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2m Elevated
Groundplane
Antenna
An antenna designed to exactly match the impedance of the
feed cable has much to recommend it. The transmitter will
develop its maximum power, losses in the feed cable will be
minimised and any risk of damage due to mismatch is avoided.
By PHILIP WATSON, VK2ZPW
62 Silicon
iliconCChip
hip
This view shows all the pieces of the antenna just before the
final assembly. The copper tube forms part of the matching
section. The materials used are all ready available and you
should be able to scrounge most of the parts for little cost.
T
HIS VHF ANTENNA was originally constructed as part of the
author’s research into the impedance of an elevated groundplane
antenna, as set out in the June 1999
issue of SILICON CHIP. In particular,
the author wanted to establish that
an impedance matching section (or
“Q” section) could be constructed, to
match the 52Ω impedance of the feed
cable to the 18Ω antenna impedance.
In fact, the finished device has
proved to be a completely practical
antenna. It is simple to construct, easy
to mount and because it provides the
correct load, it allows the transmitter
to generate maximum power.
This is important because not every
transmitting device is completely
safe from mismatch damage. Typical
commercial power amplifiers (“afterburners”) frequently carry a warning
that an SWR above a specified figure
will void the warranty, for example.
A feature of the unit is that the “Q”
section is of solid construction. It
makes a substantial “handle” which
can be lashed or clamped to a mast
or, if the mast is in tubular form, the
“Q” section can sit inside the mast,
along with the coax cable.
Another feature of the unit is the
use of screw-on radials which can be
easily detached for transport. In fact,
this antenna has proved extremely
useful as a temporary base antenna
during WICEN exercises. Alternatively, as a permanent base station
antenna, it would suit any situation
requiring an omnidirectional VHF
antenna for 2-metres (144-148MHz).
Useful background
In the June 1999 issue of SILICON
CHIP, the author presented an article
entitled “What Is A Groundplane Antenna?”. This article sought to clarify
the difference between two different
types of groundplane antenna – the
earthed variety and the elevated type.
Having established that the elevated
version has a theoretical impedance
of 18Ω, the article went on to discuss
the problem of matching the antenna
to the feed impedance (52Ω) and
briefly described a practical antenna.
However, the arrangement described in that article is not the only
approach. In that case, the idea was
to design the antenna itself to provide
the required 52Ω feed impedance. In
the current approach, the antenna is
left in its simple basic form, compatibility between the two impedances
being achieved by inserting a matching device between the cable and the
antenna.
One of the simpler forms of matching device is what is commonly called
a “Q” section; a quarter wavelength
coax section having an impedance
value intermediate between the two
impedances (ie, between 18Ω and
52Ω). A simple formula (1) is used
to calculate this value:
(1). Zq = √(Za * Zb)
where Zq = Required Q section impedance; Za = Cable Impedance; and
Zb = Antenna Impedance
In this case, the value for Zq comes
out at 30.6Ω. And so it all appears to
be delightfully simple; just insert a
quarter wavelength of Zq impedance
coax between the cable and the anten
na. It’s all too easy.
But of course, there’s a catch – just
where do you find 30.6Ω coax? You
certainly can’t get it from any of the
regular electronic outlets. In fact
no such material exists – all that is
readily available are the (nominal)
52Ω and 75Ω varieties.
Granted, there are some tricks
available – eg, two lengths of coax
connected in parallel will provide
half the impedance. From this, the
best choice would seem to be two 52Ω
parallel lengths to produce an impedance of 26Ω. That’s much better than
the gross mismatch of 2.88/1 using a
straight connection but still short of
the ideal. The error is similar using
two parallel 75Ω lengths.
A possible solution to this problem
might be to use a parallel arrangement made up of one length of 52Ω
cable and one length of 75Ω cable.
This would produce an impedance
of 30.7Ω (which is very close to the
required value of 30.6Ω) – assuming
that the simple resistances-in-parallel
law holds true. The author hasn’t tried
using this technique, however, and so
is unable to vouch for its authenticity.
