Anyone interested in magnetic loop antenna design/fabrication? If not, no need to read further….
Magnetic loop antennas are becoming increasingly popular with hams, particularly in neighborhoods with restrictions on outside antennas. Some of us have been successful with dipoles in the attic, but some hams have aluminum-backed roof insulation that renders electric-field antennas (like the classic dipole) ineffective. Enter the magnetic field loop to the rescue! Aluminum has little effect on the magnetic field of radio waves AND are very compact compared to a dipole antenna. The trade-offs come with bandwidth and voltage considerations. A magnetic loop antenna usually has a much tighter bandwidth that requires a tuning capacitor….and the voltages at the capacitor can be quite high (1000’s of volts at transmission powers of only 50 watts). A loop diameter of only 3-4 feet can make an effective antenna at 40 meters and a 6-foot diameter loop should work well on 80 meters! Antennas hidden in an attic keep neighborhood associations happy AND keep them relatively protected from storms like the hurricanes we prepare for.
A few weeks ago, Frank Rysavy (KA9PYU), kindly provided KPARC with some sections of heliax (of various diameters) to encourage making magnetic loop antennas. I’ve snagged a couple pieces but there are several left. So, if there’s interest by club members, I plan to have a workshop on making your own HF transmitting loop antenna in the near future. If you’re interested in building one, please drop by the KPARC club room (or contact me) and snag a piece. I’ve included a mag loop calculator so that you can enter your lowest design frequency and subsequent loop diameter….that will tell you how much heliax to take. The basic rule-of-thumb is to keep the loop circumference less than 1/10th of a wavelength. Also, use the calculator to determine what range of capacitance you’ll need for the tuning cap. I found a 1000pf, 3KV variable air cap on eBay….the best choice is a vacuum cap, but they’re more expensive (but will allow running higher power).
If you are interested in the heliax Frank provided, feel free to take a section. If you’re interested in a workshop on this, drop me a line and we’ll put something together (probably Jan-Feb time frame).
Want to explore this topic a bit more? There are many websites describing loop antenna projects out there. Here’s one, for starters:
Keith / KI5US
Small Loop Equations for a Copper Loop
(circular loop assumed, results may vary with other shapes)
The full Excel File can be downloaded here magloop-40m-4foot