Amateur Radio Report Card

------------------------------------ - - - -
Jim
Ke nnedy K6M IO
281 6 E. Norwich
Fresno 26, Calif.
The
Big
Sa iI
Photos by Joe De Young WA6CQL
The Ultimate for 70 Om.
The 432 Me band has over the years, offered
a fertile proving ground for antenna designs.
However, building antennas for 432 involves
some considerations and problems that aTC not
evident at lower frequencies.
The close tolerances required by parasitic
arrays such as the yagi become difficult to
h andle. Designs that work well at 144 Me
give very disappointing performance when
scaled down to 432.
In many cases, the failure of such yagi
layouts is the fact that the antennas are built
"according to the book" and not on a test
stand where the various parameters can be
varied to compensate for factors the "book"
failed to take into account.
However, even with yagis which have apparently
been tuned properly, there are stories
in circulation about antennas which showed
good patterns but poor gain, though why this
would be 1 don't know.
In any case. the yagi, even if working
p roperly, is a delicate beast and suffers from
rather severe bandwidth limitations.
T rue, if operation on 432.000 ± 3 Me is
all that is desired, then the 6 to 7 Me bandwidth
of the yagi would be acceptable-assuming
you could get your antenna to peak
on the right six megacycles. If TV or some
other broadband operation is desired, even a
working yagi leaves something to be desired.
Various types of colinear- broadside arrays
can be made to perform quite well. The extended,
expanded H arrays can and do deliver
about 15 db of gain while 32 element
INCOMING
WAVE _
FOCAL lINE -
REF L ECTOR
Fig. 1. Focus action of the cylindrical parabola.
50 13 MACAZINE