14-30 MHz Magnetic Loop Antenna

Construction of a Compact and Efficient Portable High Frequency Antenna

by Dr. Carol F. Milazzo, KP4MD (posted 03 September 2012)
E-mail: kp4md@arrl.net

The small magnetic loop antenna is a compact efficient antenna that is ideal for portable deployment or for limited spaces and that can be improvised inexpensively.  The antenna is essentially a tuned circuit with an inductor formed by a loop of wire measuring less than 1/4 wavelength and resonated to the operating frequency with a capacitor.  Due to its low radiation resistance and large circulating current, the loop must be constructed of a large outer diameter conductor of low resistance for best efficiency.  Typically these loops are built from coaxial cable, hardline, or copper or aluminum tubing.  These loops have a very narrow bandwidth and require a variable capacitor (and preferably a reduction drive) to be resonated at the operating frequency.  Air variable capacitors or vacuum variable capacitors are used due to the voltage on the order of several thousand volts that is developed across the capacitor.  In order to maintain the lowest possible series resistance, soldered connections and a "butterfly" or split-stator capacitor are preferred. The addition of a fixed capacitor in parallel with the variable capacitor will allow operation of this antenna on 7 or 10 MHz at reduced efficiency.


Materials used for the tuning enclosure:
                      3¼” x 2½” x 4½” craft storage
                      box from Michael's (2 for $2.99), 1/2"
                      conduit clamp (5/$0.99) and .08" acrylic
                      sheet ($1.99) from Lowe's, Jackson Bros. 4511 DAF
                      6:1 ball drive from my junk box.

1. Materials used for the tuning enclosure: 3¼” x 2½” x 4½” craft storage box from Michael's (2 for $2.99), 1/2" conduit clamp (5/$0.99) and .08" acrylic sheet ($1.99) from Lowe's, and a Jackson Bros. 4511 DAF 6:1 planetary reduction ball drive.

Butterfly capacitor ($5 at swap meet), 19
                      plates 1.5" diameter, 0.02" plate
                      spacing provided 24-116 pF per section, or an
                      effective 12-58 pF tuning range as we use the
                      sections connected in series. The calculated
                      voltage rating is 2-5 kV. A piece of the acrylic
                      sheet was cut and drilled with a Dremel tool and
                      mounted to the front of the capacitor.

2. Butterfly capacitor ($5 at swap meet), 19 plates 1.5" diameter, 0.02" plate spacing provided 24-116 pF per section, or an effective 12-58 pF tuning range as we use the sections connected in series. The calculated voltage rating is 2-5 kV.  A piece of the acrylic sheet was cut and drilled with a Dremel tool and mounted to the front of the capacitor.

Another
                      piece of acrylic sheet was cut and drilled to
                      mount the reduction drive needed due to the very
                      sharp tuning at resonance. Reduction drives are
                      available through The Xtal Set Society and MFJ
                      Enterprises. An angle bracket was attached to
                      mount the assembly inside the tuning enclosure.

3. Another piece of acrylic sheet was cut and drilled to mount the reduction drive needed due to the very sharp tuning at resonance.  Reduction drives are available through The Xtal Set Society and MFJ Enterprises.  An angle bracket was attached to mount the assembly inside the tuning enclosure.

The
                      assembled tuning enclosure. The shell of a SO-239
                      connector was soldered with 8 AWG stranded copper
                      wire to each stator section of the butterfly
                      capacitor. The center pins and the capacitor rotor
                      were left unconnected. The NE-2 neon lamp serves
                      as a resonance indicator. Both leads of the lamp
                      are soldered together to only one of the stator
                      sections. The lamp is in the air about an inch
                      away from the other stator section.

4. The assembled tuning enclosure. The shell of a SO-239 connector was soldered with 8 AWG stranded copper wire to each stator section of the butterfly capacitor.  The center pins and the capacitor rotor were left unconnected. The NE-2 neon lamp serves as a resonance indicator. Both leads of the lamp are soldered together to only one of the stator sections. The lamp is in the air about an inch away from the other stator section.

