HF Vertical Antenna Ground Systems Some Experiments Rudy Severns N6LF antennasbyn6lf.com

HF Vertical Antenna Ground Systems

Some Experiments

Rudy Severns N6LFantennasbyn6lf.com

• 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.

• Typical amateur antennas use:

–radials lying on the ground surface,

–or elevated radials,

–and/or small numbers of radials,

–short loaded verticals

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?

• 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.

• 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!

Power transmission

21Rr

RgRX

power

RgRr

RrPiS21antenna equivalent

circuit

antenna 1

antenna 2

E and H fields around a vertical

ground

soil equivalent

The Magnetic field (H)

The Electric Field (E)

+

-

VE field

resistor

2 2

2

, d is the distance between the platesV

EdV EPR Rd

H-Field Currents Near A Vertical

Relative Ground Current

0

1

2

3

0 0.1 0.2 0.3 0.4 0.5

r (distance from base in wavelengths)

Iz (

A)

, zo

ne

cu

rre

nt

in g

rou

d

constant radiated power =37 W

h=.1

h=.15

h=.2

h=.25

h=.3

h=.4h=.5

loss is proportional to I2!

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

H-Field Loss

0

10

20

30

40

50

60

70

80

90

100

0.00 0.10 0.20 0.30 0.40 0.50

r (wavelengths)

To

tal

H-f

ield

gro

un

d l

os

s w

ith

in r

(W

)

h=0.1

h=0.15

h=0.20

h=0.25

h=0.30

h=0.40

h=0.50

0.005/13 ground, 1.8 MHz, Pr=37 W

E-Field Loss

0.10

1.00

10.00

100.00

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

r (wavelengths)

tota

l E-f

ield

gro

un

d lo

ss (

W)

wit

hin

r h=0.10

h=0.15

h=0.20

h=0.40

h=0.30

h=0.25 Pr=37W, f=1.83 MHz, sigma = 0.005 S/m

Power transmission

21Rr

RgRX

power

RgRr

RrPiS21antenna equivalent

circuit

antenna 1

antenna 2

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.

• 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 antennatest antenna

Some experimental results

• 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.

Test Results

-30.5

-30

-29.5

-29

-28.5

-28

-27.5

-27

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

number of radials

Mea

sure

d a

mp

litu

de

(d

Bm

)

160 m test verticalAugust 2006

run 2

delta gain = 2.4 dB

A new antenna test range

Antenna under test

Test antenna with sliding height base

Adding radials to the base

Elevated radials

Elevated radials close-up

Loop receiving antenna

Receiving antenna at 40’

N7MQ holdingup the mast!

Network analyzers

HP3577A with S-boxHomebrew N2PK

note, automatic, organic, heating system

Inside the N2PK VNA

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

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

Measured improvement over a single ground stake

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 10 20 30 40 50 60 70

Number of radials

Sig

nal

imp

rove

men

t (d

B)

7.5' mobile whip

1/8-wave base loaded

1/8-wave top-loaded

1/4-wave

12 Sept 07

1/4-wavecalculated

f=7.2 MHz

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.

Measured base impedances

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

Number of radials

Rs

(Oh

ms)

7.5' mobile whip

1/8-wave base loaded

1/8-wave top-loaded

1/4-wave

12 Sept 07

Antenna resonance versus radial number

6.85

6.9

6.95

7

7.05

7.1

7.15

7.2

7.25

0 10 20 30 40 50 60 70

Number of radials

Res

on

an

t fr

eq

ue

nc

y (M

Hz)

15 June 07

Radial current for different heights

A current sensor

Radial current measurements

Measured current distribution on a radial

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

0 5 10 15 20 25 30 35

Distance from base (feet)

Rel

ativ

e cu

rre

nt

am

plit

ud

e

Sinewave trendline

1/4-wave vertical7.2 MHz4 radials

23 Sept 07

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

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?

NEC modeling prediction

-1.5

-1

-0.5

0

0.5

1

1.5

0.001 0.01 0.1 1 10

height above ground [m]

pea

k g

ain

[d

B]

resonant radials

non-resonant radials

40m gp 4rad A and C

22 April 08

• 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.

Effect of shorting radials, constant height

0

0.5

1

1.5

2

2.5

3

3.5

4

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

radial length [ft]

Gai

n in

crea

se f

rom

33'

rad

ials

[d

B]

4 radialsno ground

stake

4 radials1 ground

stake

experiment 46 May 08

radials lying on groundf= 7.2 MHz

8 radialsno ground

stake

vertical height = 34'

constant

Radial current distribution

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

0 5 10 15 20 25 30 35

Distance from base (feet)

Rel

ativ

e cu

rre

nt

am

plit

ud

e

Sinewave trendline

1/4-wave vertical7.2 MHz4 radials

23 Sept 07

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

Another experiment

33' radials 21' radials 33' radials

21' radials

number of

radials

feed-point impedance

[ohm]

feed-point impedance

[Ohm]

|S21| relative to 4, 33' radials

[dB]

|S21| relative to 4, 33' radials

[dB]

delta gain

change [dB]

4 89.8 52.5 0 3.08 +3.1 8 51.8 45.6 2.26 3.68 +1.42

16 40.5 42.8 3.76 3.95 +0.19 32 37.7 41.6 4.16 4.04 -0.12

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!

What about a few elevated radials versus a large number

of surface radials?

NEC modeling prediction

-1.5

-1

-0.5

0

0.5

1

1.5

0.001 0.01 0.1 1 10

height above ground [m]

pea

k g

ain

[d

B]

resonant radials

non-resonant radials

40m gp 4rad A and C

22 April 08

4-64 radials lying on ground surface

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70

radial number

Gai

n i

mp

rove

men

t [d

B]

17 April 08, h=33.5', radial length = 33'

no ground stakes,choke isolated

4 radials raised above ground

0

1

2

3

4

5

6

0 0.5 1 1.5 2 2.5 3 3.5 4

height of radials above ground [ft]

Gai

n i

mp

rove

men

t [d

B]

17 April 08, h=33.5', radial length = 33'

no ground stakes,choke isolated

• 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’!

Some more elevated radial experiments

configuration

number|S21|

[dB]

Zi

[Ohms]

configuration

h=33.5’

1 0 39+j6.3 base & 4 radials

elevated 48”

2 -0.47 36+j6.2 base at ground level

radial ends at 48”

3 -0.65 29-j11 gullwing, base at ground level

ends at 48”

4 -0.36 39+j0.9 base & radials at 48”

four 17.5’ radials, 2.2 uH L

5 -5.19 132+j22 base & radials at ground level

6 -1.79 51+j1.0 base & radials at ground level

four 21’ radials

7 -0.1 40-j1.2 base & radials at ground level

64, 33’ radials

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.

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.

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.

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.