qex-ground-systems-part-1.pdf

Rudy Severns, N6LF

PO Box 589, Cottage Grove, OR 97424;n6lf@arrl.net

Experimental Determination of

Ground System Performance for

HF Verticals

Part I

Test Setup and Instrumentation

This description of the test setup used by the author for a series of experiments sets

the stage for a series of articles describing his results.

HF verticals located on or near ground are

a perennial topic among amateurs. Over the

past several years this discussion has been

illuminated (and in some cases obscured!) by

the advent of really good modeling software

based on NEC (numerical electromagnetic

code). This has resulted in a vast literature

on antennas using the results of modeling.

However, these results are not without some

controversy. In particular the relative merits

of a large number of buried radials versus a

few elevated radials has been especially con-

tentious. What has been missing from the dis-

cussion are careful ield measurements done

with good instrumentation and technique to

see if the NEC predictions actually hold up

in the real world. To address this problem I

performed a series of ield experiments, over

a period of a year, to examine how different

ground system arrangements affected the

behavior of a vertical antenna and to see if

ield measurements on a real antenna would

correlate with NEC modeling.

The results of these experiments will be

presented in a series of QEX articles. There

is no pretence that these experiments will

answer all questions or even definitively

settle some of the arguments, but at least they

should give us something to think about.

In Part 1, I will discuss the test range, test

instrumentation and test procedures used for

all the experiments. Part 2, which is included

in this issue of QEX , discusses an earlier and

apparently overlooked prediction from NEC,

Figure 1— This drawing illustrates the traditional measurement scheme.

that in sparse (<10 radials) radial systems

lying close to ground, there can be a substan-

tial increase in ground loss when the radials

are made much longer than 1 ⁄ 8 wavelength.

This is a case of more copper = more loss,

which is not at all intuitive! Part 3 will com-

pare verticals with a large number of ground

surface radials to verticals with four elevated

radials. This part will directly address the

elevated radial controversy. Part 3 will also

have comparisons between several different

elevated radial configurations. Part 4 will

look at the effect of radial numbers on the

characteristics of ¼ wavelength and several

shorter loaded antennas. Part 5 will take a

look at the problems of ground systems for

multiband verticals, where a range of 7 to

30 MHz must be accommodated. Finally in

Part 6, I will report on some experiments with

a full size ¼ wavelength vertical on 160 m. In

addition, because this series will take many

months to be published, there will be lots of

time for feedback. I plan to include some of

this in Part 6.

QEX – January/February 2009 21

another transmission line.

Amplitude measurements with a pro-

fessional VNA are typically displayed to

0.001 dB, but of course nothing else in the

system is stable to that level. In practice I

found that measurements made over a short

period of time (2-3 hours) were repeatable to

within 0.05 dB. That is more than adequate

for these experiments. A weakness of this

measurement method is that as the separation

between the test antenna and the receiving

antenna is increased, the attenuation around

the transmission loop becomes quite large,

–40 to –60 dB. For instrumentation and a

physical setup with a noise loor and stray

coupling below –110 dBm, this is accept-

able but it did limit the separation distance

on 40 m to about 2.25 wavelengths for the

particular receiving antenna employed. This

is in the far ield but not by much. Another

limitation was that ± 0.05 dB repeatability

was possible only when the antenna under

test and the receive antennas were actually

stable to that level. This usually meant that

measurements had to be made in early morn-

ing when the test range was in the shade or

late in the day when things had reached ther-

mal equilibrium. It was very easy to detect

a cloud passing over by the small changes

due to temperature changes in the antennas. I

could readily detect the effect of the wind on

the vertical, causing it to move slightly. In the

end the A-B comparison measurements were

probably within a few tenths of a dB but only

when I carefully attended to all the details.

This brings us to an important point. The

purpose of the experiments was to determine

the effect of different ground system arrange-

ments from their effect on S21. All the mea-

surements were relative A-B comparisons. In

other words, they were comparisons between

two different conigurations. There was no

Test Setup

The physical layout of the test range,

the instrumentation employed and the test

procedures were all key elements in obtain-

ing reliable results. The following discus-

sion provides descriptions of these elements

which remained essentially constant for

the experiments. The majority of measure-

ments were done at 7.2 MHz although there

was some work at 160, 30, 20, 17, 15 and

10 meters. The information given here is

intended to provide information common to

all the experiments.

Test Concept

The traditional test procedure for these

kinds of measurements is well known. As

shown in Figure 1, a test antenna is excited

with a known power, and the resulting signal

is measured at a remote point. A change is

then made in the test antenna and the mea-

surement is repeated. The difference between

the two measurements is a measure of the

effect of the change in the antenna and/or

ground system on performance. The signal

transmission to antenna 2 from the excita-

tion of antenna 1 (S21) will be proportional

to the radiation eficiency of the antenna. In

other words, S21 ~ input power × Rr / (Rr +

Rg) where Rr is the radiation resistance and

Rg is the ground loss. For our purposes we

can assume that losses due to conductors are

small. Both Rr and Rg will vary as we change

the ground system but the inal goal is to see

the effect on the transmitted signal. 1

The standard way to make these measure-

ments is to use a transmitter combined with

forward and relected power meters to excite

the test antenna (antenna 1) with a known

power. A calibrated receiver is connected to a

remote receiving antenna (antenna 2) to mea-

sure the resulting signal. In my initial tests

I used both an HP3586C and an HP3585A

spectrum analyzer for the receiver. I wished

to measure the performance differences

between conigurations to within 0.1 dB if

possible, and these instruments were capable

of that. However, the limiting factor turned

out to be my ability to measure the excita-

tion power; 0.1 dB corresponds to about 2%.

