Transcript ground_system_tests.pps

HF Vertical Antenna
Ground Systems
Some Experiments
Rudy Severns N6LF
antennasbyn6lf.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
antenna 1
antenna 2
2
power
Rr
antenna equivalent
circuit
Rg
1
 Rr 

S21  Pi
 Rr  Rg 
RX
E and H fields around a vertical
ground
soil equivalent
The Magnetic field (H)
The Electric Field (E)
+
E field
V
resistor
V
E  , d is the distance between the plates
d
V
E
P 
R dR
2
2
2
H-Field Currents Near A Vertical
Relative Ground Current
loss is proportional to I2!
3
Iz (A) , zone current in groud
constant radiated power =37 W
h=.1
h=.15
2
h=.2
h=.25
1
h=.3
h=.4
h=.5
0
0
0.1
0.2
0.3
r (distance from base in wavelengths)
0.4
0.5
Electric Field Intensity Near The Base
• f = 1.8 MHz and Power = 1500 W
Total H-field ground loss within r (W)
H-Field Loss
100
0.005/13 ground, 1.8 MHz, Pr=37 W
h=0.1
90
80
70
60
h=0.15
50
40
h=0.20
30
20
h=0.25
h=0.30
10
h=0.40
0
0.00
0.10
0.20
0.30
r (wavelengths)
0.40
0.50
h=0.50
E-Field Loss
total E-field ground loss (W) within r
100.00
h=0.10
10.00
h=0.15
h=0.20
1.00
h=0.40
h=0.30
h=0.25
Pr=37W, f=1.83 MHz, sigma = 0.005 S/m
0.10
0.00
0.05
0.10
0.15
0.20
0.25
0.30
r (wavelengths)
0.35
0.40
0.45
0.50
Power transmission
antenna 1
antenna 2
2
power
Rr
antenna equivalent
circuit
Rg
1
 Rr 

S21  Pi
 Rr  Rg 
RX
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 1/4wave vertical).
• This approach takes great care and good
equipment to make accurate
measurements.
S21
• The modern alternative is to
use a vector network
rx antenna
test antenna
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.
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
delta gain = 2.4 dB
-27
160 m test vertical
August 2006
run 2
Measured amplitude (dBm)
-27.5
-28
-28.5
-29
-29.5
-30
-30.5
4
8
12
16
20
24
28
32
36
40
number of radials
44
48
52
56
60
64
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 holding
up the mast!
Network analyzers
note, automatic, organic, heating system
Homebrew N2PK
HP3577A with S-box
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
6.00
f=7.2 MHz
7.5' mobile whip
5.00
Signal improvement (dB)
4.00
1/8-wave base loaded
3.00
1/8-wave top-loaded
1/4-wave
2.00
1/4-wave
calculated
1.00
12 Sept 07
0.00
0
10
20
30
40
Number of radials
50
60
70
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
70
12 Sept 07
60
50
Rs (Ohms)
1/4-wave
40
30
1/8-wave top-loaded
20
1/8-wave base loaded
10
7.5' mobile
whip
0
0
10
20
30
40
Number of radials
50
60
70
Antenna resonance versus radial number
7.25
Resonant frequency (MHz)
7.2
7.15
7.1
7.05
7
6.95
6.9
15 June 07
6.85
0
10
20
30
40
Number of radials
50
60
70
Radial current for different heights
A current sensor
Radial current measurements
Measured current distribution on a radial
Relative current amplitude
1.400
1.200
Sinewave
trendline
1.000
0.800
0.600
0.400
1/4-wave vertical
7.2 MHz
4 radials
23 Sept 07
0.200
0.000
0
5
10
15
20
25
Distance from base (feet)
30
35
Radial current distribution
Radial number
1
Relative radial current
normalized to 1 A total
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
peak gain [dB]
resonant
radials
0.5
0
non-resonant
radials
-0.5
40m gp 4rad
A and C
22 April 08
-1
-1.5
0.001
0.01
0.1
height above ground [m]
1
10
• 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
Gain increase from 33' radials [dB]
4
3.5
4 radials
1 ground
stake
3
2.5
8 radials
no ground
stake
4 radials
no ground
stake
2
1.5
experiment 4
6 May 08
radials lying
on ground
f= 7.2 MHz
1
0.5
vertical
height = 34'
constant
0
18
19
20
21
22
23
24
25
26
27
radial length [ft]
28
29
30
31
32
33
Radial current distribution
Relative current amplitude
1.400
1.200
Sinewave
trendline
1.000
0.800
0.600
0.400
1/4-wave vertical
7.2 MHz
4 radials
23 Sept 07
0.200
0.000
0
5
10
15
20
25
Distance from base (feet)
30
35
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
number feed-point feed-point
of
impedance impedance
radials
[ohm]
[Ohm]
4
8
16
32
89.8
51.8
40.5
37.7
52.5
45.6
42.8
41.6
33'
radials
|S21|
relative
to 4, 33'
radials
[dB]
0
2.26
3.76
4.16
21'
radials
|S21|
relative
to 4, 33'
radials
[dB]
3.08
3.68
3.95
4.04
delta
gain
change
[dB]
+3.1
+1.42
+0.19
-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
peak gain [dB]
resonant
radials
0.5
0
non-resonant
radials
-0.5
40m gp 4rad
A and C
22 April 08
-1
-1.5
0.001
0.01
0.1
height above ground [m]
1
10
4-64 radials lying on ground surface
Gain improvement [dB]
6
5
4
3
2
17 April 08, h=33.5', radial
length = 33'
no ground stakes,
choke isolated
1
0
0
10
20
30
40
radial number
50
60
70
4 radials raised above ground
Gain improvement [dB]
6
5
4
3
2
17 April 08, h=33.5', radial
length = 33'
no ground stakes,
choke isolated
1
0
0
0.5
1
1.5
2
2.5
3
height of radials above ground [ft]
3.5
4
• 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.