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Rudy Severns, N6LF
PO Box 589, Cottage Grove, OR 97424; n6lf@arrl.net
Experimental Determination of
Ground System Performance for
HF Verticals
Part 2
Excessive Loss in Sparse
Radial Screens
These experimental results may surprise you, and might turn “conventional
wisdom” upside down.
In 1998, Jack Belrose, VE2CV, used
NEC
modeling to show the effect of resonant and
non-resonant radials placed very close to the
ground surface on the behavior of a ¼ wave-
length vertical.
1
One of the observations in
that article was that the use of a small number
of ¼ wavelength (free space) radials, lying on
the ground surface, could lead to much higher
losses than expected, and that shortening the
radials could actually reduce ground loss.
This seems counter to more classical analy-
ses which show that making radials too long
may be a waste of wire but does no harm.
The classic analysis, however, does not take
into account the possibility of resonances in
the radial screen that might amplify the radial
current, increasing ground loss.
The purpose of this experiment was to see
if a real antenna would actually demonstrate
the predicted behavior, and validate the
NEC
predictions experimentally.
aluminum-tubing vertical, averaging 1 inch
in diameter, with a ixed height of 34 feet.
The test frequency was 7.2 MHz. I used four
no. 18 insulated wire radials lying on the
ground surface. All four radials were of equal
length, which was varied from 33 feet down
to 18 feet. The impedance at the feed point,
the transmission gain (S21) and the current
division ratios between the radials were mea-
sured and recorded. The antenna and radials
were isolated from ground and the feed line
with a common mode choke.
2) For part 2, part 1 was repeated, irst
isolated from ground and then with one or
more ground stakes connected, to evaluate
the effect of using ground stakes at the base
of the antenna. Tests were also done without
any radials, and with just 1, 2 or 3 ground
stakes connected to the base plate.
3) Part 3 of the experiment was the same
as part 1 except with 8 radials (no ground
stakes).
4) For part 4, the antenna was changed
from the fixed tubing vertical to a
remotely adjustable SteppIR vertical. In
parts 1, 2 and 3, the antenna height was
kept constant at 34 feet, but in this part of
the experiment the height was changed
to re-resonate the antenna as the radial
number and radial lengths were changed.
The test frequency was 7.2 MHz.
5) After completing the irst four parts
of the experiment it was clear that shorten-
ing the radials from the standard free space
¼ wavelength value did indeed improve the
signal, at least in the case of 4 and 8 radials,
so I wanted to see what the effect was for 16
and 32 radials. Trimming that many radials
to gradually shorten them, however, was a
bit more work and wasted wire than I was
prepared for. Instead, I ran this part of the
experiment irst with 4, 8, 16 and 32, thirty-
three foot radials, which I had on hand, and
then with 4 ,8, 16 and 32, twenty-one foot
radials, which were also on hand. This gave
me two data points for each number of radi-
als. Again, the test frequency was 7.2 MHz,
with measurements of S21 and feed-point
impedance.
6) Part 6 of the experiment was a check to
see if the same kind of improvement would
be seen at 30, 20 and 15 m by shortening
the radials from ¼ wavelength (free space).
This part of the experiment was not nearly
as thorough as the irst ive parts but did con-
irm that the same basic behavior was pres-
ent at the higher frequencies as that seen on
40 m. The test frequencies were 10.120 MHz,
14.200 MHz and 21.200 MHz.
Description of the Experiment
The experiment was done in six parts
spread over a three week period:
1) The antenna for part 1 was a telescoping
1
Notes appear on page 52.
48
QEX – January/February 2009
Experimental Results
Part 1
Figure 1 shows the variation in |S21|
(magnitude of the transmission gain) as a
function of radial length. The amplitude scale
is normalized to 0 dB for a radial length of
33 feet, which is approximately a ¼ wave-
length in free space at 7.2 MHz. The Y-axis
shows the improvement in dB as the radials
are shortened.
The improvement is quite large, about
2.8 dB, which would have a noticeable effect
on signal strength. In Belrose’s paper the
improvement was about 3.5 dB but that was
for average soil. My average ground charac-
teristics are
approximately
σ = 0.015 S/m and
ε
r
= 30, which is quite a bit better than aver-
age ground. These values were derived from
ground probe measurements.
2
One would
expect more improvement for poorer soil.
An earlier experiment in which the
current distribution on a 33 foot radial, at
7.2 MHz, was measured, gave the results
shown in Figure 2.
A quick check was made during the pres-
ent experiment, and the current distribution
appeared to be essentially the same. From
the current distribution we can see that the
radial in Figure 2 is resonant well below
7.2 MHz. To move the current maxima back
to the base of the vertical we would have to
reduce the radial length by about 10 feet.
