14-30 MHz Magnetic Loop Antenna
Schematic Diagram of Ferrite Toroid Core
                          Feed Magnetic Loop Antenna with Variable
                          Transformer Ratio.

Magnetic Loop Antenna Parameters: 7-30 MHz

Antenna Parameters Measured with a Vector Network Analyzer

by Dr. Carol F. Milazzo, KP4MD (posted 20 September 2014)
E-mail: kp4md@arrl.net

INTRODUCTION

The small magnetic loop is a useful compromise antenna for limited space and portability. In the light of the many subjective anecdotal reports extolling their performance, comparisons of their measured operating characteristics against a validated model provide objective evidence that is needed to assure and understand efficient design and operation of small magnetic loop antennas.

Some desired goals include:

  • Non-reactive antenna to transmission line impedance matching for efficient power transfer.  This requires adjustment of the impedance transformation ratio due to variations in loop impedance over extended frequency ranges (see figure 8 below);
  • Low-resistance large-diameter loop material with minimal use of non-soldered mechanical connections;
  • Use of a high-Q vacuum or split-stator or butterfly air variable capacitor to minimize dielectric losses and to eliminate rotor contact resistance;
  • Adequate capacitor plate spacing to handle the expected voltage for the transmitter power level; and,
  • Narrow bandwidth (high Q) at resonance.  For any specific frequency and magnetic loop antenna, its Q is proportional to its radiation efficiency.  Broad bandwidth at the resonant frequency is not desirable as it indicates that power is radiated as heat (resistive loss) rather than as radio frequency energy.

These characteristics impact the material cost, size, portability, and performance of the antenna and should be available for the radio operator to make an informed choice in the purchase or construction of a magnetic loop antenna to meet their requirements.

TEST ANTENNA

The test antenna was a 0.9 m (3 foot) diameter loop of RG-8A/U coaxial cable mounted on a tripod at 1.52 m (5 feet) height above ground.  The antenna was oriented vertically with the capacitor above and feed point below as shown in the photograph.  A miniVNA Pro Vector Network Analyzer was attached directly to the antenna feed point, magnetically coupled to the loop through either 6, 8, or 10 turns of 14AWG solid copper wire in a primary winding on an FT114-43 toroid core as shown in the schematic diagram.

The antenna parameters were measured over the operating range of 7 MHz through 30 MHz.  To achieve resonance on 7 MHz a 150 pF shunt capacitor made of 1.54 m (60.5 inches) of RG-58/U coaxial cable was added in parallel, and on 10 MHz a 60 pF shunt capacitor made of 0.61 m (24 inches) of RG-58/U coaxial cable was added in parallel respectively across the 12-58 pF air variable capacitor.


This curve of the measured SWR demonstrates
                      the narrow 2:1 VSWR bandwidth of the Magnetic Loop
                      antenna when the capacitor is adjusted to
                      resonance near 7.1 MHz. The antenna should
                      function satisfactorily within 12 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.

1. This curve of the measured SWR demonstrates the 23 kHz 2:1 VSWR bandwidth of the Magnetic Loop antenna when the capacitor is adjusted to resonance near 7.103 MHz. The antenna should function satisfactorily within 12 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.


This curve of the measured SWR demonstrates
                      the 2:1 VSWR bandwidth of the Magnetic Loop
                      antenna when the capacitor is adjusted to
                      resonance near 10.1 MHz. The antenna should
                      function satisfactorily within 16 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.

2. This curve of the measured SWR demonstrates the 33 kHz 2:1 VSWR bandwidth of the Magnetic Loop antenna when the capacitor is adjusted to resonance near 10.102 MHz. The antenna should function satisfactorily within 16 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.


This curve of the measured SWR demonstrates
                      the narrow 2:1 VSWR bandwidth of the Magnetic Loop
                      antenna when the capacitor is adjusted to
                      resonance near 14.15 MHz. The antenna should
                      function satisfactorily within 10 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.

3. This curve of the measured SWR demonstrates the 20 kHz 2:1 VSWR bandwidth of the Magnetic Loop antenna when the capacitor is adjusted to resonance near 14.153 MHz. The antenna should function satisfactorily within 10 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.


This curve of the measured SWR demonstrates
                      the 33 kHz 2:1 VSWR bandwidth of the Magnetic Loop
                      antenna when the capacitor is adjusted to
                      resonance near 18.1 MHz. The antenna should
                      function satisfactorily within 16 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.