Transmission line tricks
Fortunately, some of the techniques
FEBRUARY 2001 63
How it goes together – the 6.35mm (OD) brass tube is pushed down the 12.7mm copper tube to form the matching section.
It is then soldered to the centre pin of a PL259 plug via a short length of tinned copper wire – see Fig.1.
employed by users of open wire transmission lines can be used to solve our
impedance matching problem. An
open wire transmission line can be
made with any impedance value (over
a wide range) simply by selecting a
suitable wire gauge and spacer dimensions. So, if a “Q” section is required,
it is easily made to the required value.
Coming back to the coax scene,
could the same trick be pulled there?
Could a length of “coax” be constructed to have any required impedance?
The answer is yes and a formula
is available to design it. Taken from
the ARRL Antenna Handbook, it is
as follows:
(2). Zo = 138(log D/d)
where Zo = Characteristic Impedance;
D = Inside Diameter of Outer Conductor; and d = Outside Diameter of
Inner Conductor
This, of course, is for an air-spaced
device. In theory, the use of any spacers would alter both the impedance
and the velocity factor. In practice,
this can be ignored – at least in the
context of this article and the antenna
described here.
Practical considerations
So much for the theory. Putting this
idea into practice is another matter,
since we are no longer thinking of a
flexible cable. Instead, we are talking about a rigid device which must
somehow be mounted. And, of course,
suitable materials with the appropriate dimensions must be found to build
the matching section.
Strangely enough, finding the
materials turned out to be the least
of our problems. A hunt through my
scrapmetal box soon yielded an odd
length of 12.7mm OD copper water
pipe plus a length of 6.35mm (0.25inch) OD brass tube.
Well, that was as good a place as any
from which to start. The water pipe
would serve as the outer conductor,
while the brass tube would become
the inner conductor.
As it turned out, I was lucky – when
the appropriate measurements were
The PL259 plug is connected to
the “Yorkshire” fitting at the end
of the copper tube using 1/8-inch
Whitworth screws, as shown here
and in the photo at right.
64 Silicon Chip
fed into Eqn.(2), the result came out
within a whisker.
More exactly, it worked out as follows. The diameter (d) of the brass
tube inner conductor (6.35mm) was
already known but the inside dia
meter of the outer conductor had to
be measured. Since I didn’t have an
inside micrometer or callipers, the
best I could come up with was a finely
calibrated steel rule and this gave a
figure of 10.5mm (D).
When these two figures were fed
into Eqn.(2), the charac
teristic impedance (Zo) came out as 30.14Ω
– not quite the 30.6Ω being sought
but probably “within acceptable
tolerance” as an engineer might say,
or “near enough” in layman’s terms.
Mechanical design
Now it was a matter of deciding
on a suitable mechanical design and
the physical construction. Originally,
the idea was to build the matching
section as a separate unit which
could be coupled to the antenna base
using an appropriate plug and socket
combination.
However, while assembling a rough
mock-up of the inner and outer conductors, a much simpler approach
suddenly suggested itself. If the inner
conductor was extended beyond the
antenna end of the “Q” (matching)
section, it would form the beginning
of the antenna. And by further extending this to an appropriate length, it
would form the antenna itself.
This changed the whole approach;
the tail was starting to wag the dog.
Instead of starting with an antenna
and making a “Q” section to attach to
it, we are now making a “Q” section
which also becomes the antenna.
So the logical approach is to combine the two items into one structure.
Not only is it simpler and cheaper to
build, obviating the need to supply
and fit a plug and socket assembly, but
also rather more elegant technically.
In theory, the presence of conventional plug and socket assemblies – or any
junction – in a coax cable creates a
discontinuity which increases losses.
Just how serious this is in practice
may be debatable but, anyway, every
little helps.
