50 MHz Halo Antenna

Construction and Analysis of a Low Cost Omnidirectional Horizontally Polarized Antenna for 50 MHz

by Dr. Carol F. Milazzo, KP4MD (posted 15 Mar 2014)
E-mail: kp4md@arrl.net


The
                      basic horizontally polarized antenna is a
                      half-wave dipole. It has a radiation pattern that
                      exhibits lobes and sharp nulls. The impedance at
                      its feed point is 75 ohms in free space.

1. The basic horizontally polarized antenna is a half-wave dipole. It has a radiation pattern that exhibits lobes and sharp nulls.  The impedance at its feed point is 75 ohms in free space.

In
                      order to obtain a horizontally polarized antenna
                      that radiates almost equally in all directions, we
                      may bend the half-wave dipole to form an open
                      circular loop. This configuration is known as an
                      HO loop or halo antenna. Its impedance at the feed
                      point is about 12 ohms in free space. The halo
                      antenna requires a device such as a gamma match to
                      efficiently couple its low impedance to 50 ohm
                      coaxial cable. There is high RF voltage across the
                      open ends of the halo loop, and these must not
                      touch each other.

2. In order to obtain a horizontally polarized antenna that radiates almost equally in all directions, we may bend the half-wave dipole to form an open circular loop.  This configuration is known as an HO loop or halo antenna.   Its impedance at the feed point is about 12 ohms in free space.  The halo antenna requires a device such as a gamma match to efficiently couple its low impedance to 50 ohm coaxial cable.   There is high RF voltage across the open ends of the halo loop, and these must not touch each other.

I used
                      a 112 inch (284 cm) long piece of 1/2 inch (12.5
                      mm) soft aluminum tubing coil to make the halo
                      antenna. I formed the aluminum tubing into an open
                      loop of 37 inch (94 cm) diameter.

3. I used a 112 inch (284 cm) long piece of 1/2 inch (12.5 mm) soft aluminum tubing coil to make the halo antenna.  I formed the aluminum tubing into an open loop of 37 inch (94 cm) diameter.

50 MHz
                      Halo Antenna - omnidirectional horizontally
                      polarized. The full sized halo loop is 112 inches
                      or 284 cm (37 inch or 94 cm diameter) of 1/2 inch
                      (12.5 mm) soft aluminum tubing coil. The gamma
                      match rod is 18 inches (46 cm) of 12 AWG solid
                      copper wire spaced 1.5 inches (3.8 cm) from the
                      halo loop and with a shorting clip at 13.5 inches
                      (34.3 cm). The supporting boom is 1/2 inch (12.5
                      mm) Schedule 40 PVC conduit.

4. 50 MHz Halo Antenna - omnidirectional horizontally polarized. The full sized halo loop is 112 inches or 284 cm (37 inch or 94 cm diameter) of 1/2 inch (12.5 mm) soft aluminum tubing coil. The gamma match rod is 18 inches (46 cm) of 12 AWG solid copper wire spaced 1.5 inches (3.8 cm) from the halo loop and with a shorting clip at 13.5 inches (34.3 cm). The supporting boom is 1/2 inch (12.5 mm) Schedule 40 PVC conduit.

Another
                      view of the 50 MHz Halo Antenna.

5. Another view of the 50 MHz Halo Antenna.

Close
                      up view of the feed point - SO-239 receptacle and
                      center point of halo loop are secured to the boom
                      with hose clamps. An Arco 466 mica compression
                      trimmer capacitor 105-480 pF is soldered between
                      the center pin of the SO-239 and the gamma match
                      rod.

6. Close up view of the feed point - SO-239 receptacle and center point of halo loop are secured to the boom with hose clamps. An Arco 466 mica compression trimmer capacitor 105-480 pF is soldered between the center pin of the SO-239 and the gamma match rod.

Close
                      up view of the feed point - SO-239 receptacle and
                      center point of halo loop are secured to the boom
                      with hose clamps. An Arco 466 mica compression
                      trimmer capacitor 105-480 pF is soldered between
                      the center pin of the SO-239 and the gamma match
                      rod.

7. Close up view of the feed point - SO-239 receptacle and center point of halo loop are secured to the boom with hose clamps. An Arco 466 mica compression trimmer capacitor 105-480 pF is soldered between the center pin of the SO-239 and the gamma match rod.