4. This curve of the measured SWR demonstrates the 33 kHz 2:1 VSWR bandwidth of the Magnetic Loop antenna when the capacitor is adjusted to resonance near 18.1 MHz. The antenna should function satisfactorily within 16 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.


This curve of the measured SWR demonstrates
                      the 39 kHz 2:1 VSWR bandwidth of the Magnetic Loop
                      antenna when the capacitor is adjusted to
                      resonance near 21.2 MHz. The antenna should
                      function satisfactorily within 20 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.

5. This curve of the measured SWR demonstrates the 39 kHz 2:1 VSWR bandwidth of the Magnetic Loop antenna when the capacitor is adjusted to resonance near 21.201 MHz. The antenna should function satisfactorily within 20 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.


Plot of Magnetic Loop Antenna Feed Point
                      Impedance vs. Frequency comparing 5 through 11
                      primary turns on the FT114-43 ferrite core. The
                      measurements on 14, 21 and 28 MHz were taken with
                      only the air variable tuning capacitor on the loop
                      antenna. The measurements on 7 MHz and 10 MHz were
                      taken with a 150 pF shunt coaxial capacitor and a
                      60 pF shunt coaxial capacitor respectively
                      connected in parallel with the air variable
                      capacitor. This yields the optimal turns ratio as
                      10 turns on 7 MHz and 14 MHz, 8 turns on 10 MHz
                      and 21 MHz, and 6 turns on 28 MHz.

6. This curve of the measured SWR demonstrates the 38 kHz 2:1 VSWR bandwidth of the Magnetic Loop antenna when the capacitor is adjusted to resonance near 24.913 MHz. The antenna should function satisfactorily within 19 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.


This curve of the measured SWR demonstrates
                      the 76 kHz 2:1 VSWR bandwidth of the Magnetic Loop
                      antenna when the capacitor is adjusted to
                      resonance near 28.4 MHz. The antenna should
                      function satisfactorily within 38 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.

7. This curve of the measured SWR demonstrates the 76 kHz 2:1 VSWR bandwidth of the Magnetic Loop antenna when the capacitor is adjusted to resonance near 28.405 MHz. The antenna should function satisfactorily within 38 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.


Plot of measured Magnetic Loop Antenna Feed
                      Point Impedance vs. Frequency. Comparison of feed
                      point impedances at 5 through 11 primary turns on
                      the FT114-43 ferrite core. VSWR < 1.2:1 from 42
                      to 60 ohms impedance. The measurements on 14-28
                      MHz were taken with only the air variable tuning
                      capacitor on the loop antenna. The curves are
                      skewed on 7 and 10 MHz due to the attachment of
                      the lossy 150 pF or 60 pF shunt coaxial capacitors
                      respectively on those frequencies.

8. Plot of measured Magnetic Loop Antenna Feed Point Impedance vs. Frequency.   Comparison of feed point impedances at 5 through 11 primary turns on the FT114-43 ferrite core.  VSWR < 1.2:1 from 42 to 60 ohms impedance.  The measurements on 14-28 MHz were taken with only the air variable tuning capacitor on the loop antenna.  The curves are skewed on 7 and 10 MHz due to the attachment of the lossy 150 pF or 60 pF shunt coaxial capacitors respectively on those frequencies.

Plot of measured 2:1 VSWR bandwidth vs.
                      frequency. The bandwidth values for 7 MHz and 10
                      MHz are higher than at other frequencies due to
                      the lossiness of the shunt coaxial capacitors.

9. Plot of measured 2:1 VSWR bandwidth vs. frequency.  The bandwidth values for 7 MHz and 10 MHz are higher than at other frequencies due to the lossiness of the shunt coaxial capacitors.

Plot of calculated Magnetic Loop Antenna Q
                      vs. Frequency. The Q values for 7 MHz and 10 MHz
                      are lower than at other frequencies due to the
                      lossiness of the shunt coaxial capacitors used on
                      those frequencies. (Calculator on
                      http://owenduffy.net/calc/VswrBw2AntQ.htm)

10. Plot of calculated Magnetic Loop Antenna Q vs. Frequency.   The Q values for 7 MHz and 10 MHz are lower than at other frequencies due to the lossiness of the shunt coaxial capacitors used on those frequencies.  (Calculator on http://owenduffy.net/calc/VswrBw2AntQ.htm)


Magnetic Loop Antenna Free Space Efficiency
                      vs. Frequency 7 MHz with 150 pF shunt coaxial
                      capacitor and 10 MHz with 60 pF shunt coaxial
                      capacitor. (Calculator at
                      http://owenduffy.net/calc/SmallTransmittingLoopBw2Gain.htm)

