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What is a
groundplane
antenna?
By PHIL WATSON
The term “groundplane antenna” often
means different things to different
people. There are two quite distinct
antenna designs under this heading, a
myriad of variations in between and
lots of confusion as a result.
T
HE GROUNDPLANE antenna is
probably the best known and
most commonly used of all
transmitting antennas, both in commercial and amateur roles. It is omnidirectional, simple to construct,
uses low-cost materials and is equally
suitable for base or mobile use.
In its basic form, it consists of a
quarter-wavelength vertical radiator,
mounted above four quarter-wavelength horizontal radials spaced 90°
apart. These horizontal radials form
the so-called “groundplane”. This
type of antenna is fed via coaxial
66 Silicon Chip
cable, the inner conductor going to
the radiator and the outer braid to the
groundplane assembly.
This is the configuration with
which most amateurs will be familiar
and it sounds simple enough. But confusion begins immediately we tackle
the task of matching the impedance of
this antenna to the impedance of the
transmitter. Nowadays, by conven
tion, transmitters are designed to work
into a 52Ω load and to be connected to
the antenna via a 52Ω coaxial cable.
In practice, this means that the
antenna should provide a 52Ω load.
In reality, very few antennas provide
such a load naturally and the ground
plane is no exception. As a result, we
have to modify the antenna, or the
coupling to it, to present the transmitter with the correct load.
There are many well established
ways of doing this but first, we need
to know the natural impedance of
the antenna, the mismatch that this
creates, and the best way to correct
it. So what is the natural impedance
of the groundplane antenna? Put this
question to most amateurs and nine
times out of ten they will nominate
36Ω, a figure that’s frequently quoted
in the textbooks.
However, as more than one amateur
has learned to his dismay, any attempt
to develop a matching system based
on this figure is doomed to failure.
So is this a case where theory and
practice don’t agree? This is where
things become confusing.
Groundplane development
In order to better understand the
problem, let’s first take a look at the
groundplane’s history and clarify
some of the published data.
The groundplane antenna evolved
from the basic half-wave, centre-fed
antenna; ie, a half-wavelength long
radiator, broken at the centre to form
a feed point. A half-wave centre-fed
antenna has a natural radiation resistance of 72Ω, may be polarised
horizontally or vertically, and is a
very efficient antenna in its own right.
This type of antenna is most popular in the horizontal mode, particularly at the lower frequencies. It isn’t
used as much in the vertical mode
because it would be impractically
long at low frequencies, while the centre-feed requirement is an awkward
arrangement in some applications.
The original groundplane antenna
was designed to overcome these limitations. The radiator was reduced
to a quarter-wave vertical element
and this was mounted above a large
conducting surface. In theory, the
quarter-wave element is reflected
by the conducting surface, thus providing the other half of the antenna
which would thus be equivalent to a
half-wave centre-fed system.
Theory also suggests that the reflecting surface should be infinitely
large and have zero resistance. In
practice, the Earth itself serves as the
reflector and although it isn’t perfect,
it can be made very effective. Various
tricks are often employed to enhance
its performance, such as selecting a
moist area of ground area and burying
wires in the ground, radiating outwards from the vertical element. The
theoretical radiation resistance of this
type of antenna is 36Ω (ie, half 72Ω)
but, in practice, this varies according
to the efficiency of the groundplane.
Typical examples are the antennas
used by radio stations in the broadcast band. A single mast acts as the
radiator and, in its simplest form, is
a quarter wavelength long. However,
the length may vary, with some installations embracing the five-eighth
wavelength concept or some other
means to control the radiation angle.
Often, the mast is located above moist
or swampy ground, into which many
radials are buried.
OK, so that’s the background to
what might be called the “original”
or “earth” groundplane; names deliberately chosen to avoid confusion as
we progress. It is popular with many
amateurs, particularly for the higher
HF bands – up to 30MHz – where the
physical size of the radiator is more
manageable.
However, it does have a disadvantage. Although fine for use out
in the country, in the traditional
40-acre paddock with few nearby
obstructions, it is less attractive in
suburban backyards which are often
surrounded by buildings on all sides.
And as we go higher in frequency and
the radiator becomes shorter, these
obstructions become more and more
detrimental.
The elevated groundplane
It’s here that we come to another
version of the groundplane antenna.
Known as the “elevated groundplane”, this is the version that’s most
familiar to amateurs working at VHF.
Its development is usually credited
to Dr George H. Brown and J. Epstein
of RCA and took place around 1938,
when interest in frequencies above
30MHz was increasing rapidly.
As mentioned earlier, it consists
of a quarter-wave verti
cal radiator
and four quarter-wave horizontal
radials, emanating from the base of
But at least there was agreement
on one point; the elevated groundplane has a lower impedance than
the original groundplane and this
was recognised by its creators back in
1938. They measured two values: 25Ω
for one version and 21Ω for another.
Figures like this continued to be
quoted for many years, with some
writers having a bet each way by
quoting 20-30Ω. What appears to be
one of the first references to a realistic
value is in the “RSGB Amateur Radio
Handbook”, Third Edition (page 365),
where the value is quoted as being
“less than 20Ω.”