So much for the theory. The first
construction step is to ensure that the
ends of the copper tube are cut square.
Ideally, this should be done using a
tube cutter, as used by plumbers, if
one can be begged or borrowed.
If a hacksaw is to be used, take
care in marking and cut
ting. Use
the straight edge of a piece of paper
wrapped around the tube to mark out
a cutting guide, then cut a shallow
groove right around the tube. Deepen this cut progressively by rotating
the tube a little at a time until the
operation is complete. Be sure to cut
slightly to one side of the cutting
guide, so that the end can later be
cleaned up with a file.
Don’t try to cut straight through the
tube in one operation. It will almost
certainly come out crooked if you do.
The antenna end of the tube is
fitted with a small metal plate which
supports the four radial elements. In
the writer’s case, this was made from
a piece of scrap brass, cut to about
110mm square (although this isn’t
critical) and drilled with a central
hole to match the OD of the brass tube.
The plate is simply flush-mounted
with the end of the tube and secured
by soldering (eg, using flux, a solder
Fig.1: this exploded diagram shows how the antenna is assembled. Note
that the 6.35mm OD brass tube is used as both the radiator and as part of
the matching (Q) section (ie, the brass tube is 1004mm long). An
insulating grommet isolates the radiator from the copper tube at the brass
plate end.
FEBRUARY 2001 65
The radials are tapped at one end to 4BA x 10mm to match the spacers on
the brass plate and fitted with a soldered “stopper” nut. This makes it easy to
dismantle the antenna for transport. Alternatively, for a fixed installation, the
radials can be soldered directly to the brass plate.
stick and a gas flame to provide the
heat).
An alternative form of plate is a
press-on lid as used on large coffee
tins or similar containers, painted for
protection from the weather.
The radial elements can be made
from any convenient size and type of
material. The writer used 3.175mm
(0.125in) brass rod but larger diameter
tubing could also be used. The radials
are each about 450mm long and can be
directly soldered to the four corners
of the metal plate.
Alternatively, the radials can each
be tapped at one end to 4BA x 10mm.
A brass nut is then threaded onto
each radial to act as an end stop (and
soldered in position). The radials are
then screwed into 4BA brass spacers
soldered to the corners of the metal
plate (see photo & Fig.2). The prototype used round brass spacers but
hexagonal spacers would be much
easier to position during soldering.
You can buy a pack of six from any
of the parts retailers for around $3.00.
The advantage of this latter scheme
is that it allows the antenna to be
easily dismantled and transported,
if necessary.
plumber’s “Yorkshire” fitting. (Note:
the metric dimensions are rounded to
12mm in hardware literature).
The “Yorkshire” and “Yorkway”
unions are designed to join (ie, buttjoin) two lengths of 12.7mm OD copper tube. In this case, the unit used
should be specified as a “slip fitting”
which has no stop in the centre (as
normally used to simplify correct
This close-up view shows how the end
of the brass rod is plugged and drilled
to accept the short length of 1.3mm
tinned copper wire which connects to
the PL259 plug.
Termination
The cable end of the tube is terminated with a PL259 plug. The plug
body is the same diameter as the OD
of the tube (ie, 12.7mm) and is buttjoined to the tube. It is secured using
a brass sleeve consisting of a 12.7mm
(0.5-inch) ID union – a standard
66 Silicon Chip
The insulating grommet should be a
tight fit over the inner brass tube. It
is pushed down into the copper tube
at the end of the matching Q section
during the final assembly.
positioning over a junction).
Both fittings are designed to be
soldered to the copper tube. The
“Yorkshire” fitting is supplied with
two internal rings of solder. The
copper tubes should first be cleaned
and fluxed, after which everything
is fitted together and hit with a gas
flame until solder flows right around
the end of the union.
A “Yorkway” fitting is treated
similarly, except that the solder has
to be applied externally to the ends
of the union. In the writer’s case, a
“Yorkshire” fitting was used simply
because it was on hand but it would
probably be the preferred device.