11. Plot of calculated Magnetic Loop Antenna Free Space Efficiency vs. Frequency.  (Calculator at http://owenduffy.net/calc/SmallTransmittingLoopBw2Gain.htm)


Plot of calculatated Magnetic Loop Antenna
                      Free Space Gain vs. Frequency. (Calculator at
                      http://owenduffy.net/calc/SmallTransmittingLoopBw2Gain.htm)

12. Plot of calculatated Magnetic Loop Antenna Free Space Gain vs. Frequency.  (Calculator at http://owenduffy.net/calc/SmallTransmittingLoopBw2Gain.htm)


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

13. 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).

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


SPECIFICATIONS AT RESONANCE

Impedance:  50 ohms non-reactive nominal
Standing Wave Ratio:  1.2:1 or less
Polarization:  Vertical at low angles, horizontal at high angles
Power Rating:  30-35 watts peak.  Capacitor arcing occurs as the power approaches 50 watts.
Toroid Core Warming (50 sec at 25 W):  2F at 14 MHz, 5F at 21 MHz, 8F at 28 MHz.  Toroid core temperature increase measured during a continuous 50-second transmission at 25 watts.  This is adequate for the lower intermittent duty cycles of usual operating modes such as CW and SSB.
Frequency range:

Frequency MHz Parallel
capacitor pF*
Min. Max.
13.8 33.5 None
9.3 11.8 60
6.8 7.6 150
3.67
3.78
565
3.5 3.6 715

*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.

ANTENNA PARAMETERS: Measured vs. NEC Model
Predicted Efficiency and Free Space Gain

Frequency.
MHz
2:1 SWR
BW kHz
    Q    Eff. %* Eff. %**
per Model
Free Space*
Gain dBi
Gain dBi**
per Model
28.4 76 271 56.7 66.1 -0.7 -0.2
24.9 50 374 52.7 54.5 -1.0 -1.1
21.2 39 398 33.4 37.1 -3.0 -2.7
18.1 32 399 21.7 24.4 -4.9 -4.5
14.15 20 498 12.9 11.3 -7.1 -7.8
10.1*** 33 221 2.1 3.7 -15.1 -13
7.1*** 23 198 0.6 1.1 -20.1 -18

* Antenna Q, Efficiency and Free Space Gain dBi derived from measured data with calculators by Owen Duffy.
** Efficiency and Free Space Gain dBi per Model calculated with the specified parameters for the NEC model.
*** The lossy coaxial shunt capacitors used on 7 and 10 MHz would account for the lower measured Q, efficiency and gain than the NEC model predictions for those frequencies.

CONCLUSION

The correlation of the measured parameters with those predicted by the 4nec2 Method of Moments calculations validate the magnetic loop antenna models.

REFERENCES

  1. Magnetic Loop Antenna for 80-20 Meters, Doerenberg, F, N4SPP
  2. Calculate Antenna Q from VSWR bandwidth measurement, Duffy, O, VK2OMD
  3. Calculate small transmitting loop gain from bandwidth measurement, Duffy, O, VK2OMD
  4. Ground effects on KP4MDs 0.9m loop on 28MHz, Duffy, O, VK2OMD
  5. Portable Magnetic Loop Antenna Version Two, Goff, R, G4FON
  6. Small High Efficiency Loop Antennas, Hart, T, W5QJR
  7. ZPLOTS Impedance Plots Using Excel Charts, Maguire D, AC6LA
  8. A Universal HF Magnetic Loop NEC Model, Milazzo, C, KP4MD
  9. Small Transmitting Loop Antennas, Yates, S, AA5TB
  10. Magnetic Loop Antenna Project, River City Amateur Radio Communications Society

LINKS

  1. 14-30 MHz Magnetic Loop Antenna
  2. Magnetic Loop Antenna Photo Album
  3. Magnetic Loop Antenna Measurements Album
  4. Magnetic Loop Antenna Club Project
  5. Magnetic Loop Antenna Yahoo Group

ZPLOTS DATA FILES
Frequency (Primary Turns)

  1. 7.103 MHz (10 turns)
  2. 10.102 MHz (8 turns)
  3. 14.153 MHz (10 turns)
  4. 18.1 MHz (8 turns)
  5. 21.201 MHz (8 turns)
  6. 24.913 MHz (8 turns)
  7. 28.405 MHz (6 turns)

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