Later, in the “RSGB Radio Communication Handbook”, Third Edition
(page 12.81) is what appears to be
the first mathematical explanation.
In simplified form, this states that it
is the theoretical value of a dipole
feedpoint (73Ω) divided by four, or
18.25Ω. It adds that measured values
are usually a little higher.
A later (6th) edition of this handbook expands on this theme. It quotes
the dipole feedpoint impedance as
the more usual 72Ω, thus making the
calculated value 18Ω, and provides
Any attempt to develop a matching system
based on an impedance of 36 ohms for an
elevated groundplane antenna is doomed
to failure.
the radiator. The radials form an artificial groundplane which is no longer
earthbound, allowing the com
plete
antenna system to be mounted high
above surrounding obstacles.
And so the scene was set for confusion, with two somewhat different
antenna configurations using the same
name. Granted, one evolved from the
other and for the most part, their be
haviour is similar, even when it comes
to the angle of radiation.
But one characteristic of the two antennas is significantly different – the
feedpoint impedance. So what is the
feedpoint impedance of an elevated
groundplane antenna? This is a figure
that has been difficult to accurately
pin down. Indeed, one might take the
cynical view and say that it depended
on the last reference consulted.
a more detailed explanation as to
why this value may vary somewhat
in practice.
So that’s the basic background to
the elevated groundplane antenna
and, in particular, its feedpoint impedance. And, if it appears that this
point has been unduly laboured, it
was for a very good reason – to put
to rest the confusion over feedpoint
impedance that’s occurred over the
years.
This confusion has arisen because
many well-known publications and
textbooks have failed to recognise
and make clear this all-important
distinction between the two antennas. And at least one textbook has
positively stated that the (elevated)
groundplane, clearly portrayed diagrammatically, has an impedance of
JUNE 1999 67
30-35Ω. Not only that, but it goes on
to describe a matching stub, based
on this figure, which is supposed to
match it to a 75Ω coaxial cable – this
some 30 years after the inventors,
Brown and Epstein, had suggested a
value as low as 20Ω.
Practical considerations
But the situation is really quite
clear. The original or “earth” groundplane has a theoretical feedpoint
impedance of 36Ω and a value close
to this figure can be achieved given a
favourable situation and an elaborate
setup. Otherwise, the value may vary
considerably.
On the other hand, the elevated
groundplane has a theoretical figure
of 18Ω and this value or one very
close to it can also be achieved in
practice. Between 18Ω and 20Ω is
a frequently quoted range but the
writer’s own experience suggests that
calculations based on 18Ω work out
to be extremely close.
Having said that, it is necessary to
but may call for more attention at HF.
At 14MHz (20 metres), for example,
the required clearance would be 10
metres.
Matching problems
For now, let’s settle for the true
elevated version and accept an impedance value of 18Ω. Unfortunately,
this is not exactly a convenient figure
when it comes to matching the 52Ω
impedance of the transmitter and
the associated coax cable. Indeed, it
represents a mismatch of nearly three
to one (2.88:1).
And that brings us to the practical
side – how do we match the two?
Broadly speaking, there are two
possible approaches: (1) interpose
a matching transformer (typically a
quarter-wavelength of a suitable value coax), or (2) modify the antenna
design itself so that it presents the
desired impedance.
The author has tried both approaches, with near perfect results in both
cases. However, this article will con-
By juggling the element diameters, we can
continuously vary the feed impedance over
a wide range. In short, we can design an
antenna to have exactly the impedance we
require.
point out that there can be intermediate conditions between these two
configurations. A typical example is
the mobile version – a vertical quarter-wave radiator above a vehicle body
as the groundplane. There are so many
variables here that the impedance is
anybody’s guess. It satisfies neither
the elevated version nor the earth
version.
So how long is a piece of string?
If in doubt there is only one way to
find out; measure it and see. But that’s
another story.
Another variable factor is the distance between the elevated ground
plane and the true earth, and/or other
conducting surfaces. This should be
at least 0.5 wavelengths, or greater
if possible. The most likely effect
of nearby conducting surfaces is to
raise the impedance towards the 36Ω.
Maintaining good separation is not a
difficult requirement to satisfy at VHF
68 Silicon Chip
centrate on the latter approach, mainly because it is physically simpler but
also because it has some advantages
in its own right.
In simple terms, the method is a
variation of the folded dipole concept,
except that it uses a folded monopole.
This is in no sense an original concept. It has been known and used in
both amateur and commercial circles
for many years. However, it has never
attracted much publicity.
As is well known, a folded dipole
has an impedance that’s four times
that of a simple dipole – ie, 288Ω. This
figure is usually rounded to 300Ω. The
same applies to a folded monopole,
which has a feedpoint impedance of 4
x 18Ω, or 72Ω. Admittedly, this is still
not a perfect match to a 52Ω system
but it is a good deal better than that of
a simple monopole. In fact the error
is now only 1.4:1.
To digress briefly, this approach
was used extensively during the
early days of VHF mobile radio systems, mainly for base antennas. The
transmitters of the day were designed
for a 75Ω load, using 75Ω cable. The
basic folded monopole presented an
impedance of 72Ω; as near perfect a
match as one could wish for.