Looking back at Figure 1, we see that we
are very close to the maximum |S21| when
the length has been reduced by 10 feet to
23 feet. What appears to be happening is that
we are tuning the radials to resonance (or at
least close to it) at 7.2 MHz to compensate
for the loading effect of the soil in close prox-
imity to the radial wire.
The division of current between the radi-
als was measured for 18 foot and 33 foot
Table 1
Current Division Between Radials Normalized to 1 A of Total Base Current.
Radial Number
I
n
, 33-Foot Radials (A)
I
n
, 18-Foot Radials (A)
1
0.24
0.26
2
0.24
0.25
3
0.25
0.25
4
0.27
0.24
rods) connected at the base of the antenna.
Measurements with 4 and 8 radials were
repeated in each run. This run was with a
ixed height for the vertical (34 feet). The
results are shown in Figure 3.
At all lengths, 8 radials are an improve-
ment over 4. With 8 radials, the amount
of improvement with radial shortening is
smaller but still useful. We can also see that
adding a ground stake in the case of 4 radials
also makes a substantial improvement but we
should keep in mind that my soil would be
classiied as “very good” so we would expect
ground stakes to be more effective than they
would be in poorer soil.
The results for the case of no radials and 1,
2 or 3 ground stakes, normalized to the cases
of four 33 foot radials and four 21 foot radi-
als, with no ground stakes, are given in Table
3. Vertical height was constant at 34 feet.
Part 4
In part 4 I changed to the SteppIR verti-
cal and adjusted the height to re-resonate the
vertical for each radial length. The results are
shown in Figure 4, which are very similar to
the results for constant height given in Figure
3. No ground stakes were employed.
Part 5
From the earlier test results, I could see
that the improvement due to radial shortening
decreased as the number of radials increased.
Table 2
Measured Feed Point Impedances
Radial length
Feed Point Impedance
(ft)
(
Ω
)
33
135 +
j
28
30
108 +
j
55
27
83 +
j
51
24
67 +
j
37
21
60 +
j
22
18
57 +
j
8
lengths. Table 1 shows the results. The cur-
rent division was quite uniform and the dif-
ferences too small to have signiicant effect
on the observed gain changes.
The variation of feed-point impedance as
the radial lengths were shortened (with the
vertical height constant at 34 feet) is shown
in Table 2.
Parts 2 and 3
Part 1 was done during a week of heavy
rain. Parts 2 and 3 were performed 8 days
after part 1, when the soil had drained and
dried out signiicantly so the ground charac-
teristics may have changed somewhat.
The next step in the experiment was to
expand the radial count from 4 to 8 radials
and also to investigate the effect of using
grounding stakes (4 foot copper clad steel
Figure 1 — This graph shows the improvement in |S21| as the
radials were shortened. There were four radials lying on the
ground surface.
Figure 2 — This graph shows the relative current amplitude
along a radial.
QEX – January/February 2009
49
Figure 3 — This graph shows the change in |S21| with radial length.
The vertical antenna height was a constant 34 feet.
Figure 4 — This graph shows the change in |S21| with radial length.
I adjusted the SteppIR antenna height to resonance for each radial
length.
Table 3
Test Results for no Radials and 1, 2 or 3 Stakes, Compared to 4 Radials with no Ground Stakes.
Number of Stakes
Feed Point Z (
Ω
)
Compared to Four 33-Foot Radials,
Compared to Four 21-Foot Radials
No Ground Stakes (dB)
No Ground Stakes (dB)
1
77 +
j
40
2.67
–0.95
2
69 +
j
30
3.09
–0.53
3
66 +
j
26
3.25
–0.37
Table 4
Results for 4, 8, 16 and 32 Radials, with Lengths of 33 Feet and 21 feet.
Number
33-Foot Radials
21-Foot Radials
33-Foot Radials
21-Foot Radials
Feed Point
Feed Point
|S21| Relative to Four
|S21| Relative to Four
Delta Gain Change (dB)
Impedance (
Ω
)
Impedance (
Ω
)
33-Foot Radials (dB)
33-Foot Radials (dB)
4
89.8
52.5
0
3.08
+3.08
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
In this part of the experiment the number of
radials was extended to include 16 and 32
radials to quantify that difference. The test
was conducted with sets of 4, 8, 16 and 32
thirty-three foot radials, and then repeated
with the same numbers of 21-foot radials.
The StepIR antenna was used, and its height
was adjusted to re-resonate as the radials were
altered. The results are tabulated in Table 4.
These measurements were made several
days after those used in Figure 4, so there are
some differences because of small changes in
the ground characteristics, radial layout, and
other conditions. These day-to-day variations
are a major reason for repeating some parts of
earlier experiments multiple times and trying
to do a complete experiment in a short period
of time (a couple of hours).
It should be noted that a ground system
consisting of only four radials is really laky.
