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usually are I then the radiati o n pattern of the
dipole gets into the act. Broadsid e a rrays
made up of d ipol es are usually se t up either
parallel to each other. »r end to end. as in
the " colinear" antenna. Endfire arrays of
d ipoles on th e o the r hand usually have the
an tenna conductors arranged at righ t angles
to the line of radiation, lik e th e Vagi
parasitic array (which has an endfire radiation
pattern).
A parasitic array is essentially a driven
array in which only one of the antennas is
actually drive n, and the rest pick u p their
energy by radiation from that o ne. Most
parasitic arrays a re end fire designs, since it's
sim ple to get the necessary coupling and
phase relationships from parallel dipoles.
T he phase of the energy actually radiated
fro m an antenna depends, in part, upon the
relationship of t h e exciting energy 's frequency
to the frequency at w hich the
antenna is self-reso na nt. The phasing adjustment
which, in the e ndfire driven array, was
made by adj usting feed line lengt h, is made in
a pa ras it ic array hy tuning the parastic (nond
riven ) elements to frequenci es sligh tl y differen
t from that at which the antenna is to
o pe ra te.
If an element is tuned to a frequency
slightl y lowe r than t hat at w hich operation is
desired, the phase o f it s re flec ted o r re-rad i­
ated energy will be such as to cu t down the
radiation pattern In its direction. and build it
up in the opposite direction. Such an element
is called a "reflector".
If a n cle me nt is tuned to a freq ue ncy
slightly higher than that at which the bea m
is to operate, it will build u p the radiation
pattern in it s direction and cut it down in
the opposite direction. Such an element,
since it directs the pattern in its own
d irection, is ca lle d a " director".
The spacing between the driven ele ment
and the parasitic e le me nts is j ust as critical,
in a parasitic a rray. as is the tuning of the
parasitic elements, since the a rray's performance
is determined, by relative phase
over the entire st ructu re. Distance d etermines
phasing also. For any specific tuning
of a parasitic element, there is a critical
distance as well.
Before the principles of th e parasitic
array were understood as well as they now
an.', th is led to many co nflic ti ng rules for
design of parasiti c beams and their tuning.
About 10 years ago, however, it was discovered
that the critical factor actually is the
combination of tuning and spaci ng. It is no w
NEW 51.51·5
MULTIBAND DIPOLE ANTENNA
For Amateurs
- SWL.
UTILITY
• REPLACES BALUNS
REPLACES ALL WIRE
EXCEPT COAX
OPERATES 10
THROUGH 80
METERS
IMPROVED RECEP·
TION FOR SWls
SPECIFICATIONS
MODEl 6BA - 68B
AVERAGE SWR 1.7 to
I or better.
OVERALL LENGTH
102 feet
INCLUDES 30 feet
leadin
10.15.20-40-80 Met ers WEI GHT 2-% lbs,
Mod e l 68A-I OOO W atts-2000 P.E.P 34.50
Model 68B-500 Wath·IOOO P.E.P 26.50
Model B6D-For Rec eption Only 17.95
Models 68A a nd 688 o p erat e 10 through 80 meters with
a typical dipole radiation pattern within the frequency
ra nge. A sealed ce nter unit p rovides connection to 7-~2
cop per a ntenna wire and 30 feet of heavy duty t':"'ln
lead. Twi n lead is equip ped wit h a sealed coax ~tt! ng
for connection to a random length of coax transmiss ion
line. Ma y b e used as a flat ~Hpol.e or "inv ert~ ~ V" . Not
effected b y wide c ha ng es In cl imatic cond itions.
Model 860 is for recepti on only and covers a ll shortwave
and b roadca st bands. C o nsists of 100 feet antenna
wire, 30 feet twin lead plus 25 feet of coax for
di rect con nection to recei ver.
MURCH ELECTRONICS
See-
FRANKLIN, MAINE 04634
your dlstribufor or order dired.
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PETERBOROUGH, N.H. 03458
MAY 1969
113