Amateur Radio Report Card

If
t
TYPE
uN" CONNECTOR
'n'· "'.n,, ,'"'' w","
~ 11 8 SOFT COPPER WI RE
----- , · "'''~-----­
·1
------ 6 - 5 / 8"~------_I
Fig. 4 . Balun dimension s.
long and 14 inches high should be attached
to the dipole side of the mast.
The mesh can be held in place with wire
and then spot soldered to the mast with a
heavy iron or propane torch.
The dipoles should then be soldered in
place.
It will be noted from Fig. 3 that the J<
wave phasings stubs are actually longer than
the length required and that shorting bars are
used to set the final adjustment. In all probability,
this array will perform properly if
the shorts are set to the measurements given,
however, if the time can be spared, a simple
diode test stand step up with a signal source
of some sort will allow the array to be
tweaked just to be sure.
If the test stand method is used, it should
be found that, when the shorts are tweaked
for maximum gain, the horizontal beam width
of th e antenna is at a minimum which should
be about 16 degrees.
As with many antennas, the beam direction
can be changed by altering the phasing
balance among the dipoles. Be sure to set
all the shorts the same distance. It is possible
to achieve the proper beam width and then
squirt the beam out sideways, if the shorts
are not pretty close to the same length.
To illustrate the necessity of proper phasing,
Fig. 5 shows the E plane pattern of the
first trial collinear which was built "by the
book." The screen reflector was not used in
this test. The unidirectionalness of the pattern
was apparently due to the reflecting action
of the length of tubing used to support the
dipole string.
In contrast, Fig. 6 shows the pattern obtained
in the E plane with the second trial
collinear, which was also without screen re­
Bector. In this case, the phasing stubs were
made longer and the movable shorting stubs
added and adjusted for maximum gain.
It can be seen that the second antenna has
a much cleaner pattern, lower sidelobes and
narrower beamwidth.
Though not shown, the H plane beamwidth
of this antenna is much too broad to satisfy
the feed requirements of the parabolic sheet.
Fig. 7 shows E plane plot the final array.
This was built like the second trial except
that the reflector screen was added and the
stubs retuned as required.
The sidelobes are further reduced and the
back lobe is almost completely suppressed.
The H plane beamwidth is reduced to about
100 degrees which is a quite satisfactory
value for the feeding sheet. In fact, the array
is, in itself, a fairly decent antenna, which,
if used without the parabolic reflector, will
have about 10 db gain.
The collinear is mounted at the focus of
the sheet by using four additional lengths of
TV mast. Two 5 foot lengths and two 8
foot 4 inch lengths are used to form the
collinear support which is held together and
to the sheet by antenna U clamps.
Reference to the photographs will provide
the details of this construction. The U clamp
which ties the mast behind the sheet to the
center tubing in the framework is particularly
important if the collinear support is to remain
rigid.
The collinear is then clamped at the focus
of the sheet.
T wo lengths of nylon rope were tied between
the edges of the framework and th e
ends of the collinear to insure against the
collinear twisting on its mount. If this were
to happen the pattern would be upset and,
if carried to an extreme, the effectiveness of
the antenna would be destroyed altogether.
To this point, nothing has been said about
the matching scheme used. It will be noted
that the middle dipole in the array is split
at the center and a shorted stub has been
inserted.
The center impedence of the array is rather
low and requires a balanced feed. The simplest
matching arrangement seemed to be the "beer
can balun," in this case, a soft drink can
balun" and shorted stub match.
The balun is used to transform the 50
ohm unbalanced transmission line to a 50
ohms balanced shorted line. The 50 ohms balanced
line is tapped onto the shorted stub.
The tap point and the short are positioned
for minimum VSWR.
The details of the balun are shown in
Fig. 4. These details vary slightly from the
balun shown in the photographs, however,
they are essentially the same. It is also well
to note that the short 50 ohm balanced line
is not actually 50 ohms but somewhat higher,
56 73 MAGAZINE