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"Marc Battyani" wrote in message
... Hello, I've made a Class-E amplifier which works rather well as I get 25VRMS into a 50 Ohm resistor. But heating a resistor is not really want I do ;-) I want to send this power into a loop antenna to transmit power by induction to another aantenna 10 to 20 mm away. The transmit loop antenna is in series with a capacitor and the receive antenna is in parallel with a capacitor. Both antennas are tuned to the correct frequency. The problem is that though I have 125Vpkpk at the transistor drain, I only have 15Vpkpk at my loop antenna and also 15Vpkpk in the receive antenna (not loaded) I guess I need to to some impedance matching but how do I compute the impedance of those loops antennas (is it just the L//C ?) and how do I compute the output impedance of the class-E amplifier ? Any suggestion, URL etc ? It sounds to me like you are talking about audio (or low) frequencies? I think you are correct in that you need some impedance matching. The impedance of your antenna is lower than 50 ohms so does not develop as much voltage. The best way to work out the impedance of the loop is to measure it. If you are absolutely sure it is at resonance then it will be purely resistive and you can measure the impedance of the loop with the equipment you have - if it is not at resonance there will be phase errors in the following measurement. Connect your 50 ohm resister in series with your loop and then connect this to the output of the amplifier. Measure the voltage across the output of the amplifier. If it is reading over 100v pk2pk turn the drive down so it does not saturate the output transistors. Next disconnect the voltmeter and connect it across the 50 ohm resistor and note the voltage (Vr). Disconnect the voltmeter and connect it across the loop and note the voltage (Vl). You know one resistance Rr = 50 ohms. Since the current is the same in both loads and they are in phase you can use ohms law. Vr/Rr = Vl/Rl rearranging you get R_loop = (E_loop * 50ohms) / E_resistor it doesn't really matter if you measure RMS or pk2pk good luck and let us know how you go. Thanks, Marc Battyani |
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