When I was building my own I came across this information which I found very useful from the DX Zone.....Enjoy!
Construction guide of
the G5RV (II),
by Louis Varney, G5RV
The Antenna
The dimensions of the
antenna and its matching section are shown in the drawing displayed below. The
"flat-top" should, if possible, be horizontal, run in a straight
line, and should be erected as high as possible above ground.
In describing the
theory of operation, it has been assumed that it is generally possible to erect
the antenna at an average height of about 10.35m (34 ft), which happens to be
the optimum radiation efficiency on 160, 80 and 40m bands for any horizontal
type antenna, in practice few amateurs can install masts of the optimum height
of half a wavelength on 80 or 40m, and certainly not on 160m.
If, due to limited
space available, or to the shape of the garden, it is not possible to
accommodate the 31.1m (102 ft) top in a straight line, up to about 3m (10 ft)
of the antenna wire at each end may be allowed to hang vertically or at some
convenient angle, or be bent in a horizontal plane, with little practical
effect upon performance. This is because, for any resonant dipole antenna, most
of the effective radiation takes place from the centre two-thirds of its length
where the current antinodes are situated. Near to each end of such an antenna,
the amplitude of the current standing wave falls rapidly to zero at the outer
extremities; consequently, the effective radiation from these parts of the
antenna is minimal.
The antenna may also
be used in the form of an inverted-V (L). However, it should
be borne in mind that, for such a configuration to radiate at maximum
efficiency, the included angle at the apex of the L should not be less
than about 120° (although many authors consider an opening between 90-120°
only); The use of #14 AWG (inner Ø1.63mm) enameled copper wire (stranded) is
recommended for all models, although thinner gauges such as #16 (inner Ø1.31mm)
or even #18 AWG (inner Ø1.04mm) solid wire can be used. If the wire has to
sustain a strong traction, a stranded steel wire of #14 AWG protected with PVC
is also suitable.
Electrical tables:
The Matching Section
This should be,
preferably, of open-wire feeder construction for minimum loss. Since this
section always carries a standing-wave of current (and voltage) its actual
impedance is unimportant. A typical, and very satisfactory, form of construction
is shown below. The feeder spreaders may be made of any high-grade plastic
strips or tubing; the clear plastic tubing sold for beer or wine siphoning is
ideal.
If it is desired to
use 300W
ribbon type feeder for this section, it is strongly recommended that the type
with "windows" (ladder line) be used because of its much lower loss
than that with solid insulation throughout its length, and its relative freedom
from the "detuning" effect caused by rain or snow. If this type of
feeder is used for the matching section, allowance must be made for its
velocity factor in calculating the mechanical length required to resonate as a
half-wave section electrically at 14.150 MHz. Since the velocity factor
of standard 300W ribbon feeder is 0.82, the mechanical
length should be 8.5m (28 ft). However, if 300W ribbon with
"windows" is used, its velocity factor will be almost that of
open-wire feeder, say 0.90, so its mechanical length should be 9.3m (30.6 ft).
This section should hang vertically from the centre of the antenna for at least
6.1m (20 ft) or more if possible. It can then be bent and tight off to a
suitable post with a length of nylon or terylene cord so as to be supported at
above head-height to the point where, supported by a second post, its lower end
is connected to the feeder.
Can we use a coaxial
for the matching section ? Louis Varney didn't really extend on this question
but we can easily find the benefits and drawbacks of using a coaxial vs. a
ladder line. At first sight the coaxial looks indeed more convenient and more
resistant. But if both types of lines connected to an antenna tuner can achieve
a very low SWR (any value below 2:1 is fine), both systems are not equivalent.
The ladder line is not
as easy to install as coaxial. First it must be kept clear of large pieces of
metal (at least 10 cm away) to avoid detuning. Unlike coaxial, once installed
you cannot bend and shape a ladder line to accomodate your installation. A
ladder line doesn't tolerate either repeated flexing as well as coaxial cable
or the line may break. At last the built-in antenna tuner of a transceiver is
not designed to work with open-wire feed lines. However, a compromise is
possible, like using an external balun or an external antenna tuner between the
ladder line and the transceiver (one side is attached to the ladder line, the
other one receiving a short coaxial that runs to the radio).
But contrarily to
coaxial, a ladder line has two big advantages on the coaxial. First, as we told
previously a ladder line offers the lowest RF loss on HF frequencies, even when
the SWR is relatively high on the feed line to the antenna. Then, connected to
an external antenna tuner, a ladder line doesn't require pruning to render the
lowest SWR on each operating band. Simply attach your ladder line at your wire
antenna and let the antenna tuner worry about providing the lowest SWR for the
transceiver. This is simple and efficient !
In other words, a ladder line is much less complex to use that a
coaxial; you have only to sit down in front of your radio to go on the air,
without squinting at the antenna tuner's SWR meter and twisting several knobs
to accomodate the lowest SWR. Using a coaxial you will probably find that you
need to readjust the tuner when you go away from the central frequency (over
200 kHz or so) and surely when you will change of band.
So, even if most hams don't use the open-wire for convenience reasons,
you can never beat a ladder-line fed dipole in terms of simplicity.
The Feeder
The antenna can be fed
by any convenient type of feeder provided always that a suitable type of
antenna tuner is used. In the original article describing the G5RV
antenna, then in RSGB bulletin November 1966, it was suggested that if
coaxial cable feeder was used, a balun (a balanced-to-unbalanced transformer)
might be employed to provide the necessary impedance transformation at the base
of the matching section. This was because the antenna and its matching section
constitute a balanced system, whereas a coaxial cable is an unbalanced type of
feeder. However, later experiments and a better understanding of the theory of
operation of the balun indicated that such a device was unsuitable because of
the highly reactive load it would "see" at the base of the matching
or "make-up" section on most HF bands.
