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15-6 Various tuning schemes: (A) parallel, (B) series resonant, (C) series or parallel
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Shielded loop antennas 309 windows But this approach is not really needed today We can use varactor voltagevariable capacitance diodes to tune the circuit A varactor works because the junction capacitance of the diode is a function of the applied reverse-bias voltage A high voltage (such as 30 V) drops the capacitance, whereas a low voltage increases it Varactors are available with maximum capacitances of 22, 33, 60, 100, and 400 pF The latter are of most interest to us because they have the same range as the tuning capacitors normally used with loops Figure 15-7 shows how a remote tuning scheme can work with loop antennas The tuning capacitor is a combination of a varactor diode and two optional capacitors: a fixed capacitor (C1) and a trimmer (C2) The dc tuning voltage (Vt ) is provided from the receiver end from a fixed dc power supply (V ) A potentiometer (R1) is used to set the voltage to the varactor, hence also to tune the loop A dc blocking capacitor (C3) keeps the dc tuning voltage from being shorted out by the receiver input circuitry
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OK, sports fans, what do you do when the best game of the week is broadcast only on a low-powered AM station and you live at the outer edge of their service area where the signal strength leaves much to be desired You use the sports fan s loop antenna, that s what! I first learned of this antenna from a friend of mine, a professional broadcast engineer, who worked at a religious radio station that had a pipsqueek signal but lots of fans It really works one might say it s a miracle The basic idea is to build a 16-turn, 60-cm2 tuned loop and then place the AM portable radio at the center so that its loopstick is aimed such that its null end is broadside of the loop When you do so, the nulls of both the loop and the loopstick are in the same direction The signal will be picked up by the loop and then coupled to the radio s loopstick antenna Sixteen-conductor ribbon cable can be used for making the loop For an extra touch of class, place the antenna and radio assembly on a dining room table lazy Susan to make rotation easier A 365-pF tuning capacitor is used to resonate the loop If you listen to only one station, then this capacitor can be a trimmer type
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The loop antennas discussed thus far in this chapter have all been unshielded types Unshielded loops work well under most circumstances, but in some cases their pattern is distorted by interaction with the ground and nearby structures (trees, buildings, etc) In my own tests, trips to a nearby field proved necessary to measure the depth of the null because of interaction with the aluminum siding on my house Figure 15-8 shows two situations In Fig 15-8A we see the pattern of the normal free space loop, ie, a perfect figure-8 pattern When the loop interacts with the nearby environment, however, the pattern distorts In Fig 15-8B we see some filling of the notch for a moderately distorted pattern Some interactions are so severe that the pattern is distorted beyond all recognition
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15-7 Remote tuning scheme
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15-8A A normal free space loop
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15-8B Filling of the notch
The solution to the problem is to reduce interaction by shielding the loop, as in Fig 15-9 Loop antennas operate on the magnetic component of the electromagnetic wave, so the loop can be shielded against voltage signals and electrostatic interactions In order to prevent harming the ability to pick up the magnetic field, a gap is left in the shield at one point There are several ways to shield a loop You can, for example, wrap the loop in adhesive-backed copper-foil tape Alternatively, you can wrap the loop in aluminum foil and hold it together with tape Another method is to insert the loop inside a copper or aluminum tubing frame Or the list seems endless
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