POTOMAC INSTRUMENTS INC. FIM-21 User manual
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WARRANTY
FOR
EQUIPMENT
AND
ACCESSORIES
Potomac
Instruments
inc,
warrants
each
new
equipment
to
be
free
of
defects
in
material
and worlananship,
for
a
period
of
one
(l
)
year
after
date
of
original
ship
m
ent,
except
for
tubes,
lamps,
fuses,
transistors,
and
diodes
which
will
be
wa:rranted
for
a
period
of
ninety
(90)
days.
Any
instrument
which
is
fom1d
with-
in
one
year
not
to
meet
the
foregoing
standards
after
examination
by
our
factory,
will
be
repaired
or
at
the
option
of
Potomac
Instruments
replaced
without
charge.
This
warranty
does
not
apply
to
equipment which
has
been
altered,
improperly
handled,
or
damaged
in
any
way.
CHANGES
TIJ
SPECJJi'ICATIONS
The
right
is
reserved
to
change
the
published
specifications
of
equipment
at
any
time,
and
to
furnish
merchandise
in
accordance
with
current
specifications,
without
incurring
any
liability
to
modify equipment
previously
sold.
RETURNING
EQUIPMENT
TO
THE
FACTO
RY
Factory
service
is
available
for
repair
and
adjustment
of
this
instrument;
however, a
description
of
the
trouble,
plus
the
model and
serial
number(s)
of
the
Wlit
should
be
forwarded
first,
whereupon
instructions
for
correcting
the
trouble
or
authoriza
ti
on
for
returning
the
equip
m
ent
will
be
given.
This
applies
to
all
Wlits,
whether
under
warranty
or
not
. Shipment
in
an
improper
containe
r
may
resul
t
in
damaged equipment
thereby
voiding
warranty
,
ii
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TABLE
CONTENTS
Paragraph
Page
Ll
1.2
1.3
2.1
2.2
2.3
2.4
2.5
2.6
3.1
3.2
3,3
4.1
4.2
4,3
4,4
4.
5
4,6
4.7
4,8
4,9
4.10
SECTION
1.
GENERAL
DESCRIPTION
Scope
of
Manual
••••••
Purpose
of
Equipment
•••
:Equipment Spec
ifications
. . . ... .. .. . . .
SECT
ION
2,
OPERATION
1-1
1-1
1-1
General
. . . . . . . . . . . . . . . . .
2-1
Operating
Controls
and
Indicators.
• • • • • . • • •
2-1
FIM
Calibration
. • . . • • • • • • • • • • • •
2-2
M
easuring
Field
Strength
• • • . • • . • • • • • • • 2-4
Measur
ing
Harm
onic
Ratio
in
Dec
ibels
• • • • • • • • 2-5
R. F. Bridge Measurem
ents.
(Tuned
Voltmeter)
• • • • 2-6
SECTION
3•
THEORY
OF
OPERATION
General
. . . . . . . . . . . . . . .
Block Diagram
Descript
ion.
Circuit
Description
•••••••••
SECTION
4.
MAJNTENANCE
General
. . . . . . . .. . . .
Batt
ery
Replacement . . . . .
Battery
Types . . . . . . .. .
LaJJIP
R
eplac
ement .
To
Rem
ove
FIM
fro
m Case .
. . . .
To
Operate
the
FIM
With Case
Rem
oved
IC
Re
place
ment . . .
.
Preventive
Maintenance .. .
Troubleshooting
and
Re
pair
. . . . .
Recalibration
. . . . . . . . .
SECTION
5.
SCHEMATIC
DIAGRAM
General
...••
iii
... ..
. . . .
...
. . .
. . . . .
.
. . . . .
. . .
....
. .
. .
. .
. .
. .
. .
.
.
.
.
.
•
3-1
3-1
3-3
4-1
4-1
4
-1
4-1
4-2
4-3
4-3
4-3
4-5
4-S
5.1
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LIST
OF
ILLUSTRATIONS
Figure
2-1
Operating
Controls
and
Indicators.
• • • • • • .
2-3
3-1
Field
Strength
Meter,
Functional
Block Diagram
3-2
4-1
4-2
4-3
5-1
Table
1-1
1-2
2-1
Troubleshooting
Voltage
Charts
••.•••
Field
Strength
Meter, Component
Locations.