Measurements vary signiicantly with small
variations in radial layout, changes in soil
moisture, placement of the feed line relative
to the radials, and so on. Shortening the radi-
als does seem to reduce this sensitivity, but
even so, a four radial system should only be
an emergency measure.
As expected, as the number of radials is
increased the change due to radial shorten-
ing gets much smaller. Over the very good
ground on which these measurements were
made, shortening the radials gave only a
modest advantage when more than 8 radials
were used. Over poorer soils, however, radial
shortening with 16 radials might be worth
doing. The lower value for feed point imped-
ance (Z
i
) with 33-foot radials is at least in
part due to the shorter height needed to reso-
nate. For 21-foot radials the height had to be
increased to re-resonate the antenna.
It is interesting to note that with 32 radi-
als, the 33-foot radials were actually slightly
better (0.12 dB) than 21-foot radials. Quite
probably there was some optimum length
in-between that may have been slightly
higher than either, but that is not likely to be
very large and I decided it wasn’t worth the
trouble to cut up a set of 32 radials to ind
out. The important point is that the changes
in gain, input impedance and height varia-
tion to re-resonate all get much smaller when
more radials are used. I would think that with
32 or more radials you wouldn’t worry about
resonances in the radial screen. The problem
is only important when fewer than 16 radials
are deployed.
Table 5 shows the antenna height (h) in
inches. This is the reading from the control
box. The actual height is about 12 inches
longer due to the height above ground of the
50
QEX – January/February 2009
Table 5
Indicated Height of the Vertical.
Table 6
30 m, ¼ Wavelength Free Space = 24.3 Feet.
Number
33-Foot Radials
21-Foot Radials
Radial Length (ft) Z
i
(
Ω
)
|S21| (dB)
h (in)
of Radials
h (inches)
h (inches)
21
44.4
–62.31
260
4
357
381
20
41.6
–61.12
261
8
366
382
18
41.0
–61.84
264
16
374
382
16
42.6
–61.78
267
32
377
382
reel and the lengths of connecting wires, plus the length of radials
from the reel box to ground surface. The columns for h do, however,
give an idea of the change in height. In the case of 33-foot radials the
change is quite large (20 inches) between 4 and 32 radials. On the
other hand with 21-foot radials the change in h with radial number is
very small, factions of an inch. The values in the Table are rounded
off to the nearest inch.
Part 6
In the inal part of this experiment the effect of radial shortening
on 30, 20 and 15 m was examined. This was really just a quick look
using radials left over from the earlier parts of the experiment, cut
down from them rather than making up a new set of ¼ wavelength
(free space) radials for each band.
In all three cases 8 radials were
used
. The test frequencies were: 10.120 MHz, 14.200 MHz and
21.200 MHz. The corresponding free space ¼ wavelengths would
have been, 24.3 feet, 17. 3 feet and 11.6 feet respectively. The results
are shown in Tables 6, 7 and 8. The value for |S21| is the actual mea-
surement.
One oddity in this data was that the best radial length on both
30 and 20 m was the same, about 15 feet. There is some dispersion
(variation with frequency) in the soil characteristics but I don’t think
that’s a full explanation. In all cases the optimum length was well
short of the free space ¼ wavelength. I think this part of the experi-
ment needs to be rerun cutting down from full length radials. This
will be done at some future time.
Figure 5 — Here is a comparison between
NEC
modeling run 1 and
the experimental data using 4 radials taken on May 8, 2008.
NEC
Modeling
At this point it was clear that Belrose’s original work was basi-
cally confirmed experimentally, but I was curious to see how
closely this data could be replicated using
NEC4-D
modeling soft-
ware (
EZNEC Pro + MultiNEC
). The irst trial model employed
4 radials with lengths from 6.4 m (21 feet) to 10 m (33 feet). The
wire table for this model is given in Table 9. The radials were placed
5 mm above 0.01/14 soil. The test frequency was 7.2 MHz and the
vertical height was adjusted to maintain resonance as the radial num-
ber was changed.
We can compare the maximum gain data against the experimental
data for 4 radials (from Figure 4) as shown in Figure 5.
The match in gain data is very good, as was the current distribu-
tion on the radials. The impedance data was also close. We can also
see what
NEC
predicts about the current distribution on a radial as
we change the length. Figure 6 shows the current distribution on a
33-foot radial for
NEC
model 1.
Figure 6 looks very similar to the experimental measurement
shown in Figure 2. When we shorten the radials to 21 feet, we get
the current distribution shown in Figure 7. This is very close to reso-
nance.
The match in gain and current distribution, however, is really too
good to be believed. First of all, this is not an exact model of the real
antenna. The vertical uses a strip of beryllium-copper, not a no. 12
wire, and I believe my ground characteristic is better than the 0.01/14
used in the model. Models with wires very close to the ground sur-
Figure 6 — This graph shows the current distribution on a 33 foot
radial (
NEC
model).
