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MFJ MFJ-249 - Page 10

MFJ MFJ-249
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MFJ-249
SWR
Analyzer
Instruction
Manual
Measuring
the
Feedpoint
Resistance
of
Antennas
The
feedpoint
resistance
of
a
low
impedance
(10-100
ohm)
resonant
HF
antenna
or
load
can
be
measured
with
the
MFJ-249.
A
low
value
non-inductive
potentiometer
(250
ohms)
and
a
conventional
ohm
meter
can
be
used
to
make
these
measurements.
1
Connect
the
MFJ-249
directly
across
the
terminals
of
the
unknown
impedance.
If
the
load
is
unbalanced
be
sure
that
the
ground
is
connected
to
the
SO-239
"ANTENNA"
connectors
ground.
Adjust
the
MFJ-249
until
the
SWR
reads
the
lowest
value.
7
3
Ifthe
SWR
is
not
unity
(1:1)
place
the
potentiometer
in
par
.
el
with
the
load.
Adjust
the
potentiometer
until
the
SWR
is
as
good
as
possible.
If
the
SWR
only
becomes
worse
go
to
step
7.
.
°
5S
Ifthe
SWR
reached
unity
remove
the
potentiometer
and
measure
it's
resistance.
Nr
PSS
6
The
resistance
of
the
load
is
found
by
using
the
formula
beiow.
R,
UK
R,,
-50
Re
eAnteana
resistance
R,
=
Potentiometer
resistance
7
If
the
earlier
steps
did
not
work
put
the
potentiometer
in
series
with
the
center.
pin
(ungrounded
terminal)
of
the
SO-239
"ANTENNA"
connector.
8
Adjust
the
value
of
the
potentiometer
until
the
SWR
is
unity
(1:1).
9
Remove
the
potentiometer
and
measure
the
resistance
of
the
setting
used
in
step
8.
Subtract
this
value
from
50
to
determine
the
load
resistance.
TESTING
AND
TUNING
STUBS
AND
TRANSMISSION
LINES
The
proper
length
of
quarter
and
half
wave
stubs
or
transmission
lines
can
be
found
with
this
unit
and
a
S50Q
carbon
resistor.
Accurate
measurements
can
be
made
with
any
type
of
coaxial
or
two
wire
line.
The
line
does
not
have
to
be
50
ohms.
The
stub
to
be
tested
should
be
attached
with
a
500
noninductive
resistor
in
series
to
the
center
conductor
of
the
"ANTENNA"
connector
with
a
coaxial
line.
The
shield
should
be
grounded
to
the
connector
shell.
For
two
wire
lines
the
5JQ
resistor
connects
in
series
between
the
ground
shell
of
the
PL-259
and
one
conductor.
The
other
conductor
of
the
balanced
line
connects
directly
to
the
center
pin
of
the
connector.
Coaxial
lines
can
lay
in
a
pile
or
coil
on
the
floor,
two
wire
lines
must
be
suspended
in
a
straight
line
a
few
feet
away
from
metallic
objects
or
ground.
The
lines
must
be
open
circuited
at
the
far
end
for
odd
multiples
of
1/4
wave
stubs
(.e.
1/4,
3/4,
1-1/4,
etc.)
and
short
circuited
for
half
wave
stub
multiples
(like
1,
1-1/2,
etc.)

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