Field
Strength
Meter,
Component
Locations.
Field
Strength
Meter, Schematic Diagram
••
LIST
OF
TABLES
. . .
Specifications:
Specifications:
Type
FIM-21
Field
Strength
Meter
Type
FIM
-41
Field
Strength
Meter
Operating
Controls
and
Indicators
..••••
• •
iv
4-1+
4-6
4-7
5-1
1-1
1-.3
2-1
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SECTION
l .
GENER.AL
DESCRIPTION
1.1
Scope
of
Manual
This
manual
provides
a comprehe
ns
ive
technical
description
of
the
operating
and
physical
characteristics
of
the
FIM
-2
1
and
FIM-41
Field
Strength
M
eters.
It
provides
sufficient
information
conc
er
ning
the
operation
and
func
-
t
ional
theory
of
operation
to
permit
effective
operation
of
these
instruments.
1.2
Purpose
of
Equipment
Field
Strength
Meters,
FIM
-
2l
and
FIM-41
are
light
-
weight
portable
ins
truments
for
the
me
asurements
of
a
wide
range
of
radio
signal
intensities
in
the
.54
to
5.0
MHz
fr
eq
uency
spectrwn.
The
FIM
-
21
covers
the
broadcast
band
(540
to
1600
KHz)
while
the
FIM-41
covers
the
broadcast
band and harmonic
fre-
quencies
to
5
.O
MHz.
Both
units
are
rugged
test
instrumen
ts
that
are
equally
effective
for
interference
studies
at
low
signal
strength
and
for
close-in
meas-
ur
ements on
high-power
directional
arrays.
In
addition
, a
special
input
jack
permits
the
receiver
to
be
used
as
a
null
detector
for
RF
Bridge
measurements.
l.J
Equipment
Specifications
(See
specifications,
FIM-21,
Table
1-1,
FIM-41,
Table
1-2)
Table
l-l.
S
pec
ifications:
Type FIM-21
Field
Strength
Me
ter
Frequency Range:
Fi
eld
Intensity
Range:
Accuracy
of
Calibration:
Accuracy
of
Range
Attenuator:
Se
l
ectivity
B
andwidth
(6
db):
IF
R
ejection:
Image
Rejection:
Adjacent
Channel
Re
jection:
Panel
Meter:
Antenna:
535
KHz
to
1605
KHz
lO
microvolts
per
meter
to
10
volts
per
meter
1
percent,
referenced
to
NBS
Standard
Field
2
percent
over
entire
FI
range
and
tuning
band
7
KHz
nominal
with
m
ulti-pole
hybrid
fi
l
ter
.
1-
1
75
db minimum
80
db minimum
50
db minimum 10
KHz
above
and
below
tuned
station
4n
mirrored
scale,
logarithmic
graduations
l
to
10,
taut
band
meter
movement,
2%
linearity
Shielded
loop,
integral
part
of
hinged
cover
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SECTION
l .
GENERAL
DESCRIPTION
(Cont.)
Table
1
-1.
S
pe
ci
f
ications:
Typ
e
FIM
-21
Field
Strength
Meter (Cont. )
Audio
Outputs
:
Recorder
Output:
Lights:
Batteries
(6
re
q
uir
e
d):
Battery
Life:
Environmental:
Dimensions:
Weight:
External
RF
In
p
ut:
RF
Input
Switch:
METER
Switch:
Logarithmic
Dynamic Range:
1- 2
Front
panel
loudspeaker,
weather
treated
cone
Headphone j
ack,
high
and
low
Z
(disconnects
speaker)
0.4
to
4 v
olts
DC
proportional
to
field
i
ntensity
for
each
attenuator
range,
2000
ohm
source
resistance
Frequency
dial
and m
et
e
r;
panel
switch
for
night
use
Standard
l½
volt
C-~ unrr
cells
or
l½
volt
.Alkaline
cells
for
extended
life
at
very
low
temperat
ur
es
Greater
than
1000
FI
readings
(re
-
duced
life
with
frequent
use
of
l
ights
and/or
high
speaker
volume)
Continuous e:xposure -
10°
F
to
+130°
F;
lower
temperature
ope
ration
practical
for
11
reading
time
11
expos
ur
es
8-3/4
11
high,
11-1/2
11
wide,
5-
1/8
11
deep
with
cover
closed;
3-7/8
11
deep
with
cover
open
(gr
i
pping
width)
Approximately
ll-1/2
pounds
535
KHz
to
1605
KHz,
lO
microvolts
to
10
vol
ts
Selects
loop
antenna
(
ANT)
or
Panel
Connector
(EXT)
Selects
normal
linear
(
LJN)
or
l
oga-
rithmic
(LOG)
operation
60 db compressed
input
range
for
FI
meter
and
recorder
output
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SECTION
1.
