DID TEURS

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RL Gruner W4UFT
3179 Boone 51.
Memphi5 TN 38127
Perfect Parabolas
- use some solar technology on your
next dish antenna
Ibecame interested in
building an MD S TV converter
after read ing an article
in the August. 1979,
issue of 73 Magazine entitled
"You Can Watch Those
Sec re t TV Cha nnels,"
After building the converte
r and coffee-can feedhorn
, I was concerned over
the number of trees between
m y location and that
of the MDS tr ansmitting
station. A tine-of-sigh t path
is normally required at the
frequencies used for MDS
TV and trees grea tly attenuate
sig na ls. I sus pected that
Pho to A. Front view of completed fo ur-foot dish showing
rib structure.
90 73 Magazine . April,1981
I would need an antenna
wit h more gai n than tha t of
the " snow -saucer" type.
Whenever U HF or microw
ave frequencies are rnenti
oned, I form a mental image
of a parabo lic d ish .
However, they are expensive
and I kn ew nothing
about design ing o r building
them. $0 w hat do I do now ?
Research
After a long and fruitless
search through my limited
libra ry of radio publications,
I was ready to give up
when I spotted my Solar
Energy Handbook . It contains
info rm ation on collecting
light waves with parabolic
reflectors . Having
read that t he properties of
high frequency rad io w aves
and light w aves are sim ilar,
I decided to design and
build a parabolic reflector
based upon the information
in the book. This inform a­
tion is shown in Figs. l (a),
l (bl, and l (c) and il lustrates
three approaches that can
be used to produce a parabolic
curve.
Design m ater ial s needed
are:
• Several sheets of linear
graph paper
• Straight edge or ru ler
• Com pass for drawing
circles
• Pen cil
• Piece of wood - 1" X 6"
X 24"
• Approximately 5' of string
. 4' X 4' piece of plywood
or heavy cardboard
Dimensions
Due to my limited knowledge,
my choice of dimensions
was based upon comm
o n sense and convenience.
Choosing a d iamete
r of four feet for high
gain and a focal length of
26 inches so t he feedhorn
mounting t ube cou ld be
sho rt and rigid, I fol lowed
the in structions show n in
Fig. l (a). I let each square
on the graph paper represent
one inch (not shown for
purposes of clarity).