The Magnetic Loop Antenna |
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The much misunderstood magnetic loop antenna is a phenomenon that hams are finding intriguing. It is a methodology of covertly getting on the air in situations that prohibit unsightly antennas. Instead of employing electrical energy to generate electromagnetic waves, the mag loop uses magnetic energy. The magnetic loop seems to go against the nature of antennas. It's small, not anything near the size of a normal antenna. Also, instead of being placed as high as one can, they work best close to the ground. Are they efficient? Relative to a well placed dipole, they are said to not be. Their claim to fame is their small size. I wanted to get on the 160-meter band. Well, with the above loop sitting as pictured indoors in my woodshop, I was able to carry on QSOs hundreds of miles from my home - this with all kinds of heavy metal cast iron machinery right next to the antenna. Imagine what it can do outdoors away from anything ferrous metal. I have two other loops - a 6' single loop for 40 to 20 meters, and a 36" loop for 17 to 12 meters. Note. It is not really possible to build a magnetic loop antenna that will efficiently perform on more than two bands.
The above loop is 4 pieces of 1" x 10' schedule L copper pipe. All connections have been brazed.
I used a Harbor Freight tubing roller with a set of 1" pipe size (1.125) roller dies from ... I can't remember where I got them. Either I modified om the lathe a smaller set that cane with the HF or I may have got them from a flea market.
The coupler between large the aluminum pulley and the Vacuum variable capacitor is a 2" piece of Delran. The secondary switches in a parallel 20pf Vacuum capacitor that extends the antenna's range enabling it to tune up on the 160 meter band. Also, the tuning of the loop between 80 and 160 meters is faster with the switchable capacitor. With a duel loop design, there is additional capacitive reactance between the two loops.
The three magnetic loops use toroidal matches with multiple taps. This enables a near 1:1 match on all bands. Each loop has a stepper motor that drives a 30-amp 6 position selector switch for teh match and a stepper for the variable capacitor.
There is no digital logic in the antennas - just analog relays and a single stepper driver that is switched between the two steppers. Everything is opto-isolated.
Yes, I'm finally ready to install my stealth magnetic loop antennas in my HOA regulated backyard. I spent almost three years in researching, designing, experimenting, and building these magnetic loop antennas as well as perfecting each of the control mechanisms and PC interface.
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