The PL259 plug was secured into
the sleeve using two 1/8-inch Whit
worth screws. Matching holes are
drilled through the sleeve and the
plug body, initially to suit a 1/8-inch
Whitworth tap. The holes in the plug
are then tapped, while the holes in
the sleeve are enlarged to provide
clearance.
The same arrangement can be used
to secure the sleeve to the tube or, as
in this case, the sleeve can be secured
by soldering.
The distance from the end of the
PL259 plug to the start of the pin is
about 20mm (as measured inside the
plug) and the internal diameter of its
body is similar to, but not identical
with, that of the tube. But although
not exact, it really is close enough
considering that only 20mm is involved.
As a result, the plug can be treated as an extension of the tube. This
means that the tube must be cut
20mm shorter than the calculated
section length (ie, to 494mm instead
of to 514mm, as quoted later on).
The cable end of the inner conductor (ie, the 6.35mm tube) has to be
joined to the pin of the PL259 plug.
This was done by first plugging the
end of the tube using a 3/16-inch
brass machine screw. This screw was
soldered into place with its head cut
off and filed square with the end of
the tube.
The screw was then drilled longitudinally to accept 16g (1.3mm) tinned
copper wire (about 25mm long), using
a No.55 or 5/64-inch drill, to a depth
of about 6mm. The 16g wire was then
soldered in place. When the antenna
is later assembled, this 16g tinned
copper wire slides into the plug pin
and is soldered.
Longitudinal drilling can be
a tricky operation unless one
has access to a lathe. However,
provided care is taken, it can
be done using a hand drill
(eggbeater). Just be sure to accurately centre-punch the end
before drilling.
Small off-centre errors are
easily corrected simply by
bending the wire. Larger errors
can be corrected by drilling an
oversize hole and accurately
positioning the wire prior to
soldering.
The inner conductor is secured where it emerges from
the outer conductor (ie, at the
ground
plane) using a simple
plastic grommet. This insulator should have a bore size of
6.35mm (0.25-inch) to accept
the inner conductor and should
be a tight fit into the 12.7mm
outer brass tube.
Scrounging the
copper tube
Obtaining a suitable length of
12.7mm copper water pipe for
the outer conductor shouldn’t
present any problems. Normally, plumbers buy it in standard
6-metre lengths but most hardware stores will sell it to you
by the metre.
There are also two other likely
sources of scrap lengths: (1) a local
plumber and (2) a scrap metal yard.
A scrap metal yard will also usually
have brass rod and tubing in a range
of sizes and this can be cheaper than
buying commercial lengths from a
hardware store.
Fig.2: the four tapped brass-rod radials screw into threaded brass spacers (or
standoffs) which are soldered to the four corners of the mounting plate.
Antenna dimensions
The exact dimensions of the antenna assembly will depend on the particular frequency to be favoured. The
antenna described here was designed
for 146MHz which equates to a freespace quarter-wavelength dimension
of 514mm. This means that the outer
copper tube in the “Q” (matching)
section had to be cut to 514 - 20 =
494mm, as mentioned previously.
The calculated length of the radiator, after allowing for end effect or
“K” factor, is 490mm (ie, 514/1.05)
and so the 6.35mm brass rod is cut
to 514 + 490 = 1004mm.
And how did all this work out in
practice? Extremely well, as indi-
This photo shows the finished antenna with the four radials screwed into
position. Also visible is the insulating grommet (red) at the end of the matching
section. Use silicone sealant to seal around this grommet.
cated by the following SWR figures,
which speak for themselves.
144MHz ..................... 1.02/1
145MHz ..................... 1.02/1
146MHz *.................... see note
147MHz *.................... see note
148MHz ..................... 1.02/1
* Too Low To Measure
So that’s it; a near perfect antenna
- well, impedance-wise anyway. SC
FEBRUARY 2001 67
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