Schematic diagram of ferrite toroid core feed
                      Magnetic Loop Antenna with variable transformer
                      ratio. The transformer couples the very low
                      impedance of the loop to the 50 ohm impedance of
                      the transmission line. As described by G4FON on
                      http://www.g4fon.net/MagLoopTwo.htm2, a 3 position
                      switch was wired to vary the transformer ratio,
                      selecting taps at either 5, 7 or 9 turns to
                      achieve the lowest SWR on each frequency band.

5. Schematic diagram of ferrite toroid core feed Magnetic Loop Antenna with variable transformer ratio.  The transformer will couple the very low impedance of the loop to the 50 ohm impedance of the transmission line.  As described by G4FON on http://www.g4fon.net/MagLoopTwo.htm2, a 3 position switch was wired to vary the transformer ratio, selecting taps at either 5, 7 or 9 turns to achieve the lowest SWR on each frequency band.

A
                      wide-spaced 9-turn coil of 14 AWG solid copper
                      wire was wound on an FT140-43 ferrite toroid core
                      threaded over the loop wire (a 112 inch length of
                      RG-8A/U coaxial cable terminated in PL-259
                      connectors). Taps would later be soldered on the
                      5th and 7th turns after mounting in the coupling
                      enclosure. Spreading the coil turns over the full
                      circumference of the core improves coupling
                      efficiency, antenna Q and radiated field
                      strength.

6. A wide-spaced 9-turn coil of 14 AWG solid copper wire was wound on an FT140-43 ferrite toroid core threaded over the loop wire (a 112 inch length of Alpha Wire 9219A RG-8A/U coaxial cable terminated in PL-259 connectors).  Taps would later be soldered on the 5th and 7th turns after mounting in the coupling enclosure.  Spreading the coil turns over the full circumference of the core improves coupling efficiency, antenna Q and radiated field strength.

A view
                      of the coupling enclosure on the assembled
                      antenna. The 3 position slide switch selects 5, 7
                      or 9 turns of the primary winding on the ferrite
                      toroid core. SWR was 1:1 or better at resonance.
                      Best match occurred with the 5 turn tap on 25-28
                      MHz, the 7 turn tap on 21-25 MHz and the full 8
                      turns on 14-21 MHz.

7. A view of the toroid core in the coupling enclosure. The 3 position slide switch selects 5, 7 or 9 turns of the primary winding on the ferrite toroid core.  The SWR was 1.2:1 or better at resonance.  The best match occurred with the 5 turn tap on 25-28 MHz, the 7 turn tap on 21-25 MHz and the full 9 turns on 14-21 MHz.

A view
                      of the bottom of the coupling enclosure showing
                      the 3 position switch and BNC connector. This 3
                      position slide switch was salvaged from a defunct
                      blow dryer.

8. A view of the bottom of the coupling enclosure showing the 3 position switch and BNC connector.  This 3 position slide switch was salvaged from a defunct blow dryer. 

The
                      magnetic loop antenna assembled on a 5 foot length
                      of 1/2 inch PVC conduit and mounted on a tripod.
                      The 112 inch loop of RG-8A/U cable is attached to
                      the SO-239's on the tuning enclosure. The
                      transmission line is RG-58/U coaxial cable with 2
                      turns threaded through ferrite cores at the feed
                      point to form a choke balun. The capacitor is
                      adjusted using a vinyl mini-blind wand attached to
                      the shaft of the reduction drive with a small hose
                      clamp.

9. The magnetic loop antenna assembled on a 5 foot length of 1/2 inch PVC conduit and secured to a tripod with Velcro straps. The ends of the 112 inch loop of RG-8A/U cable are attached to the SO-239's on the tuning enclosure. The transmission line is RG-58/U coaxial cable with several turns wound into a coil at the feed point to form a choke balun.  The capacitor is adjusted using a vinyl mini-blind wand attached to the shaft of the reduction drive with a small hose clamp.

