I know absolutely nothing about Magnetic Loop Antennas! The OT's used to say if you want to make contacts then build a dipole, put it in the clear, up 100 feet in the air and you are there. The thought of a 40 inch in diameter copper loop sitting 2 feet above ground does not sound like a DX antenna, or any kind of antenna.
Gulp! A $2500 Mag Loop from Italy sold by DX Engineering
The best analogy I thought of for those using a loop: Taking your cousin to the prom and convincing yourself... she is a YL; but in truth is sort of unattractive, not a 44DD and your cousin.
So, why Mag Loops? I did do a bit of research on Mag Loops and found in truth they do work and are good in high noise environments and where hams are subject to space limitations or HOA's. They are not a one for one substitute for dipoles in the clear, at 100 feet so don't get too excited!
Basically, a Mag Loop is really two loops with the outer loop being in the range of 10 to 15 feet in diameter with the better performing loops being at least 0.5 inch in diameter copper tubing with the ends connected to a HV Variable Cap. The best caps are vacuum variables. More on that.
The second loop is 1/5 the length (20%) of the main loop and this connects to your transceiver or transmitter. Now the configurations seem to vary but most common has the smaller loop at the top dead center of the larger loop, and the feed point at the bottom of the smaller loop. The Variable loop tuning cap is at the bottom of the larger loop. The larger Loop is supported at the bottom typically on a small stand or tripod often only a few feet above ground. One feature of the Mag Loop not easily done with a wire dipole at 100 feet... it is rotatable for nulling out noise.
Now for the shocker: the loops are Hi Q with a limited tuning range once set... like 20-30 kHz (or smaller range). So, if you QSY (move frequency for the BTE's) outside that range you need a retune.
The other big shocker if you run 100 watts into the small loop, the circulating current in the big loop might be 10 to 15 amps and the voltage across the Variable Capacitor might be the close to 3500 Volts. That is the why of the large transmitting cap or vacuum variable. The reason is that the two loops form a step-up transformer where magnetic coupling (the Mag part of Mag Loop) connects the two loops.
VK3YE, Peter Parker had a good solution for the HV capacitor problem and that is a chunk of LMR coax. Coax cable has a certain capacitance per foot. His trick: Find an approximate length of coax to resonate the loop and solder each conductor at the ends of the outer loop and at the other end of the coax it is open. Two feet is maybe like 50 to 60 Picofarad. But this limits you to one band and one small tuning range.
Below are two you tube videos. I must have sampled about a dozen how to make it You Tube Videos and for someone unfamiliar with how to build one, these two seemed to hit the mark.
The second one answered a question for me. So, you got the expensive copper tubing, and now the problem how you make it into a perfectly round circle? His answer was he covered his dining room table with newspaper that was taped down. Then using a stick as a radius went around the newspaper with a felt tip pen forming a visual template. The final step has the tubing follow the pattern. Nice.
Bozos has 0.5-inch copper tubing in a 10-foot coil in the $50 range. A 40 Inch in diameter loop is 40X3.14159 in circumference or 125.7 inches. Divide that by 12 and you get 10.47 feet. The open ends of the outer loop are connected to the capacitor, so you make that opening 5.64 inches (use an insulated spacer) and you have a perfect 40 inch in diameter loop.
So after watching the two videos most hams conclude that it is too hard to build one (only 1% of hams homebrew) and so what can you get for less than $2500. Here is one commercial loop that is about 20% of the Italian offering.
Here is a link to the Alpha antenna website. By the time you are done adding in everything to make it work likely you will have spent close to the cost of an ICOM IC7300 (earlier version). But the Mag Loop may be the only solution for those who have significant noise issues and where other antennas may be impossible to install, such as an apartment or where there are prevailing HOA restrictions.
Safety Precautions: That circulating current can cause serious RF burns. Keep your hands off the loop while transmitting. One install caused me to shudder. The loop was made of large diameter copper > 1 inch in diameter and was installed on the ceiling of a shack centered over the operating position. I saw a high risk of frying the operator's brain... well, I think he WAS from the W5 call area.
Them that know, now don't care if you know.
73's
Pete N6QW
