.net barcode reader component download Two small loop antennas at right angles to each other in Software

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15-3 Two small loop antennas at right angles to each other
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15-4 An untuned loop antenna
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It is common practice to make a small loop antenna with two loops rather than just one Figure 15-5 shows such a transformer loop antenna The main loop is built exactly as discussed above: several turns of wire on a large frame, with a tuning capacitor to resonate it to the frequency of choice The other loop is a one- or two-turn coupling loop This loop is installed in very close proximity to the main loop, usually (but not necessarily) on the inside edge not more than a couple of centimeters away The purpose of this loop is to couple signal induced from the main loop to the receiver at a more reasonable impedance match The coupling loop is usually untuned, but in some designs a tuning capacitor (C2) is placed in series with the coupling loop Because there are many fewer turns on the coupling loop than on the main loop, its inductance is considerably smaller As a result, the capacitance to resonate is usually much larger In several loop antennas constructed for purposes of researching this chapter, I found that a 15-turn main loop resonated in the AM BCB with a standard 365-pF capacitor, but the two-turn
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15-5 A transformer loop antenna
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Tuning schemes for loop antennas 307 coupling loop required three sections of a ganged 3 365-pF capacitor connected in parallel to resonate at the same frequencies In several experiments, I used computer ribbon cable to make the loop turns This type of cable consists of anywhere from 8 to 64 parallel insulated conductors arranged in a flat ribbon shape Properly interconnected, the conductors of the ribbon cable form a continuous loop It is no problem to take the outermost one or two conductors on one side of the wire array and use them for a coupling loop
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Loop performance is greatly enhanced by tuning the inductance of the loop to the desired frequency The bandwidth of the loop is reduced, which reduces front-end overload Tuning also increases the signal level available to the receiver by a factor of 20 to 100 times Although tuning can be a bother if the loop is installed remotely from the receiver, the benefits are well worth it in most cases There are several different schemes available for tuning, and these are detailed in Fig 15-6 The parallel tuning scheme, which is by far the most popular, is shown in Fig 15-6A In this type of circuit, the capacitor (C1) is connected in parallel with the inductor, which in this case is the loop Parallel resonant circuits have a very high impedance to signals on their resonant frequency and a very low impedance to other frequencies As a result, the voltage level of resonant signals is very much larger than the voltage level of off-frequency signals The series resonant scheme is shown in Fig 15-6B In this circuit, the loop is connected in series with the capacitor A property of series resonant circuits is that they offer a high impedance to all frequencies except the resonant frequency (exactly the opposite of the case of parallel resonant circuits) As a result, current from the signal will pass through the series resonant circuit at the resonant frequency, but off-frequency signals are blocked by the high impedance There is a wide margin for error in the inductance of loop antennas, and even the precise-looking equations to determine the required values of capacitance and inductance for proper tuning are actually only estimations The exact geometry of the loop as built determines the actual inductance in each particular case As a result, it is often the case that the tuning provided by the capacitor is not as exact as desired, so some form of compensation is needed In some cases, the capacitance required for resonance is not easily available in a standard variable capacitor, and some means must be provided for changing the capacitance Figure 15-6C shows how this is done The main tuning capacitor can be connected in either series or parallel with other capacitors to change the value If the capacitors are connected in parallel, then the total capacitance is increased (all capacitances are added together) If the extra capacitor is connected in series, however, then the total capacitance is reduced The extra capacitors can be switched in and out of a circuit to change frequency bands Tuning of a remote loop can be a bother if it is done by hand, so some means must be found to do it from the receiver location (unless you enjoy climbing into the attic or onto the roof) Traditional means of tuning called for using a low-rpm dc motor, or stepper motor, to turn the tuning capacitor A very popular combination was the little 1- to 12-rpm motors used to drive rotating displays in retail store show
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