This approach to a 72Ω load require
ment is suggested in the “RSGB Radio
Communication Handbook”, 3rd
edition, p12.82 (Fig.12.123(d)) and
further confirms the 18Ω basic value.
The 52Ω standard is not quite so
easily accommodated but we have
another trick up our sleeve. In its basic form, the folded radiator uses the
same diameter conductors for both
the active and passive elements. And
in this form the spacing between the
elements is not critical. But when we
use different diameter conductors for
the two elements, the picture changes.
The spacing now becomes a factor
in determining the feed impedance
and by also juggling the element dia
meters, we can continuously vary the
feed impedance over a wide range.
In short, we can design an antenna
to have exactly the feed impedance
we require.
A formula and a graph, which can
be used to calculate the design of a
folded dipole, have appeared in several publications, including the “ARRL
Antenna Book”, 14th Edition (p2-29)
and this is equally applicable to the
folded monopole concept.
The formula is as follows:
r = [1 + log(2S/d1)/log(2S/d2)]2
where S = spacing between elements
d1 = driven element diameter
d2 = passive element diameter
r = impedance ratio
As can be seen, in this configuration
the formula solves the impedance
ratio for any nominated combination
of element diameters and spacing.
Unfortunately, this is not the most
convenient way of going about things
because, given the element diameters
and the required impedance ratio, it is
necessary to make a series of trial and
error calculations to find the correct
spacing.
In practice, we would prefer to directly calculate the element spacing
to give the required ratio, after first
nominating the element diameters we
wish to use. These diameters will in
¼ λ
Fig.1: basic concept of an elevated
groundplane antenna. It consists of a
quarter-wave vertical radiator plus
four quarter-wave horizontal radials,
which form an artificial groundplane,
emanating from its base.
turn depend on the material to hand or
on what can be obtained. Unfortunately, transposing this equation so that
we can directly calculate the spacing
(S) is not straightforward.
This problem was solved by sticking to the trial and error approach
but letting a spreadsheet do all the
calculations. This method was used
to produce a list of ratios from given
element diameters, with the spacing
increasing in 1mm steps. Although
this approach might seem a little
clumsy, it works very well and was
used for the practical design described
below.
Note that this calculation gives the
space between the element centres.
This means that, in some cases, the
physical spacing between the two
elements will be quite small when
their diameters are taken into consideration. In fact, it may even be
impossible to space them correctly,
since the theoretical figure would
require the two elements to overlap.
The answer here, of course, is to
recalculate the ratios using elements
with different diameters.
Putting all this theory into practice
resulted in the following dimensions
for an antenna designed for 146MHz
and measuring 470mm. Using a 2.89
(ie, 52 ÷ 18) multiplication factor and
taking advantage of available materials, a prototype was constructed using
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a 5/16-inch diameter driven element
(made of brass tube), a 1/8-inch brass
rod passive element, and a spacing of
28mm between the outside diameters.
And the result? Although the
prototype was rather hurriedly constructed, it came up with an SWR
ranging from 1.05:1 to 1.1:1 across
the 2-metre band. So the theory and
practice can be made to agree very
closely. And had it been considered
worthwhile, the spacing could have
been juggled a fraction to come even
closer to optimum.
And that brings us to the other advantages of this arrangement, hinted
at earlier. First, the folded element is
inherently broadband, so rather than
suffering any trade-offs with this arrangement, we actually score a bonus.
Secondly, it is at earth potential in
the DC sense, a valuable feature where
there is a risk of a lightning discharge.
In this case, the discharge is directed
directly to earth, rather than via the
equipment.
The actual construction details are
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Truscott’s
ELECTRONIC WORLD Pty Ltd
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27 The Mall, South Croydon, Vic 3136
email: truscott<at>acepia.net.au
www.electronicworld.aus.as
References
(1). Harold C. Vance, Sr. K2FF. “The
Ground Plane Antenna: Its History
and Development.” Ham Radio, January 1977, pages 26-28
(2). Amateur Radio Techniques,
6th Edition. Pat Hawker, G3VA. Pages
242-243. Published by R.S.G.B.
(3). RSGB Amateur Radio Handbook. 3rd Edition. Pages 364-365.
(4). RSGB Radio Communication
Handbook. 3rd Edition Page 12-81
(18.25Ω)
(5). RSGB-Radio Communication
Handbook. 4th Edition. September
1968. Page 13-69 (20Ω or less)
(6). RSGB-Radio Communication
Handbook. 6th Edition.
(7). Radio Handbook, 17th edition,
1967, edited by William I. Orr, W6SAI.
Published by Editors & Engineers.
Pages 359 & 407
Acknowledgements
Many fellow amateurs contributed
to this article. There are too many to
mention individually but the following deserve special mention: W. A.
(Blue) Easterling, VK4BBL (ex VK2
ABL); I. Pogson, VK2AZN; A Walker,
VK2ZEW; C. Wallis, VK6CSW (ex
VK2DQE); J. Yalden (ex VK2YGY).SC
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