A
                      close-up view of the tuning enclosure with the
                      NE-2 neon lamp glowing at resonance. The range of
                      this capacitor allowed tuning from 13.82 MHz to
                      33.5 MHz. Attaching a 150 pF fixed capacitor in
                      parallel across the butterfly capacitor allowed
                      tuning over the 7 MHz band. A 47 pF capacitor in
                      parallel allowed operation on the 10.1 MHz band.

10. A close-up view of the tuning enclosure with the NE-2 neon lamp glowing at resonance on 14 MHz at 5 watts.

The
                      capacitor is first adjusted for maximum received
                      noise on the operating frequency, then fine tuned
                      for lowest SWR. Tuning for maximum brightness on
                      the the neon lamp coincided with maximum
                      deflection of the field strength meter and was
                      sufficiently close to minimum SWR.

11. The capacitor is first adjusted for maximum received noise on the operating frequency, then fine tuned for lowest SWR while transmitting.  Tuning for maximum brightness on the the neon lamp coincided with maximum deflection of the field strength meter and was sufficiently close to minimum SWR.

A test
                      run of the magnetic loop antenna using 5 watt WSPR
                      transmissions with a FLEX-1500 on 14 MHz yielded
                      these confirmations of reception and transmission
                      with Japan, Hawaii, Venezuela, Canada and the U.S.
                      between 2226 UTC 01 JAN and 0356 UTC 02 JAN 2012.

12. A test run of the magnetic loop antenna using 5 watt WSPR transmissions with a FLEX-1500 on 14 MHz yielded these confirmations of reception and transmission with Japan, Hawaii, Venezuela, Canada and the U.S. between 2226 UTC 01 JAN and 0356 UTC 02 JAN 2012.

Here is
                      a high voltage shunt capacitor made from 24 inches
                      of RG-58/U cable (approximately 60 pF). The 3 foot
                      diameter magnetic loop antenna tuned from 9.35 to
                      11.8 MHz with this capacitor attached across the
                      butterfly capacitor stator terminals. On 30 May
                      2013, a JT65 contact was successfully completed
                      over 5,745 miles distance with EA7AH in Huelva,
                      Spain, on 10.138 MHz using 30 watts and this
                      capacitor on the indoor magnetic loop antenna.

13. Here is a high voltage shunt capacitor made from 24 inches of RG-58/U cable (approximately 60 pF). The 3 foot diameter magnetic loop antenna tuned from 9.35 to 11.8 MHz with this capacitor attached across the butterfly capacitor stator terminals. On 30 May 2013, a JT65 contact was successfully completed over 5,745 miles distance with EA7AH in Huelva, Spain, on 10.138 MHz using 30 watts and this capacitor on the indoor magnetic loop antenna.

Here is
                      a high voltage shunt capacitor made from 60.5
                      inches of RG-58/U cable (approximately 150 pF)
                      attached to the indoor 3 foot magnetic loop
                      antenna. The antenna tuned from 6.86 to 7.67 MHz
                      with this capacitor attached across the butterfly
                      capacitor stator terminals. It handled 30 watts
                      transmitter power with only some perceptible
                      warming after several JT65 transmissions of 50
                      second duration.

14. Here is a high voltage shunt capacitor made from 60.5 inches of RG-58/U cable (approximately 150 pF) attached to the indoor 3 foot magnetic loop antenna. The antenna tuned from 6.86 to 7.67 MHz with this capacitor attached across the butterfly capacitor stator terminals. It handled 30 watts transmitter power with only some perceptible warming after several JT65 transmissions of 50 second duration.

The
                      NE-2 neon lamp glows brightly during a 30 watt
                      JT65 transmission on 7.076 MHz.

15. The NE-2 neon lamp glows brightly at resonance during a 30 watt JT65 transmission on 7.076 MHz.

Azimuth Radiation Patterns for Magnetic Loop
                      Antenna modeled for 14, 21 and 28 MHz at 5 feet
                      above Average Ground (4nec2 model).

16. Azimuth Radiation Patterns for Magnetic Loop Antenna modeled for 14, 21 and 28 MHz at 5 feet above Average Ground (4nec2 model).

