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I MODEL 4OO TRANSCEIVER
B.
Installation (Cont)
3 .
4 .
The SWR bridge
is a useful in6tru-
ment,
but unfortunately
it is quite
often mis unde rstood and overrated
in importance.
Basically,
the SWR
bridge wil.l indicate how closely the
antenna load imPedance matches
the
transmis sion line,
With long
t r a n s r n i s s i o n l i n e s , s u c h a s w i l l b e
used in many fixed station installa_
t i o n s , : t r s d e s i r a b l e ! o k e e P t l e
impedance match fairly
close i.n
o r d e r 1 0 l i m i t
p o w e r l o s s .
T h i s i s
particularly
true at the higher fre-
quencies.
The longer the llne,
and
the higher the frequeDcy,
the rnore
important
SWR becornes,
However,
i n m o b i l e I n s t a l l a t i o n s t h e t r a n s -
r n i s s i o n l i n e s e l d o m e x c e e d s 2 0 f e e t
in length,
and an SWR oI even 4 to I
a d d s v e r y l i t t l e t o P o w e r l o s s '
T h e
only time SWR will indicate a low
figure
is when the antenna presents
a toad close to 50 ohrns, bot nany
mobile
antennas will have a base
impedancc
as low as l5 or 20 ohrns
at their
!esonant frequencY.
In
e u c h a c a s e , S W R w i U i n d i c a t e 3
or 4 to I, and yet the system will
be radiating
efticiently'
The really
important
tactor
in your
mobile antenna i5 that it shoold be
carefully
tuned to resonance at the
d e s i r e d f r e q u e n c y .
T h e J a l l a c Y l n
using an SWR b!idge
lie6 in the {act
that it is sometimes
Pos sible to
r e d u c e t h e S W R r e a d i n 8 b Y d e t u n i n g
the antenna.
Field
strength may
actually be reduced in an effort to
bring SWR down,
Since tield
strength is the Prirnary
Soal' we
recornmend
a FieId Strength Meter
Ior antenna tuning.
I'or
antenna aajustments,
the Swan
400 rnay be loaded lightlY to about
lO0 !na. cathode current
in6tead of
the us\ral 500 rna.
This wiU limit
tube dis sipation duling
adjrstrnents,
a n d w i l t a l s o h e l p r e d u c e i n t e r l e r -
ence on the frequency.
In any case,
d o n o t l e a v e t h e t r a n s r n i t t e r o n f o r
very long at one time.
Turn it on
just long enough to tune and load,
and get a Jield strength
reading.
Start o\rt with the antenna whiP at
about the center of its adjustment
range,
Set the VFO to the desired
operating
Irequency
and then adjust
P . A , T U N c J o r d i P , a n d P . A '
LOAD for I00 ma,
Then observe
the tield strength
reading.
The
Field Strength Meter
rnay be set
on top oI the dash, on the hood, or
at an elevated location
sorne distance
from the car.
Change the whip length a half inch,
o r s o , a t a t i m e ,
r e t u n e t h € P . A .
for I00 rna. loading each time,
and
check field strength.
Continue this
p r o L e d u r c u n t i l t h e p o i n t o f m a x i r n u m
Iield strength is found.
This adjust-
ment will be Inost critical
on 75
r n e t e r s , s o m e w h a t l e 6 6 c r i t i c a l
o n
4 0 , e t c . ,
u n t i l o n l 0 r n e t e r s t h e
adjustment
will
be quite broad.
AJter tuning the antenna to resonance'
load the P. A. to IuIl Power'
5 .
t 0
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