To make repeatable measurements to 0.1 dB

you would need to measure power to better

than 1%.

To get around that problem I decided to

use the instrumentation scheme illustrated in

Figure 2. I chose to make the measurements

with a vector network analyzer (VNA) in

the transmission mode (S21 is the response

at port 2 due to the excitation at port 1).

The transmission path was from the VNA

output port, out to the test antenna via a

transmission line, from there to the receive

antenna and back to the VNA input port via

Figure 2 — This diagram shows the vector

network analyzer approach for measuring

antenna performance.

Figure 3— A view of the test antenna area and test equipment shelter. The receiving antenna

is at the far end of the pasture.

1 Notes appear on page 25.

22 QEX – January/February 2009

Figure 5 — Here is the test antenna base at ground level, with 64 radials.

Figure 4 — This photo shows a typical test

antenna and center post support.

attempt to measure absolute signal strengths

or radiation patterns. The separation distance

between the test antenna and the receiving

antenna was suficient to place the receiving

antenna outside the reactive near ield but the

groundwave was still signiicant. This was

not a problem for the type of measurements

being made. The presence of a metal pump

house and a travel trailer, both of which are

small in terms of a wavelength might have

had an impact on pattern measurements but

should not have affected the type of A-B

measurements being made in this series of

experiments.

Physical Arrangement

The test range was set up in a ield as

shown in Figure 3, with an area for the test

antennas (including ground systems), a

remote receiving antenna (in the far distance)

and a small travel trailer to provide shelter for

the instrumentation.

The eight poles, in an 80 foot diameter

circle around the test antenna, were used to

support elevated radials as needed. When

more than eight elevated radials were needed,

a ½ inch Dacron line was stretched around

the posts at the desired height and tightened

with a turn-buckle. Each post has a backstay

to a buried deadman anchor so the radials

could be well tensioned. Radial heights on

each post were located using a laser level to

keep the radial fan lat around the circle.

In the center of the circle there is a support

post (PVC pipe) as shown in Figure 4, with

Dacron support lines attached to the top. This

post is intended to hold the antenna under test

and allow it to move up and down to vary the

height for elevated radial tests. An example

Figure 6 — The base plate is in position for elevated radials.

of the base plate at ground level with 64 radi-

als attached is shown in Figure 5.

The base plate is isolated from ground but

there are three ground stakes (4 foot copper-

clad steel rods) close to the plate for those

tests where grounding is desired. The ground

stakes have short pig-tail leads to connect to

the base plate when desired.

Figure 6 shows an example of the base

plate positioned for elevated radial tests.

The base plate, the radials and the entire test

antenna are elevated by sliding them along

the support pipe. This arrangement made it

very easy to change the height of the radi-

als in small increments up to 4½ feet above

ground. The radials lying on the ground in

Figure 6 were not present during elevated

radial tests!

As shown in Figures 5 and 6, a coaxial

common mode choke (balun) was used to

isolate the transmission line from the test

antenna. This was done for all measurements

whether or not ground stakes were engaged.

The choke has an impedance of >3 kΩ at

7.2 MHz. For those tests in which the

SteppIR vertical was employed, the balun

that comes with that antenna was used in lieu

of the choke shown in the photos.

The receiving antenna was a 3-turn dia-

mond loop with a diagonal dimension of

QEX – January/February 2009 23

24 inches, as shown in Figure 7. The loop

was resonant at 8.2 MHz. This loop was

installed at the top of a 40 foot mast, as

shown in Figure 8.

The distance from the base of the test

antenna to the receiving loop is a little over

300 feet, about 2¼ wavelengths at 7.2 MHz.

The elevation angle from the base of the test

vertical is about 8°.

The coax from the VNA output port to the

base of the test antenna was ½ inch Andrews

Heliax with N connectors. The coax from

the receiving antenna back to the VNA was

LMR400. Low loss coax was used because it

provided better shield attenuation to reduce

coupling and in the case of the heliax run-

ning out to the test antenna, the very low loss

removed the need for an additional correction

factor for the change in cable loss with varia-

tions in SWR.

on top! Critical for maximum accuracy! The

common mode choke in the photo is undergo-

ing characterization for transmission loss and

series impedance at 7.2 MHz. It turned out

however, that the impedance of the choke was

much greater than the 50 Ω reference imped-

ance of the VNA. Above about 2 kΩ even

an HP VNA becomes inaccurate for a direct

measurement. For choke measurements,

I used an HP4815 analyzer, which is well

suited for high-impedance measurements.

