Magnetic Loop Antenna Project
The small magnetic loop antenna is a compact efficient antenna that is ideal for portable operation or limited spaces and can be improvised inexpensively.
The antenna is essentially a tuned circuit with the inductor formed by a loop of wire measuring less than 1/4 wavelength in circumference and resonated to the operating frequency with a capacitor. Since its radiation resistance is low and circulating current large, the loop must be constructed of a low resistance large outer diameter conductor for best efficiency. Typically, magnetic loop antennas can be built from coaxial cable, hardline, or solid copper or aluminum tubing or ribbon.
These loops also have a very narrow bandwidth, requiring a variable capacitor for tuning to the operating frequency. As voltages on the order of thousands of volts develop across the capacitor, air variables or vacuum variable capacitors are used. To maintain the lowest series circuit resistance, the connections are preferably soldered and a split-stator or "butterfly" capacitor is preferred.
Most of the parts required to build the antenna are inexpensive and easy to find and a typical construction is described in this photo album and handout from our January 2012 presentation.
The required parts are:
A length of PVC pipe, or other insulating material suitable for the main support. The larger loops made of coaxial cable may also require a horizontal support.
A suitable length of coaxial cable, hardline or solid copper or aluminum tubing or ribbon for the radiating loop
A smaller diameter coupling loop of approximately 1/5 the length of the radiating loop (or alternatively a ferrite toroid core* or gamma match coupling)
A high voltage variable capacitor
Suitable connectors, enclosures and mounting hardware for the components
Planetary reduction drive
*In our tests, 6 to 10 turns of wire (depending on frequency) on an FT114-43 or FT140-43 toroid core coupling provided a 50 ohm impedance match and no excessive core heating at usual duty cycles up to about 50 watts for an FT114-43 core (100 watts for the FT140-43 core). Spreading the coil turns over the full circumference of the core distributes the magnetic field and thermal losses throughout the core, raising the effective core saturation threshold, improving coupling efficiency, antenna Q and radiated field strength. If needed, ferrite cores may be stacked to increase the power capacity. Larger diameter ferrite cores provides equally tight coupling and permits an easier fit over thicker loops. Careful snug winding against the core allowed a PL-259 connector to pass through the center of an FT140-43 core for easy assembly and disassembly of the loop.
A small coupling loop one fifth the size of the larger loop can tolerate higher power levels without heating and can be repositioned on each frequency band to provide the closest match to 50 ohm impedance.
The gamma match coupling method is reported to offer a good impedance match, but is best suited for rigid loop antennas.
Steve Yates, AA5TB, has very neatly documented the operating principles and his experience with magnetic loop antennas on his web page. Lewis McCoy W1ICP described these antennas in "The Army Loop in Ham Communication" in March 1968 QST, pp. 17-18 150-151, and Ted Hart W5QJR analyzed its operation in "Small, High Efficiency Loop Antennas" in June 1986 QST, pp. 33-36.
The recommended loop diameters for good efficiency are 1m (3 feet) for 14-30 MHz, 1.5m (4.2 feet) for 10-22 MHz and 2m (6 feet) for 7-14 MHz. Coaxial cable loops larger than 1m (3 feet) may require cross supports to maintain their shape. The graphs below for loops of 0.4" diameter conductor (RG-8/U cable) were made with the AA5TB calculator and show that the loops may be resonated at lower frequencies than these by increasing the capacitance but with reduced efficiency. Solid conductor loop material such as aluminum or copper strap or tubing or heliax cable offer lower resistance and higher efficiency than the braided shield of coaxial cable.
3 foot diameter loop antenna efficiency
6 foot diameter loop antenna efficiency
The tables below list the required capacitance ranges and number of sets of capacitor plates as calculated with the AA5TB spreadsheet calculator, [The Monarch Capacitors online calculator is no longer online 11/18/2021]. An alternative magnetic loop calculator at 66pacific.com yields results that vary somewhat from the AA5TB calculator. The actual frequency range of the finished antenna may vary depending on your construction.
