Amateur Radio Report Card

'7:1
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Side view of collinear feed.
units have been measured at over 17 db.
Though they are much easier to make work
than the yagi, the large colinears have one
drawback in common with the yagi and any
other multi-element array-the more elements,
the higher the Q and the narrower the bandwidth.
Unlike the yagi, you can reliably increase
the gain of the colinear by adding
elements. However, the bandwidth will be
reduced, though it is difficult to say to what
precise figure.
When it was decided to build some sort
of "ultimate" antenna for 432, several basic
requirements were established. It was concluded
that the antenna should have a minimum
honest gain of 18 db referenced to a
dipole, the widest bandwidth possible up to
the limit of the band edges; and electrical
and mechanical simplicity that would guarantee
ease of tune up and reliability of operation.
The yagis were immediatly discounted on
the basis of a number of previously unsatisfactory
experiments.
The collinear broadside was given considerable
thought but the idea was set aside on
the basis of the feed harness nightmare involved
in feeding what would have to be
a minimum of 64 elements.
The high gain, broad bandwidth requirement
suggested a parabolic dish. Certainly
nothing could be much simpler to feed
than a single dipole and the reliability of the
antenna should be considerably higher than
any of the other types considered.
However, the conventional dish (paraboloid
of revolution) is mechanically difficult to
construct. The curve in two planes makes it
difficult to get a smooth surface on the dish
without considerable trouble.
Another, less familiar, form of the paraboloid
reflector-the parabolic sheet or cylindrical
parabola-was then studied. This is merely
a sheet of reflecting material bent into a
parabolic shape in one plane only. Meehancally,
such a device is very easy to construct
with simple tools and a minimum of expense.
As has probably occurred to many readers,
the sheet parabola has a distinct electrical
difference from the dish parabola, that is,
its focus is not a point, but, rather, a line,
as diagramed in Fig. 1. In order to extract
the maximum amount of energy incident on
the reflector, it is necessary to place an energy
extractor (antenna) all the way along this
focus line. Some sort of simple colinear or
broadside configuration would seem ideal depending
on whatever other considerations
might arise.
The original gain requirement put forth was
18 db. A quick investigation of some antenna
charts shows that a 9 foot circular dish would
provide just about 18 db computed by radar
formula. The area of a 9 foot circular dish
is 63.6 square feet. A square parabolic sheet
8 feet on a side has an area of 64 square feet
and should, if properly fed, produce about
the same gain. The approach seemed to hold
considerable promise and it was decided to
build such an antenna and give it a try.
One of the first problems was the choosing
of the focal length of the dish. In order
to minimize electrical coupling between the
reflector and the driven element, it is desirable
to use a focal length of several wave
lengths with all parabolic reflectors. Though
a little short, a focal length of 4 feet-2
wave lengths- was settled upon. In order
to operate properly at 432 Me it is necessary
that the "peak to peak" deviation of the
dish surface from the true paraboloid shape not
exceed 1/8 wave length or about 11> inches.
This is necessary, if the performance is not
to be degraded by phase cancellation. This
tolerence is very easy to maintain. In fact,
the tolerence achieved on the sheet was better
than Jf inch by a fair amount and no
extrordinary care was necessary. The reflector
itself would be quite satisfactory for 1296 Me