Close
                      up view of the feed point - SO-239 receptacle and
                      center point of halo loop are secured to the boom
                      with hose clamps. An Arco 466 mica compression
                      trimmer capacitor 105-480 pF is soldered between
                      the center pin of the SO-239 and the gamma match
                      rod.

8. Close up view of the feed point - SO-239 receptacle and center point of halo loop are secured to the boom with hose clamps. An Arco 466 mica compression trimmer capacitor 105-480 pF is soldered between the center pin of the SO-239 and the gamma match rod.

Side
                      view of the feed point. A BNC to UHF adapter is on
                      this SO-239 receptacle.

9. Side view of the feed point. A BNC to UHF adapter is on this SO-239 receptacle.

Side
                      view of the feed point. A BNC to UHF adapter is on
                      this SO-239 receptacle.

10. Another view of the feed point. A BNC to UHF adapter is on this SO-239 receptacle.

An
                      alligator clip serves as a temporary shorting bar.
                      The capacitance and the position of the shorting
                      bar are adjusted alternately until a 50 ohm
                      non-reactive impedance is achieved at the feed
                      point (1:1 standing wave ratio). This occurred at
                      13.5 inches (34.3 cm) from the feed point on my
                      antenna.

11. An alligator clip serves as a temporary shorting bar.  The capacitance and the position of the shorting bar are adjusted alternately until a 50 ohm non-reactive impedance is achieved at the feed point (1:1 standing wave ratio).  This occurred at 13.5 inches (34.3 cm) from the feed point on my antenna.

A split
                      bolt connector soldered to the shorting bar
                      replaces the alligator clip for a durable
                      adjustable connection to the gamma match rod.

12. A split bolt connector soldered to the shorting bar replaces the alligator clip for a durable adjustable connection to the gamma match rod.

The
                      open ends of the halo loop are supported inside 3
                      inch (7.6 cm) lengths of PVC conduit on the Tee
                      fitting. The one inch markings on the aluminum
                      tubing indicate the gap distance which determines
                      the resonant frequency. A gap distance of 2.5
                      inches (6.4 cm) yielded a resonant frequency of
                      50.3 MHz. Small sheet metal screws through the PVC
                      secure the gap spacing.

13. The open ends of the halo loop are supported inside 3 inch (7.6 cm) lengths of PVC conduit on the Tee fitting. The one inch markings on the aluminum tubing indicate the gap distance which determines the resonant frequency. A gap distance of 2.5 inches (6.4 cm) yielded a resonant frequency of 50.3 MHz. Small sheet metal screws through the PVC secure the gap spacing.

Small
                      sheet metal screws through the PVC secure the gap
                      spacing between the open ends that determines the
                      resonant frequency.

14. Small sheet metal screws through the PVC secure the gap spacing between the open ends that determines the resonant frequency.

A
                      disposable styrofoam beverage cup is cut to fit
                      over the PVC boom and feed point with gamma match
                      arm.

15. A disposable styrofoam beverage cup is cut to fit over the PVC boom and feed point with gamma match arm.

The
                      disposable styrofoam beverage cup fit over the PVC
                      boom and feed point with gamma match arm to
                      protect it from the weather.

16. The disposable styrofoam beverage cup fit over the PVC boom and feed point with gamma match arm to protect it from the weather.

The
                      homebrew 6 meter halo antenna was secured with a
                      U-bolt atop a 10 foot (3 m) length of 1" (2.5
                      cm) Schedule 40 PVC pipe and mounted on the roof
                      about 18 feet (6 m) above ground level for the
                      2014 ARRL VHF Sweepstakes contest.

17. The homebrew 6 meter halo antenna was secured with a U-bolt atop a 10 foot (3 m) length of 1" (2.5 cm) Schedule 40 PVC pipe and mounted on the roof about 18 feet (6 m) above ground level for the 2014 ARRL VHF Sweepstakes contest.

The
                      homebrew 6 meter halo antenna was secured with a
                      U-bolt atop a 10 foot (3 m) length of 1" (2.5
                      cm) Schedule 40 PVC pipe and mounted on the roof
                      about 18 feet (6 m) above ground level for the
                      2014 ARRL VHF Sweepstakes contest.

