HF Vertical Antenna Ground Systems Some Experiments

1. HF Vertical Antenna Ground SystemsSome Experiments Rudy Severns N6LF antennasbyn6lf.com

2. We’ve been using verticals for over 100 years. • Is there really anything new to be said about ground systems for verticals? • Yes! • Little attention has been given to HF (2-30 MHz) ground systems like those used by amateurs. • Soil behavior at HF is different from BC.

3. Typical amateur antennas use: • radials lying on the ground surface, • or elevated radials, • and/or small numbers of radials, • short loaded verticals

4. Some typical questions • How much of ground system is it worth putting down? • What will I gain (in dB) by adding more radials? • Does it matter if I lay the radials on the ground surface? • Are a few long radials useful? • Are four elevated radials really as good as lots of buried radials? • How well do “gullwing” elevated radials work?

5. We can use modeling or calculations to answer these questions but most people don’t have a lot confidence in mathematical exercises. • High quality field measurements on real antennas are more likely to be believed. • Over the past year I have done a series of experiments on HF verticals with different ground systems. • That is the subject of today’s talk.

6. What’s the purpose of the ground system? • It’s there to reduce the power absorbed by the soil close to the antenna (within a ¼-wave or so). • The ground system increases your signal by reducing the power dissipated in the soil and maximizing the radiated power. • Any practical ground system will not affect the radiation angle or far-field pattern!

7. Power transmission antenna 1 antenna 2 antenna equivalent circuit

8. E and H fields around a vertical ground soil equivalent

9. The Magnetic field (H)

10. The Electric Field (E) + E field V - resistor

11. H-Field Currents Near A Vertical

12. Relative Ground Current loss is proportional to I2!

13. Electric Field Intensity Near The Base • f = 1.8 MHz and Power = 1500 W

14. H-Field Loss

15. E-Field Loss

16. Power transmission antenna 1 antenna 2 antenna equivalent circuit

17. Measurement schemes • The classical technique is to excite the test antenna with a known power and measure the resulting signal strength at some point in the far field (>2.5 wavelengths for 1/4-wave vertical). • This approach takes great care and good equipment to make accurate measurements.

18. The modern alternative is to use a vector network analyzer (VNA) in the transmission mode. This approach is capable of reliable measurements to <0.1 dB. The VNA will also give you the input impedance of the antenna at the feed-point. S21 rx antenna test antenna

19. Some experimental results

20. The first experiment was a 160 m, ¼-wave wire vertical with two ground stakes and 4 to 64 radials. • Measurements were made with a spectrum analyzer as the receiver.

21. Test Results delta gain = 2.4 dB

22. A new antenna test range

23. Antenna under test

24. Test antenna with sliding height base

25. Adding radials to the base

26. Elevated radials

27. Elevated radials close-up

28. Loop receiving antenna

29. Receiving antenna at 40’ N7MQ holding up the mast!

30. Network analyzers note, automatic, organic, heating system Homebrew N2PK HP3577A with S-box

31. Inside the N2PK VNA

32. Test antennas • A 1/4-wave 40m tubing vertical. • An 1/8-wave 40m tubing vertical with top loading. • An 1/8-wave 40m tubing vertical resonated with a base inductor. • A 40 m Hamstick mobile whip. • SteppIR vertical

33. 1/8-wave, top-loaded, 40 m vertical

34. Measured improvement over a single ground stake f=7.2 MHz

35. Caution! • Your mileage may vary! • My soil is pretty good but for poorer soils expect more improvement with more radials. • The degree of improvement will also depend on the frequency: • soil characteristics change with frequency, • at a given distance in wavelengths the field intensity increases with frequency.

36. Measured base impedances

37. Antenna resonance versus radial number

38. Radial current for different heights

39. A current sensor

40. Radial current measurements

41. Measured current distribution on a radial

42. Radial current distribution

43. Field day scenario • You want a 40 m vertical for field day. • ¼-wave = 33’. So you start with about 33’ of aluminum tubing for the radiator and four 33’ wire radials. • You erect this, with the radials lying on the ground and it’s resonant well below the band! • What to do? • Nothing, use a tuner and move on, • Shorten vertical until it’s resonant, • add more radials • or, shorten the radials until the antenna is resonant. • Which is best?

44. NEC modeling prediction

45. Lets do an experiment: • isolate the base of the antenna with a common mode choke (a balun). • lay out sixty four 33’ radials and adjust the vertical height to resonate (reference height). • remove all but four of the radials • Measure S21 with the reference height. • Measure S21 with the vertical shortened to re-resonate. • Measure S21 with the reference height as we shorten the radials.

46. Effect of shorting radials, constant height

47. Radial current distribution

48. Direct measurement of several options • Do nothing: G= 0 dB • Shorten height: G=-0.8 dB • Shorten radials: G=+3.5 dB • Use 16 radials: G=+4 dB • Use 64 radials: G=+5.9 dB

49. Another experiment

50. An observation • When you have only four radials the test results are always a bit squirrelly: • small variations in radial layout, • coupling to other conductors, • like the feed-line, • all effect the measurements making close repeatability difficult between experiments. • The whole system is very sensitive to everything! • This nonsense goes away as the number of radials increases!