Figure 7 — Here is the current distribution on a 21 foot radial (
NEC
model).
QEX – January/February 2009
51
Table 7
20 m, ¼ Wavelength Free Space = 17.3 Feet.
Table 8
15 m, ¼ Wavelength Free Space = 11.6 Feet.
Radial Length [ft]
Z
i
(
Ω
)
|S21| (dB)
h (in)
Radial Length [ft]
Z
i
(
Ω
)
|S21| (dB)
h (in)
9
27.3
–60.34
60
16
37.8
–62.03
178
8
30.0
–60.29
60
15
36.0
–61.84
179
7
34.3
–60.11
60
14
35.0
–61.91
181
6
41.0
–60.46
60
Table 9
Model Wire Table
End 1
Show Lengths in
•
m O wl
End 2
Diameter
Segs
X (m)
Y (m)
Z (m)
X (m)
Y (m)
Z (m)
(mm or #)
(359)
Wire
Length
Seg Len
40 m gp 4 rad A
0.000
0.000
0.005
0.000
0.000
10.306
#12
103
W1
10.301
0.100
0.000
0.000
0.005
6.400
0.000
0.005
#12
64
W2
6.400
0.100
0.000
0.000
0.005
0.000
6.400
0.005
#12
64
W3
6.400
0.100
0.000
0.000
0.005
-6.400
0.000
0.005
#12
64
0.000
0.000
0.005
0.000
–6.400
0.005
#12
64
Table 10
Z
i
and Peak Gain
Freq (MHz)
L
M
R at Src1
X at Src1
SWR(50
Ω
)
Max Gain
7.200
9.056
10
83.15
0.03
1.663
–4.41
7.200
9.275
9.45
65.72
0.01
1.314
–3.22
7.200
9.535
8.84
54.59
0.00
1.092
–2.12
7.200
9.757
8.23
49.83
–0.01
1.003
–1.45
7.200
9.955
7.62
48.23
–0.02
1.037
–1.04
7.200
10.136
7.01
48.48
0.01
1.031
–0.81
7.200
10.306
6.4
49.91
–0.02
1.002
–0.70
Where L is the height of the vertical in meters and M is the length of the radials in meters.
face are very sensitive to small changes in
the model and wire segmentation. A change
in height as small as 1 mm when the wires
are at 5 mm above ground, makes a very
substantial change in the results. By diddling
the model, I can get the kind of match shown
in Figure 5, but when I go the other way and
attempt to use the model to predict the behav-
ior of the real antenna, the results could be
way off. When it comes to wires very close
to ground — distances comparable to the
wire diameter —
NEC
replicates the general
behavior but you do not know enough of the
details of the real antenna and it’s immedi-
ate environment to expect exact quantitative
results from the model.
In addition, the characteristics of real soil
vary widely even at a ixed location: verti-
cally, horizontally and over time. The soil
will very likely have grass (weeds?) over it,
which varies in length and water content dur-
ing the year. We will seldom have more than a
general idea what our ground characteristics
are even with ground probe measurements.
We will also not really know the height above
ground to a fraction of mm! The radials will
be buried somewhere in the grass, so who
knows what the effective height really is.
down next to the ground surface, I have
found
NEC
predictions to be very good when
I went out and built the actual antenna.
We have a couple of ways to attack the
problem of radial resonance and excess
ground loss: irst, cut the radials to be near
resonance while lying on the ground. That
works if you have the instrumentation, but is
hardly a practical approach in general. The
second and much more practical approach is
to use at least 16, or better yet, 32 radials. As
I pointed out earlier, ground systems using
only a few radials are a poor idea for many
reasons.
Final comments
The effect that showed up initially in
Belrose’s article and in later
NEC
model-
ing appears to be real. I think it is clear that
in a sparse radial system lying directly on
the ground surface, it is possible to incur
substantial additional ground losses over
what we might expect. The prediction from
NEC
modeling of this effect appears to be
conirmed, at least qualitatively. I have been
able to reproduce it experimentally mul-
tiple times, on multiple bands, with different
antennas.
While
NEC
predicts the effect, you can’t
rely on
NEC
modeling for exact predictions.
You will have to do inal adjustment in the
field. This is not a general indictment of
NEC
. When the antenna has not been right
Notes
1
J. Belrose, VE2CV, “Elevated Radial Wire
Systems For Vertically Polarized Ground-
Plane Type Antennas, part 1 — Monopoles,”
Communications Quarterly
, Winter 1998,
pp 29-40.
2
R. Severns, N6LF, “Measurement of Soil
Electrical Parameters at HF,” ARRL,
QEX
,
Nov/Dec 2006, pp 3-9.
52
QEX – January/February 2009
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