Elevation Radiation Patterns for Magnetic
                      Loop Antenna modeled for 14, 21 and 28 MHz at 5
                      feet above Average Ground (4nec2 model).

17. Elevation Radiation Patterns for Magnetic Loop Antenna modeled for 14, 21 and 28 MHz at 5 feet above Average Ground (4nec2 model).

This curve of the measured SWR demonstrates
                      the narrow bandwidth of the Magnetic Loop antenna
                      when the capacitor is adjusted to resonance at
                      28.4 MHz. The antenna should function
                      satisfactorily within 50-100 kHz of this resonant
                      frequency and significantly decrease noise and
                      interference from undesired signals outside of
                      this frequency range. The capacitor requires
                      adjustment for operation on frequencies outside of
                      this range.

18. This curve of the measured SWR demonstrates the narrow bandwidth of the Magnetic Loop antenna when the capacitor is adjusted to resonance at 28.4 MHz.  The antenna should function satisfactorily within 50-100 kHz of this resonant frequency and significantly decrease noise and interference from undesired signals outside of this frequency range.  The capacitor requires adjustment for operation on frequencies outside of this range.

SPECIFICATIONS AT RESONANCE

Impedance:  50 ohms non-reactive nominal
Standing Wave Ratio:  1.2:1 or less
2:1 VSWR Bandwidth:  ~150 kHz at 28.4 MHz
Polarization:  Vertical at low angles, horizontal at high angles
Frequency range:

Frequency MHz Parallel
capacitor pF*
Min. Max.
13.8 33.5 None
9.3
11.8 60
6.8 7.6 150

*A fixed capacitor connected in parallel across the butterfly capacitor allowed operation over either the 7 MHz or 10.1 MHz bands.  These capacitors should be rated at 5 kV or better for 30 watt transmissions.

POWER CAPACITY

Power Rating:  30 watts peak
Capacitor arcing occurs as the power approaches 50 watts.

Toroid Core Warming*
Frequency
MHz
ºF
14 2
21 5
28 8
Typical Gain**
Freq. MHz Gain dBi
28 -0.8
21 -0.2
14 -7
10
-11
7 -15

*Toroid core temperature increase measured during a 50-second JT65 transmission at 25 watts.  This performance should be adequate for the lower intermittent duty cycles of usual operating modes.
** Maximum gain calculated with the specified NEC model parameters.

NEC Model Parameters

Height above ground 5 feet
Simulated ground type
Average
Loop diameter 3 feet
Loop NEC model segments 18
RG-8A/U cable outer diameter
0.405 inches
Outer Jacket material
PVC
Outer braid diameter
0.32 inches
RG-8/U braid conductivity*
4500000 mhos/m
Capacitor Q (typical) 2000
*RG-8A/U braid conductivity derived from measured impedances.

REFERENCES

  1. Magnetic Loop Antenna for 80-20 Meters, Doerenberg, F, N4SPP
  2. Portable Magnetic Loop Antenna Version Two, Goff, R, G4FON
  3. Small High Efficiency Loop Antennas, Hart, T, W5QJR
  4. Small Transmitting Loop Antennas, Yates, S, AA5TB
  5. A Universal HF Magnetic Loop NEC Model, Milazzo, C, KP4MD
  6. Magnetic Loop Antenna Project, River City Amateur Radio Communications Society

LINKS

  1. Photo Album
  2. Magnetic Loop Antenna Club Project
  3. Magnetic Loop Antenna Yahoo Group

4nec2 Model Files

These 4nec2 model files have been validated for this magnetic loop antenna.

  1. Magnetic Loop Antenna on 7 MHz
  2. Magnetic Loop Antenna on 10 MHz
  3. Magnetic Loop Antenna on 14 MHz
  4. Magnetic Loop Antenna on 18 MHz
  5. Magnetic Loop Antenna on 21 MHz
  6. Magnetic Loop Antenna on 25 MHz
  7. Magnetic Loop Antenna on 28 MHz
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