I’ve been interested in the magnetic loop antennas
after having read how efficient they are, and in some ways are “Tesla-esque” being one of the oldest antenna forms out there behind the end-fed Zepp and the simple dipole. Having just moved to more urban digs, it seemed like a good project. The venerable magnetic loop antenna. which is basically a large copper coil with a smaller resonant loop inside can be incredibly efficient, but its drawbacks are that it’s also incredibly finicky – the increased Q
(narrow range of resonant frequency) means that you have to tune the thing for every 2-3 khz change in frequency. And to do that, you need about 250-1300 picoFarads of capacitance to tune out the inductive reactance of what is basically a tuned LC circuit
. The proper way to do it is to mount the capacitor directly to the loop (to minimize R) and to move the capacitor stator arm back and forth with a motorized contrivance of some flavor. Because the antenna is so sensitive to capacitance, even body capacitance throws it out of tune while you’re trying to tune it, so remote operation at the very least is desired. In my case, I’m an Arduino buff and you can build the whole thing for under $20 USD with an Arduino Nano and a MAX232 and ULN2003A bipolar stepper driver (darlington array). This gives you an RS232 controllable motor controller with memory that is perfect to control a capacitor remotely. Moreover, you can write those positions to memory and recall specific positions for specific frequency recall. You could then, say send a frequency command in ASCII over an RS232 connection and make the antenna tune for resonance as this guy did
. Instead, I would like to measure for VSWR
and thus be able to tune by performance, like an automatic antenna tuner.
The first part for testing was to find a capacitor with sufficient voltage handling. Even at small “QRP” power levels, the voltage potential can be several hundreds of volts, so the preferred capacitor is either a vacuum variable or a Butterfly capacitor. I was working cheap though, and vacuum variable caps start at about $120 USD, so I found an air variable capacitor that was fairly cheap ($20USD) and robust on eBay with a simple shaft connection and now in the process of assembling the loop. Stay tuned…In the meantime, here are some embedded videos of what others are doing with remote capacitance tuning of magnetic loop antennas:
my “cheap” capacitor – lining up with a stepper I have in my shop to see how the shafts line up.
Example of a design I am trying to mimic