GENERAL
DESCRIPTION
(Cont.)
Table
l -
2.
Specifications:
Type FlN-41
Field
Str
ength
Meter
Frequency Range:
Field
Strength
Rang
e:
Accuracy
of
Calibration
:
Accuracy
of
Range
Attenuator
:
Selectivity
Bandwidth (6
db);
IF
Rejection
:
Image
Rejection:
Adjacent Channel
Rejection:
Harmonic Measurement:
Panel
M
eter:
Antenna:
Audio
Outputs:
Recorder
Output:
Lights
:
Batteries
:
1-3
.54
MHz
-
5.0
MHz
in
two bands
Band
11
A
11
•
535 -
1.61
MHz
Band
11
B
11
1.58
-
5.0
MHz
10
microvolts
pe
r
meter
to
10
volts
per
meter
1
percent,
referenced
to
NBS
Standard
Field
2
percent
over
entire
FI
range
and
t
uning
band
7
KHz
nominal
with
multi-pole
hybrid
filter
75 db
mini.nn.un
80
db
(min.)
at
540
KHz
decreasing
to
50 db
(min.)
at
4.8
MHz
50
db
minimum
10
KHz
above and below
tuned
station
Capable
of
measuring
harmonics
in
excess
of
80 db below fundamental
4
11
mirrored
scale,
logarithmic
grad-
uations
1
to
10
and
linear
db
.
scale;
taut
band meter movement,
2%
linearity
Shielded
loop,
integral
part
of
hinged
cover
Front
panel
loud
speaker,
weather
treated
cone -headphone
jack,
high
and
low
Z
(disconnects
speaker
)
0.4
to
4
volts
DC
proportional
to
field
st
rength
for
each
attenuator
range,
2000
ohm
source
resistance
Frequency
dial
and m
eter
with
panel
switch
for
night
use
Standard
1-1/2
volt
C-
Zn
11
D
11
cells
or
l-1/2
volt.
Alkal...ine
cells
for
extended
life
at
very
low
tempera-
tures
(
six
req..uired)
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SECTION
1.
GENER.AL
DESCRIPTION
(Cont.)
Table
1-2.
Specifications:
Type
FIM-41-
Field
Strength
Meter (Con
t.)
Battery
Ll.fe:
Environmental:
Dimensio
ns:
Weight:
External
RF
Input:
RF
Input
Switch:
METER
Switch:
Logarithmic Dynamic Range:
Accessories
Available:
1-4
Greater
than
1000
readings
(reduced
life
with
frequent
use
of
lights
and/or
hig
h
speaker
volume)
Continuous e:xposure
-1
0°
to
+130°
F;
lower
temperature
operation
practical
for
11
reading
time
11
e:xposures
8-3/4
11
high,
11-1/2
11
wide,
5-1/8
11
deep
with
cover
closed;
3-7/8
11
deep
with
cover
open
(gripping
width)
Approximately
12
pounds
540
KHz
to
5.0
MHz,
10
microvolts
to
10
volts
Selects
loop
antenna
(ANT)
or
Panel
Connector
(EXT)
Selects
normal
linear
(LThT)
or
loga~
rithmic
(LOG)
operation
60 db compressed
input
range
for
FI
met
er
and
recorder
output
Carrying/Shipping
dase
Unipod
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SECTION
2.
OPERATION
2.1
General
The
FIM
-21 and FIM-4l
operating
controls
and
indicators
are
de
-
scribed
in
this
section.
