HF Vertical Antenna Ground Systems Some Experiments

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HF Vertical Antenna Ground SystemsSome
Experiments

• Rudy Severns N6LF
• antennasbyn6lf.com

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• Weve 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.

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

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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?

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• We can use modeling or calculations to answer
  these questions but most people dont 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 todays talk.

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• Whats the purpose of the ground system?
• Its 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!

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Power transmission
antenna 1
antenna 2
antenna equivalent circuit
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E and H fields around a vertical
ground
soil equivalent
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The Magnetic field (H)
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The Electric Field (E)

E field
V
-
resistor
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H-Field Currents Near A Vertical
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Relative Ground Current
loss is proportional to I2!
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Electric Field Intensity Near The Base

• f 1.8 MHz and Power 1500 W

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H-Field Loss
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E-Field Loss
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Power transmission
antenna 1
antenna 2
antenna equivalent circuit
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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 (gt2.5 wavelengths for 1/4-wave
  vertical).
• This approach takes great care and good equipment
  to make accurate measurements.

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S21

• The modern alternative is to use a vector network
  analyzer (VNA) in the transmission mode.
• This approach is capable of reliable measurements
  to lt0.1 dB.
• The VNA will also give you the input impedance of
  the antenna at the feed-point.

rx antenna
test antenna
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Some experimental results
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• 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.

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Test Results
delta gain 2.4 dB
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A new antenna test range
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Antenna under test
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Test antenna with sliding height base
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Adding radials to the base
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Elevated radials
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Elevated radials close-up
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Loop receiving antenna
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Receiving antenna at 40
N7MQ holding up the mast!
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Network analyzers
note, automatic, organic, heating system
Homebrew N2PK
HP3577A with S-box
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Inside the N2PK VNA
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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

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1/8-wave, top-loaded, 40 m vertical
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Measured improvement over a single ground stake
f7.2 MHz
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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.

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Measured base impedances
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Antenna resonance versus radial number
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Radial current for different heights
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A current sensor
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Radial current measurements
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Measured current distribution on a radial
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Radial current distribution
Radial number Relative radial current normalized to 1 A total
1 0.239
2 0.239
3 0.252
4 0.269
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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 its resonant well below the band!
• What to do?
• Nothing, use a tuner and move on,
• Shorten vertical until its resonant,
• add more radials
• or, shorten the radials until the antenna is
  resonant.
• Which is best?

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NEC modeling prediction
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• 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.

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Effect of shorting radials, constant height
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Radial current distribution
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Direct measurement of several options

• Do nothing G 0 dB
• Shorten height G-0.8 dB
• Shorten radials G3.5 dB
• Use 16 radials G4 dB
• Use 64 radials G5.9 dB

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Another experiment
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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!

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What about a few elevated radials versus a large
number of surface radials?
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NEC modeling prediction
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4-64 radials lying on ground surface
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4 radials raised above ground
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• 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!

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Some more elevated radial experiments
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configuration number S21 dB Zi Ohms configuration h33.5
1 0 39j6.3 base 4 radials elevated 48
2 -0.47 36j6.2 base at ground level radial ends at 48
3 -0.65 29-j11 gullwing, base at ground level ends at 48
4 -0.36 39j0.9 base radials at 48 four 17.5 radials, 2.2 uH L
5 -5.19 132j22 base radials at ground level
6 -1.79 51j1.0 base radials at ground level four 21 radials
7 -0.1 40-j1.2 base radials at ground level 64, 33 radials
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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.

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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.

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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.

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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.