After careful comparisons between the HP

and N2PK VNAs, the N2PK was selected for

Test Instrumentation

Feed point impedance, transmission gain

(S21) and radial current measurements were

all made using a VNA. Two analyzers were

available: an HP3577A with an HP35677A

S-parameter test box and an N2PK analyzer

with dual fast detectors. Figures 9 and 10 are

photos of these instruments.

Note the organic automatic heating unit

Figure 7 — This photo shows the

loop receiving antenna.

Figure 9 — HP3577A with an HP35677A S-parameter test box.

Figure 8 — Here is the receiving antenna

atop a 40 foot mast. N7MQ assisting!

Figure 10 — Here is my test bench, showing the N2PK VNA with the associated laptop

computer and HP calibration loads.

24 QEX – January/February 2009

Figure 11 — This photo shows a typical

shielded current transformer.

most of the measurements because its perfor-

mance was very close to the HP and had the

advantage of direct readout to a computer,

which made data reduction much easier. The

N2PK VNA was also much lighter than the

HP (70+ pounds!) and much more suitable

for ield measurements.

On several occasions it was necessary to

measure the current division ratios between

the radials and in some cases, the relative

current distribution along a radial. To make

these measurements a set of shielded current

transformers, like the one shown in Figure

11 were used.

To make a current measurement, a radial

wire was passed through the current trans-

former, as shown in Figure 12. Current trans-

formers were placed in the same location

simultaneously on all the radials during a

measurement. The transformer being used to

sense current was terminated in 50 Ω by the

instrumentation, so all of the dormant current

transformers were also terminated in 50 Ω.

This was done to compensate for any interac-

tion introduced by the current transformer. At

the very least, the effect of the current trans-

former would be the same on all radials. The

active current transformer was isolated with

a choke as shown in Figure 12.

Even with this degree of care, the current

measurements were still a bit tricky because of

the residual interaction between the cable from

the current transformer and nearby radials. In

some cases I actually used four identical cables

in a symmetrical layout to try to minimize

imbalance due to this interaction. I believe the

resulting measurements were reasonable and

useful but not especially precise!

The relative value of the current was

determined by using the VNA in the trans-

mission mode, measuring S21 for the loop

from the VNA output port to the base of the

antenna, out along the radial to the current

transformer and back to the VNA input port.

This was a convenient way to measure the

Figure 12 — Here is the test setup for a typical radial current measurement.

current division between radials and the rela-

tive current distribution along a radial.

Comments on test procedures

A good physical setup and professional

instrumentation are a very good start, but to

obtain reliable data great care must be exer-

cised in using and calibrating this equipment.

For feed point impedance measurements, at

the beginning and end of every test run an

OSL (open, short, reference load) calibration

was performed with the calibration plane at

the test antenna feed point. At the beginning

and end of each test run a transmission cali-

bration was also performed.

In addition, before beginning a series

of measurements a measurement of stray

coupling and possible interference was per-

formed. The procedure was to disconnect the

feed line from the base of the test antenna, ter-

minate the feed line with a 50 Ω load and then

measure the transmission gain of the entire

system in this state. Throughout the series of

experiments, this transmission level was never

higher than –110 dBm and usually –115 dBm

or lower, at 7.2 MHz. As a further check on

results, most experiments were run several

times to verify consistency and repeatability.

All of this was very time consuming but abso-

lutely necessary to assure the best possible

measurements. I did not delude myself, how-

ever, into thinking the measurements were per-

fect and cannot be improved on. I do believe

the results make sense, fit well with NEC

modeling predictions, give useful insights into

vertical antenna/ground system behavior, and

potentially can be of practical help in optimiz-

ing a given antenna installation.

Acknowledgement

This experimental work was inspired by

the earlier work of Jerry Sevick and Arch

Doty. 2, 3 Some of my experiments were

a repeat of their earlier work with more

advanced instrumentation. I would also

like to thank Mark Perrin, N7MQ and Paul

Thompson, W8IEB for the many hours of

help they provided during the experiments.

Without their help, I would still be out in the

ield taking measurements!

Notes

1 R. Severns, N6LF, “Radiation Resistance

Variation With Radial System Design,” Sep

2008. Available at: antennasbyn6lf.com .

2 J. Sevick, W2FMI, The Short Vertical

Antenna and Ground Radial , CQ

Communications, Inc, 2003. Jerry’s work

also has appeared in a number of QST

articles.

3 A. Doty, K8CFU, “Improving Vertical Antenna

Eficiency, A Study of Radial Wire Ground

Systems,” CQ Magazine , April 1984, pp

24-31. This article also has a very nice

list of earlier references related to ground

systems for verticals.

Rudy Severns, N6LF, was irst licensed as

WN7WAG in 1954 and has held an Extra class

license since 1959. He is a consultant in the

design of power electronics, magnetic compo-

nents and power-conversion equipment. Rudy

holds a BSE degree from the University of

California at Los Angeles. He is the author of

two books and over 80 technical papers. Rudy is

an ARRL Life Member, and also an IEEE Fellow.

QEX – January/February 2009 25

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