It is now known that
if a balun is connected to a reactive load presenting a SWR of more than about
2:1, its internal losses increase, resulting in heating of the windings and
saturation of its core (if used). In extreme cases, with relatively high power
operation, the heat generated due to the power dissipated in the device can
cause it to burn out as well as the PL jacks. However, the main reason for not
employing a balun in the case of the G5RV antenna is that, unlike an antenna
tuner which employs a tuned circuit, the balun cannot compensate for the
reactive load condition presented to it by the antenna on most of the HF
bands, whereas a suitable type of antenna tuner can do this most effectively
and efficiently.
Recent experiments by
Louis Varney to determine the importance or otherwise of "unbalance"
effects caused by the direct connection of a coaxial feeder to the
base of the matching section had a rather
surprising result. They proved that, in fact, the HF currents measured at the
junction of the inner conductor or the coaxial cable with one side of the
(balanced) matching section and at the junction of the outer coaxial conductor
(the shield) with the other side of this section are virtually identical on all
bands up to 10m, where a slight but inconsequential difference in these
currents has been observed. There is, therefore, no need to provide an
unbalanced-to-balanced device at this junction when using coaxial feeder.
However, the use of an
unbalanced-to-unbalanced type of antenna tuner between the coaxial output of a
transceiver and the coaxial feeder is essential because of the reactive
condition presented at the station end of this feeder which, on all but the 20m
band, will have a fairly high to high SWR on it. This SWR, however, will result
in insignificant losses on a good-quality coaxial feeder of reasonable length;
say, up to about 21.3m (70 ft). Because it will, inevitably, have standing
waves on it, the actual characteristic impedance of the coaxial cable is
unimportant, so that either 50W or 80W type can be used.
Another very
convenient type of feeder that may be used is 75W Twin-Lead. However, because
of the relatively high loss in this type of feeder at frequencies above about
40m, especially when it has a high SWR on it, it is recommended that not more
than about 15 to 18m (50 to 60 ft) of this type feeder be used between the base
of the matching section and the antenna tuner. Unfortunately the 75W Twin-Lead
in the UK is the receiver type; the much less lossy transmitter type is
available in continental Europe and in the U.S.A.
Impedance of a ladder
line
Assuming that you use round wires, and
air dielectric, the impedance is approximatively :
W = 120 ln (2 . D/d)
with W, the impedance in
ohm
ln, the natural
logarithm
D, the wire spacing,
center to center (from 30 to over 150 mm)
d, the wire diameter (from #14 to #18
AWG)
So, to get W ohms for a given wire
diameter d, apply the next formula :
D = d.e(W/120)/2
D = d.e(W/120)/2
By far the most
efficient feeder is the "open-wire" type, although it is rarely used.
A suitable length of such feeder can be constructed in exactly the same way as
that described for the open-wire matching section. If this form of feeder is employed,
almost any convenient length may be used from the centre of the antenna right
to the antenna tuner (balanced) output terminals. In this case, of course, the
matching section becomes an integral part of the feeder. A particularly
convenient length of open-wire feeder is 25.6m (84 ft), because such a length
permits parallel tuning of the antenna tuner circuit on all bands from 80 to
10m with conveniently located coil taps in the antenna tuner coils for each
band, or, where the alternative form of antenna tuner employing a three-gang
500 pF/section variable coupling capacitor is used the optimum loading
condition can be achieved for each band. However, this is not a rigid feeder
length requirement and almost any length that is mechanically convenient may be
used. Since this type of feeder will always carry a standing wave, its
characteristic impedance is unimportant, and sharp bends, if necessary, may be
used without detriment to its efficiency. It is only when this type of feeder
is correctly terminated by a resistive load equal to its characteristic
impedance that such bends must be avoided.
Coaxial cable HF choke
Under certain
conditions, either due to the inherent "unbalanced-to-balanced"
effect caused by the direct connection of a coaxial feeder to the base of the
(balanced) matching section, or to pick-up of energy radiated by the antenna, a
current may flow on the outside of the coaxial outer conductor. This effect may
be considerably reduced, or eliminated, by winding the coaxial cable feeder
into a coil of 8 to 10 turns about 15 cm (6") in diameter immediately
below the point of connection of the coaxial cable to the base of the matching
section. the turns may be taped together or secured by nylon cord. However this
construction can become the source of problems during thundery weathers.
Water-proof you said ?
It is important, of
course, that the junction of the coaxial cable to the matching section be made
thoroughly water-proof and stays dry by any of the accepted methods; binding
with several layers of plastic insulating tape or self-amalgamating tape and
then applying two or three coats of polyurethane varnish, or totally enclosing
the end of the coaxial cable and the connections to the base of the matching
section in a sealant such as epoxy resin. At WiMo for example the 4:1 balun coming with
their G5RV is designed such a way that it protects the SO-239 connector from
the rain when it is hanged in vertical position, the coaxial being screwed over
2 cm inside the balun. However if you work with high power I warmly suggest you
to place the balun and its connector inside a small home-maded weatherproof box
or, better, to use a small electrical box for oudoor installations so that all
connexions stay dry. Even if it rains or if there is fog only once a month in
your country, moisture is your worst enemy.
Have fun with G5RV !
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