Swann 400 - Using SWR Bridge or Field Strength Meter

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
transrnission
lines,
such
as
will be
used
in many
fixed
station
installa_
tions,
:t
rs desirable
!o
keeP
tle
impedance
match
fairly
close
i.n
order
10 limit
power loss.
This
is
particularly
true at
the higher
fre-
quencies.
The
longer
the llne,
and
the
higher
the frequeDcy,
the rnore
important
SWR
becornes,
However,
in mobile
Installations
the trans-
rnission
line seldom
exceeds
20
feet
in length,
and an SWR
oI even 4
to I
adds
very
little
to
Power
loss'
The
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
euch
a
case, SWR
wiU
indicate
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
desired
frequency.
The JallacY
ln
using
an SWR
b!idge
lie6
in the
{act
that
it is sometimes
Pos
sible
to
reduce
the SWR
readin8
bY detuning
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,
and
wilt also
help
reduce
interler-
ence
on the frequency.
In
any case,
do not
leave
the transrnitter
on
for
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, TUNc
Jor
diP, and
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,
or so, at a
time,
retune th€
P.A.
for
I00 rna.
loading
each time, and
check
field strength.
Continue
this
proL edurc
until the
point of maxirnum
Iield
strength
is found.
This
adjust-
ment will
be Inost
critical
on
75
rneters,
somewhat
le66
critical
on
40,
etc., until
on
l0 rneters
the
adjustment
will
be
quite broad.
AJter
tuning
the antenna
to
resonance'
load
the P. A.
to IuIl
Power'
5.
t0