Magnetic Loop


Magnetic Loop Antenna

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 copper or aluminum tubing.
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 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.  

6:1 Reduction Drive (optional)
The required parts are:
  1. 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.
  2. A suitable length of coax, hardline or copper or aluminum tubing for the radiating loop
  3. 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)
  4. A high voltage variable capacitor
  5. A 6:1 planetary reduction drive (not mandatory but preferred) available through The Xtal Set Society or MFJ Enterprises.
  6. Suitable connectors, enclosures and mounting hardware for the components
*In our tests, the FT-114-43 toroid core coupling provided a 50 ohm impedance match and no excessive core heating at usual duty cycles up to about 50 watts.  A small coupling loop one fifth the size of the larger loop can tolerate higher power levels without heating and can be positioned to provide a 50 ohm match.  The gamma match coupling method is reported to give a good impedance match, but is best suited for rigid loop antennas.  

Magnetic Loop Design

Steve Yates, AA5TB, has very neatly documented the operating principles and his experience with magnetic loop antennas on his web page.  The original 1986 article by Ted Hart that describes the small magnetic loop antenna is at http://www.we0h.us/Amateur_Radio_stuff/Transmitting-Loops/High_Efficiency_Transmit_Loop.pdf

Homebrew Butterfly Capacitor
The tables below list the required capacitance ranges and number of sets of capacitor plates as calculated with the AA5TB spreadsheet calculator and the Monarch Capacitors online calculator.  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 recommended loop diameters are 3 feet for 14-30 MHz, 4.2 feet for 10-22 MHz and 6 feet for 7-14 MHz.  Coaxial cable loops larger than 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.

Efficiency of 3 foot diameter magnetic loop antenna Efficiency of 6 foot magnetic loop antenna
 3 foot diameter loop antenna efficiency  6 foot diameter loop antenna efficiency

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 that we ordered.
  • 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.
We used the tables at the right or this worksheet to estimate the required number of sets of Monarch capacitor plates:   (Click here for the antenna parameters we measured with our Monarch prototype capacitor.)  Your antenna's actual frequency range will vary depending on its construction.
  1. Choose the table for your desired frequency range 
  2. Select the diameter of the wire or tubing you plan to use 
  3. On the line for your maximum power, find the number of needed 3” sets OR 5” sets
  4. 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.

QRP Ferrite Core Coupled Magnetic Loop Antenna 14-30 MHz

Antenna Party
21 April 2012

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.

QRP Ferrite Core Coupled Magnetic Loop Antenna 14-30 MHz Plans

ANTENNA BUILDING IDEAS    ASSEMBLY & TEST RESULTS

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 (suitable for low power operation).

The club had coaxial cable on hand for loop material. You may provide your own building materials if you have another preference, such as hardline or copper tubing 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 was available for testing the loops.

At the time of this project the estimated parts costs were:

  • 8, 5, and 3 in. Monarch Capacitor plates$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 Antenna Party


Although the event has passed, you may e-mail us with specific questions about planning your antenna.
 




Freq. Range 14-30 MHz
Loop dia. 3 ft. Circ. 113 in.
Wire dia. 0.4” RG-8/U
Cap. range: 10-50 pF
Wire dia. 1” tubing
Cap. range: 13-62 pF
Power
Watts
Max
Voltage
Plate
spacing
No. of
3” sets
No. of
5” sets
No. of
3” sets
No. of
5” sets
50 3000 .04” 1 mm 3 1 4 2
100 4300 .06” 1.5 mm 4 2 5 2
200 5900 .08” 2 mm 6 2 7 3

Freq. Range 10-22 MHz
Loop dia. 4.2 ft. Circ. 158 in.
Wire dia. 0.4” RG-8/U
Cap. range: 13-65 pF
Wire dia. 1” tubing
Cap. range: 16-80 pF
Power
Watts
Max
Voltage
Plate
spacing
No. of
3” sets
No. of
5” sets
No. of
3” sets
No. of
5” sets
50 3000 .04” 1 mm 4 2 5 2
100 4300 .06” 1.5 mm 5 2 7 3
200 5900 .08” 2 mm 7 3 9 3

Freq. Range 7-15 MHz
Loop dia. 6 ft. Circ. 226 in.
Wire dia. 0.4” RG-8/U
Cap. range: 18-86 pF
Wire dia. 1” tubing
Cap. range: 22-104 pF
Power
Watts
Max
Voltage
Plate
spacing
No. of
3” sets
No. of
5” sets
No. of
3” sets
No. of
5” sets
50 3000 .04” 1 mm 5 2 6 2
100 4300 .06” 1.5 mm 7 3 9 3
200 5900 .08” 2 mm 9 4 11 4


Please e-mail us if you wish assistance in designing a magnetic loop with other specific parameters.

Radio Frequency HazardCAUTION: Due to the high voltages and strong radiation field this antenna may produce, take care to avoid close proximity to this antenna while transmitting significant power levels.  Refer to Table 17 in the FCC Compliance Bulletin
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Carol Milazzo,
Mar 7, 2012, 12:50 PM
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Carol Milazzo,
Feb 13, 2012, 12:03 AM
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