Also
provided
are
the
basic
operating
instructions
which
include
battery
voltage
testing,
meter
calibration,
meas
uring
field
strength,
and
use
of
these
instruments
as
null
detectors
fo
r
RF
Bridge
meas-
urements,
2.2
Operating
Controls
and
Indicators
All
operating
controls
and
indicators
are
located
on
th
e
front
panel
of
the
FJN-21 and FJN-41
as
shown
in
figure
2-1.
Table
2-1
lists
these
controls
and
indicators
and
the
associated
functions.
Table
2-l.
Op
erating
Controls
and
Indicators
Control
or
In
dicator
Function
FUNCTION
selector
switch
(S2)
FULL
SCALE
selector
switch
(Sl)
AUDIO
control
and
switch
(R3)
Selects
the
function
to
be
displayed
on
the
front
panel
meter
and
connects
battery.
OFF
-
battery
disconnected
from
circuit,
and
direct
connection
mad
e
across
meter
terminals
to
protect
meter.
BATT
-
battery
output
voltage
FI
CAL
TUNE
-
field
strength
of
antenna
input
de
veloped
from
dete
c
ted
output
of
receiver
IF
amplifier
.
CAL
NuJ.l -compares
detector
outputs
from
receiver
and
calibrating
oscillator.
S
elects
the
amount
of
attenuation
in
RF
and
IF
stages,
in
10
to
1 (20 db)
ratio
steps.
The
panel
is
marked
with
the
full-scale
direct-reading
field
strength
value
for
each
position.
CAL
-
applies
power
to
calibrating
oscil-
lator,
and
selects
1
volt
range
of
attenu-
ator.
Adjusts
level
of
audio
in
speaker
and
head
-
phones.
Switch
disables
audio
amplifier
in
OUT
position.
2-1
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SECTION
2.
OPERATION
(Cont.)
Table
2
-1.
Operating
Controls
and
;Indicators
(Cont.)
Control
or
Indicator
Function
GAJN
control
(R2)
Adjusts
gain
of
receiver
by
regulating
gain
of
IT
amplifier.
CAL
OSC
tuning
control
(Rl)
RCVR
tuning
control
(Cl)
FIELD
-
STRENGTH
meter
(Ml)
LIGHT
switch
(S3)
RF INPUT
switch
MHz
switch
(FIM-41
only)
(S7)
METER
switch
(s5)
Fine
tunes
calibrating
oscillator.
(Oscillator
frequency
coarse
tuning
is
pre-set
by
receiver
tuning
control
.)
Tunes
rece
i
ver
and
calibrati
ng
oscillator
by
rotating
capacitors
Cl
and C2. Fre-,-
quency
of
receiver
signal
is
read
on
dial.
Indicates
field
strength
in
volts
per
meter,
in
conjunction
with
full
scale
switch.
DJ.uminates
m
eter
and
dial.
Switches
receiver
input
to
loop
antenna
(ANT)
or
external
BNC
input
jack
(EXT).
Selects
frequency
band:
A.
~54
to
1.6
MHz
B.
1.6
to
5.0
MHz
Provides
either
logarithmic
(LOG)
or
linear
(LIN)
relation
between
meter
reading
and
re-
ceiver
input,
the
former
for
use
when
the
meter
is
to
operate
in
conjunction
with
re
-
cording
equipment
or
when
relative
measure-
ments
of
greater
than
one
decade
are
desired.
Replacement
of
batteries
is
described
in
the
maintenance
section
of
this
manual.
2.3
FIM
Calibration
The
FIM
should
always
be
calibrated
at
the
frequency
of
the
signal
to
be
measured.
This
eliminates
any
error
due
to
frequency
sensitive
components
in
the
circuits.
Calibrate
the
meter
as
follows:
1.
Place
or
hold
the
field
strength
meter
in
a
vertical
position,
with
the
top
surface
in
a
horizontal
plane.
Open
the
cover
and
swing
it
to
a
vertical
position.
2-2
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SECTION
2.
OPERATION
(Cont.)
2.3
FIM
Calibration
(Cont.)
2..
Rotate
the
FUNCTION
switch
to
the
BATT
position
and
see
that
the
rea
d
ing
is
above
5.0.
3.
