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Multi-band regenerative loop antenna - The RadioBoard Forums Post by qrp-gaijin » Sun Jan 26, 2020 I'm planning a multi-band regenerative loop antenna, controlled by computer (specifically, a Raspberry Pi). I want to cover 500 kHz to 30 MHz. Here's what I've come up with so far. Comments would be welcome. ml.png (18.87 KiB) Viewed 1243 times The basic idea is to have a multi-turn loop antenna with taps on the various turns. The loop will be fairly small, maybe 20 cm in diameter (the width of a standard piece of paper -- similar to the dimensions of the Kiwa Loop, http://www.dxing.info/equipment/kiwa_mw_loop_plimmer.dx). Unused turns would be left floating. The portion of the coil that is in use is selected by the switches (only one of which is closed) on the left-hand side of the schematic. These will be computer-controlled relays. The portion of the coil that is in use (selected by one of the switches) is regenerated by the BJT Q-multiplier Q1/Q2. RF is taken from the emitter and is intended to be fed into the receiver. The receiver might be an SDR or another commercial receiver with 50-ohm input. The emitter is a low-impedance node and I have never tried taking RF off of the emitter of a cross-coupled pair, so I don't know if there will be enough RF signal here to feed the receiver. But it seemed easiest to take the RF off of the emitter. An alternative approach, though more mechanically complicated, would be to situate a small coupling loop inside the main loop, and to take off the RF signal via magnetic coupling, as is often done in small transmitting loops. There will be some stray capacitance through the switch from the Q-multiplier to the non-switched-in windings. There will also be stray capacitance between the connecting wires on the taps -- physically, each coil's tap must be located next to the other coil taps, and hence the connecting wires from all the taps to all the switches (relays) will be running parallel to each other in close proximity. Hopefully this won't cause a problem in practice. The first turn of the multi-turn loop -- which is always in the circuit -- will be a low-loss, wide copper strap. This can be thought of as the "main" transducer portion of the antenna which is primarily responsible for intercepting the incoming EM wave. The other turns can be made of thin wire and can be thought of as a "series loading coil" that is optionally added in series with the "main" loop. I took this idea from the CTL antenna (viewtopic.php?f=10&t=8611), which also recommended using one wide-strip loop wired in series with a number of thin-wire windings. In addition to computer control of the switches (to select the coil tap and hence the band of operation), the computer will also control the tuning voltage (by generating a PWM signal that will be filtered and amplified), and the regeneration control VR1 (by generating a control voltage to modify the resistance of a vactrol/light-dependent resistor). Finally, a computer-controlled pan-tilt mechanism is also planned to allow remote control of the physical orientation of the antenna. Any comments, or alternate designs for a bandswitched and regenerative loop antenna?