18. The homebrew 6 meter halo antenna was secured with a U-bolt atop a 10 foot (3 m) length of 1" (2.5 cm) Schedule 40 PVC pipe and mounted on the roof about 18 feet (6 m) above ground level for the 2014 ARRL VHF Sweepstakes contest.

MODEL ANALYSIS AND MEASUREMENTS

4nec2
                      Calculations for the 50 MHz Halo Antenna at 118
                      inches or 3 m (0.5 λ) above simulated ground. I
                      used the high-pass L-network tool to simulate the
                      gamma match to match for 50 ohms impedance.

19. 4nec2 Calculations for the 50 MHz Halo Antenna at 118 inches or 3 m (0.5 λ) above simulated ground. I used the high-pass L-network tool to simulate the gamma match to match for 50 ohms impedance.

50 MHz
                      Halo Antenna model - 4nec2 calculated standing
                      wave ratio over 49.5 - 51.5 MHz. The 2:1 SWR
                      bandwidth is 1 MHz from 49.8 to 50.8 MHz.

20. 50 MHz Halo Antenna model - 4nec2 calculated standing wave ratio over 49.5 - 51.5 MHz. The 2:1 SWR bandwidth is 1 MHz from 49.8 to 50.8 MHz.

For
                      comparison - a shortened 80 inch circumference
                      halo antenna with a 3 pF loading capacitor across
                      the gap. Note that the 2:1 SWR bandwidth has
                      decreased to 300 kHz.

21. For comparison - a shortened 80 inch circumference halo antenna with a 3 pF loading capacitor across the gap (similar to Hi-Par Saturn 6 antenna).  Note that the 2:1 SWR bandwidth has decreased to 300 kHz.

50 MHz
                      Halo Antenna prototype - Standing wave ratio and
                      Return Loss over 49.5 - 51.5 MHz measured with
                      miniVNA Pro. This full size halo antenna provides
                      a 2:1 SWR bandwidth of 1 MHz.

22. 50 MHz Halo Antenna prototype - Standing wave ratio and Return Loss over 49.5 - 51.5 MHz measured with miniVNA Pro. This full size halo antenna provides a 2:1 SWR bandwidth of 1 MHz.

50 MHz
                      Halo Antenna prototype - Resistance and Reactance
                      over 49.5 - 51.5 MHz measured with miniVNA Pro.

23. 50 MHz Halo Antenna prototype - Resistance and Reactance over 49.5 - 51.5 MHz measured with miniVNA Pro.

50 MHz
                      Halo Antenna prototype - Impedance and Phase shift
                      over 49.5 - 51.5 MHz measured with miniVNA Pro.

24. 50 MHz Halo Antenna prototype - Impedance and Phase shift over 49.5 - 51.5 MHz measured with miniVNA Pro.

50 MHz
                      Halo Antenna 4nec2 model - Three dimensional view
                      of the antenna current and radiation pattern at
                      118 inches or 3 m (1/2 wavelength) height above
                      simulated good ground.

25. 50 MHz Halo Antenna 4nec2 model - Three dimensional view of the antenna current and radiation pattern at 118 inches or 3 m (1/2 wavelength) height above simulated good ground.

50 MHz
                      Halo Antenna model - 4nec2 azimuth radiation
                      pattern at 118 inches or 3 m (1/2 wavelength)
                      height above simulated good ground.

26. 50 MHz Halo Antenna model - 4nec2 azimuth radiation pattern at 118 inches or 3 m (1/2 wavelength) height above simulated good ground.

50 MHz
                      Halo Antenna model - 4nec2 elevation radiation
                      pattern at 118 inches or 3 m (1/2 wavelength)
                      height above simulated good ground.

27. 50 MHz Halo Antenna model - 4nec2 elevation radiation pattern at 118 inches or 3 m (1/2 wavelength) height above simulated good ground.

REFERENCES

  1. Some Preliminary Notes on the Gamma Match, Cebik, LB, W4RNL
  2. Horizontally Polarized Omni-Directional Antennas: Some Compact Choices, Cebik, LB, W4RNL
  3. 144 MHz Halo Antenna, Milazzo, CF, KP4MD

APPENDIX: NEC Model File

  1. 50 MHz Halo Antenna 4nec2 Model

LINKS

  1. 50 MHz Halo Antenna Photo Album
  2. 144 MHz WSPR Propagation Study
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