The peak RF voltage on the capacitor increases in proportion to the square root of the power and is roughly equal to 425 times the square root of the power in watts for our 3 foot dia. loop of coaxial cable. The required plate spacing for a single stator capacitor is directly proportional to the maximum voltage, with 3 kV/mm being breakdown voltage of dry air. Half of that spacing is adequate for a split stator capacitor as the voltage is evenly divided between the two stator sections.
High voltage variable capacitors are now difficult to find, so these are building plans for a capacitor using either 3" diameter or 5" diameter CNC laser machined .040" gauge aluminum capacitor plates. You may order capacitor plate sets from Bob Leschyna, VE3UK of Monarch Capacitors or Vasile Pop, VA6POP.
<This diagram> from construction details for the butterfly capacitor frames are available from Monarch Capacitors.
Chuck W6FT provided the cut and drilled polycarbonate capacitor frame end plates and we assembled the capacitors.
The 3” and 5” rotor plates were assembled on a threaded rod with a shaft of 1/4" metal tubing tapped and screwed onto the threaded rod. Bob VE3UK of Monarch Capacitors suggested this standard aluminum hex rod for the rotor shaft, but this did not couple with the reduction drives we used.
High voltage variable capacitor.
Choose the table for your desired frequency range
Select the diameter of the wire or tubing you plan to use
On the line for your maximum power, find the number of needed 3” sets OR 5” sets
In our experience, we did achieve .09" plate spacing using measured standard nuts and washers for spacers. Closer spacing would have required more precision and uniformity of spacers to assure correct plate alignment throughout rotation.
The River City ARCS Magnetic Loop Build Party was held on Saturday, April 21, 2012 9:00 AM - 2:00 PM at the Sacramento County Corp Yard, 5026 Don Julio Blvd, Sacramento, CA 95842.
The Club purchased and had the following kit of parts on hand at the party for those who signed up and pre-ordered:
RG-8/U cable for loops and RG-58 or RG-59 cable for coupling loops
two PL259 and SO239 UHF connectors for each loop,
one female BNC or SO239 feed line connector for each loop,
RCA plugs and jacks (optional for coupling loop)
NE-2 neon glow lamp,
necessary bolts, nuts, spacers and washers,
½ inch PVC conduit clamps
6:1 reduction drive
butterfly capacitor plates
polycarbonate end plates for capacitor frames (pre-cut and drilled by W6FT)
polypropylene enclosures for capacitors
Our default feed method was a small coupling loop of RG-58/U cable, 1/5 the size of the main loop. You may provide and use your own ferrite toroid core if you prefer that coupling method.
The club had free donated coaxial cable on hand for loop material. You may provide your own building materials if you have another preference, such as hardline, or aluminum or copper tubing or strap, or weatherproof and UV resistant enclosures. See the tables on this page as loops thicker than RG-8/U can require more capacitor plates. A feed line and antenna analyzer were available for testing the loops.
At the time of this project the estimated parts costs were:
$4 for each 3 inch capacitor plate set*
$6 for each 5 inch capacitor plate set*
$10 for each 8 inch capacitor plate set*
$11 for the 6:1 reduction drive
The cost of the other parts was added on (about $20).
* Each capacitor plate set consists of one rotor plate and two stator plates.
Magnetic Loop Antennas on 3.5 MHz and Below
Magnetic loops at 3.5 MHz and below are a special case. If you were to scale our 1" tubing model to the 3.5 MHz frequency, you would need a 12 foot diameter loop and a 215 pF capacitor to achieve a 40% radiation efficiency.
Our 7-15 MHz 6 foot diameter model is half that size and could be resonated to 3.5 MHz with a 500 pF capacitor but it's efficiency would drop to 7.7%, similar to an 80 meter mobile antenna. You would need 15 of the 5" plate sets ($90) or 6 of the 8" plate sets ($60) to achieve that much capacitance and handle 100 watts.
Jeri Ellsworth AI6TK and Amy Herndon AI6ZU built a two turn magnetic loop antenna for 160 and 80 meters .
Jeri Ellsworth AI6TK demonstration of voltage and current distribution in magnetic loop antennas.