna’s actual performance surpasses it occupies when the free space the performance.
Figure 3 the human body to increases with the signal-to-noise ratio influence
ActivvTM driven antenna’s performance
Figure 4 explanation driven antenna ActivvTM design concept. The radiating element is a circle half-ring return route, a radiating element’s end in ledger wall’s one side earth, radiating element’s another termination to the amplifier on. May also use the multi-circle radiating element, it can enhance radiating element’s resistance. However in the most situations, it function is not very big. Return route’s inductance and the amplifier input end (between electronics grid and source) on the parallel electric capacity has the resonance, does this besides the enhancement gain, but also increased antenna’s equivalent impedance resistive component (it also to enhance stability), thus strengthened the noise match. The Pre-amplifier is altogether the source electric circuit, the use modern microwave FET transistor, reduced the noise. Entire amplifier when 3V consumes the electric current is 3mA, its gain is quite big, the linearity is also quite good.
The bias spot is feeds back stably depending on the cocurrent, the noise which produces the biasing circuit reduces through the design to is zero nearly. Because amplifier’s design is coordinates the antenna to carry on, the amplifier does not need the unconditional stability (regarding independent amplifier, request is unconditional stability). This is the driven antenna design concept huge merit, because only then adds on the resistive load in the majority situations to be able to achieve the unconditional stability. But the resistive load will reduce antenna’s gain seriously, and will increase antenna’s noise. What the people worried generally, antenna’s impedance is not the fixed constant, but changes along with the near field environmental variation. However in this case, only then magnetic material (, because it is short circuit ring circuit) affects radiating element’s input impedance, this is quite unusual.
Figure 4 uses half return route radiating element’s ActivvTM driven antenna schematic diagram
This also indicated that the antenna cannot lose because of the nearby object influence harmonious. The antenna reduced the crosstalk which in resonance frequency’s response characteristic the GSM transmitting message causes, designed the radiating element also to reduce this crosstalk properly. An antenna’s end is the short circuit, in another end parallel capacitor (, thus has resonance), exchanges the signal as a result of this capacitor, but with place short circuit, therefore antenna when the GSM antenna E to be biggest short-circuits to the place, thus guaranteed that the crosstalk is very low. Is determines like this to the GSM crosstalk’s sensitivity: (824~960MHz and 1710~2170MHz) place nearby the telephone a reference dipole antenna, and receives it on a high efficiency CW launcher. Measuring signal worsened situation, when 824MHz (signal variation most serious frequency) probably is 36dBm, is higher than GSM by far the peak power output. Shown in Figure 5 is Nokia 6125 ActivvTM antenna’s G/T loss value and the gain, is (the worst situation) the time observed value relative gain says in the close up position, G/T is smooth, this is because amplifier’s noise figure and the unbalance degree’s relations are misalignment. Therefore when designs the amplifier may free somewhat, be possible to enhance driven antenna’s G/T band width in the design, but this is the passive antenna cannot achieve. The ActivvTM driven antenna may use in the frequency harmonious (, if may obtain FM frequency modulation receiver’s control signal). Receiver, in frequency band both sides signal-to-noise ratio several dB (particularly in needs to cover entire 76~108MHz in the situation), also enhanced in the tolerance frequency band the signal jamming degree. However this belongs to the selection function, not necessarily will obtain the very good performance.
Figure 5 in Nokia 6125 telephone’s ActivvTM antenna G/T loss and gain measurement result
When launch, the radiating element plays the standard the passive antenna role, it uses a single-pole single-throw (SPST) on-off control to carry on the connection (to see Figure 4). The Pre-amplifier occupies the Rx condition, still with antenna connection (, but dump). With an interface resistance for 50Ω when supply oscillator supplies signal, unbalance half-turn antenna’s gain determination value as shown in Figure 7. The wide band scope’s average increases (or efficiency) achieves - 53~-49dB. In the switch and the frequency modulation launch between the module to turn on a matching network, or choice suitable power amplifier output impedance, then may relax many to output’s request.
Figure 6 Nokia in 6125 telephone’s ActivvTM driven antenna’s picture
Figure 7 Nokia 6125 telephone half-turn radiating element in launch (Tx) under condition gain determination value
Why can use the low gain the small antenna to receive the FM frequency-modulated signal?
May know by the scene test, when receives the FM frequency-modulated signal, the ActivvTM driven antenna’s performance is equal to the earphone connection linear antenna, although these two kind of antenna’s size has the very wide difference. Considered driven antenna’s volume is small, its radiation resistance approximately only then 1mΩ, but the parasitic loss resistance has at least 1Ω about, increases in - the 30~-50dBi scope. As for majority Engineer RF, such gain is very difficult to accept. When had a factor to relax enormously received the FM frequency-modulated signal to the gain request, namely very high ambient noise temperature. Figure 8 is a dead room (to obtain conservative estimate) after the shielding system takes away noise level. The majority wireless system work are higher than 1GHz in the frequency the scope, the ambient noise close room temperature, increases for - 10dBi, was equal reduced 10dB in the signal-to-noise ratio, in the FM frequency band, in the urban most districts, as a result of man-made noise’s influence, the noise level high 20dB (regarding amplitude modulation has been able approximately higher). Therefore, compares with the perfect double-pole electric wire, the effect rate difference’s antenna receives the signal is weak, the noise is small. Figure 9 has given the gain differently (0, - 20, - 40dB) time three kind of antenna different noise temperature G/T value. All three kind of antennas receive the noise figure are on the 6dB receivers. (The G/T value reduces 46dB regarding the gain for - the 40dB actual antenna under room temperature), when the situation is the same, the performance enhanced 19dB under the 23.000K typical temperature. Regarding the driven antenna, through suppresses receiver’s noise figure, may enhance again the performance 6dB. Therefore, looking from the signal-to-noise ratio aspect, the gain is the -40dB passive antenna, only compared to perfect double-pole day linear discrepancy 27dB.
Figure 8 the ambient noise which surveys in the city.
Figure 9 passive antenna’s under different ambient noise temperature G/T value
