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HF Vertical Antenna Ground Systems Some Experiments Rudy Severns N6LF

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Presentation on theme: "HF Vertical Antenna Ground Systems Some Experiments Rudy Severns N6LF"— Presentation transcript:

1 HF Vertical Antenna Ground Systems Some Experiments Rudy Severns N6LF

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 equivalent circuit antenna 1 antenna 2

8 E and H fields around a vertical ground soil equivalent

9 The Magnetic field (H)

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

11 H-Field Currents Near A Vertical

12 Relative Ground Current loss is proportional to I 2 !

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 equivalent circuit antenna 1 antenna 2

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 HP3577A with S-box Homebrew N2PK note, automatic, organic, heating system

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 Radial numberRelative radial current normalized to 1 A total

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!

51 What about a few elevated radials versus a large number of surface radials?

52 NEC modeling prediction

53 4-64 radials lying on ground surface

54 4 radials raised above ground

55 NEC modeling predicts that four elevated radials will perform as well as 64 radials lying on the ground. In this example, measurements show no significant difference in signal strength between 64 radials lying on the ground and 4 radials at 4’!

56 Some more elevated radial experiments

57 configuration number |S21| [dB] Zi [Ohms] configuration h=33.5’ 1039+j6.3base & 4 radials elevated 48” j6.2base at ground level radial ends at 48” j11gullwing, base at ground level ends at 48” j0.9base & radials at 48” four 17.5’ radials, 2.2 uH L j22base & radials at ground level j1.0base & radials at ground level four 21’ radials j1.2base & radials at ground level 64, 33’ radials

58 More on elevated radials If you use more than 4 radials in an elevated system: –the screen resonances and radial current asymmetries decrease. –the reactive part of the feed-point impedance changes more slowly as you add radials so you have a better SWR bandwidth. –the ground loss does not improve much however.

59 Summary Sparse radial screens (less than 16 radials) can have a number of problems: –increased loss with longer radials –unequal current distributions between radials. –system resonance shifts. –A few long radials can be worse than shorter ones. –screen resonances can alter the radiation pattern as the radials begin to radiate substantially.

60 Summary continued Try to use at least 8 radials but 16 is better. The more radials you use, the longer they can be. A number of 1/8-wave radials will be better than half that number of ¼-wave radials. At least until you have 32 or more radials. In elevated systems: –try to use at least 8 radials –you can use radials shorter than ¼-wave and either re-resonate with a small L or make the vertical taller or add some top loading. – the “gullwing” geometry can work.

61 Some advice Try to use more radials. Four is just not enough. All the funny business goes away with more radials! 16 radials are a good compromise.

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