Rotate
the
FUNCTION
switch
to
the
FI
position.
4.
Switch
LOG-LIN
switch
to
LIN
position.
5.
Set
the
FULL
SCALE
switch
to
the
range
which
covers
the
ex-
pected
value
of
field
strength.
6.
Turn
on
the
AUDIO
control
and
tune
the
RECEIVER.
d
ial
to
the
signal
to
be
measured.
Use
the
meter
indication
to
obtain
peak
tuningJ
adjusting
the
GAIN
control
and
FUIL
SCALE
switch,
and
rotating
the
instrument
to
obtain
a
reading
within
the
range
of
the
meter
. Use
the
audio
signal
to
ident
ify
the
sta-
tion.
The
RCVR
Tuning
Control
has
vernier
drive
for
3/4th
of
one
turn;
it
changes
to
direct
drive
and becomes
harder
to
turn
beyond
the
vernier
range,
Adjust
it
so
that
the
desired
station
i'a.lls
in
the
vernier
range.
7.
Ro
t
ate
the
instrument
to
obtain
reading
below
10
mV.
In
strong
fields
the
m
eter
reading
may
be
reduced
below 10
mV
by
holdi
ng
the
FIM
horizontally
or
by
changing
RCVR
Tuning away from
the
desired
signal
by
no more
than
10
KHz.
(This
is
permissible
because
the
receiver
gain
variation
with
frequency
is
very
grad
-
ual.)
An
indistinct
dip
in
the
meter
reading
in
Step
8
indicates
inade
qua
te
reduction
of
the
receive
d
signal,
or
interference.
8.
Place
the
FULL-SCALE
switch
in
the
CAL
position:
adjust
GAL
OSC
Tuning
for
highes
t
meter
reading,
or
an
audio
beat
note
low
enough
in
frequency
to
be
inaudible.
(The
meter
reading
should
be
3.5-
4.8
when
the
Gain
control
is
properly
set.)
9.
Rotate
FUNCTION
switch
to
CAL
NULL
position:
adjust
GAIN
control
for
lo·,rest
meter
reading.
The
meter
reading
should
be
below
3.0.
10.
Return
FUNCTION
switch
to
FI-CAL-TUNE
. Retune
RCVR
tuning
for
ma.x:i.Jnummeter
reading
on
the
desired
signal.
The
Field
Strength
Meter
is
now
calibrated
at
the
frequency
to
which
it
was
origi-
nally
tuned.
2.4 Measuring
Field
Strength
In
use,
the
field
meter
is
generally
held
in
the
hand,
or
mounted
on
a
tripod
or
unipod.
Of
the
latter
two,
the
u.nipod
is
preferred,
since
it
can
be
easily
rotated,
and
can
remain
attached
to
the
instrument.
A
plate
ha
ving
a
hole
tapped
for
a¼
-20
screw
is
~
astene
d
to
the
bottom
of
the
case,
for
attachment
to
a
support
.
www.SteamPoweredRadio.Com
SECTION
2.
OPERATION
(Cont.)
2.4
Measuring
Fie
l d
Strength
(Cont.)
The
field
meter
should always
be
operated
in
a
vertical
position
when making
field
strength
measurements.
After
calibrating
as
described
in
the
previous
section,
proceed
as
follows:
2.5
1.
Set
the
FULL
SCALE
switch
to
the
range
approxi.mating
the
signal
strength
e:xpected.
2.
With
the
FUNCTION
switch
set
at
FI,
rotate
the
unit
to
orient
the
loop
antenna
and
obtain
maxi.mum
deflection
on
the
panel
meter,
changing
the
position
of
the
FULL
SCALE
switch
if
nec
-
essary
to
keep
the
panel
meter
indication
on
scale.
3.
Read
the
field
intensity
directly
from
the
panel
meter,
using
the
position
of
the
FULL
SCALE
switch
as
a
guide.
For
example,
with
the
FULL
SCALE
switch
on 100
mv,
a
full
scale
reading
of
10
on
the
m
eter
means 100
millivolts
per
meter,
and a
reading
of
5.6
indicates
a
field
strength
of
56
millivolts
per
meter.
4.
Multiply
the
reading
obtained
in
step
3
by
the
K
factor
for
the
operating
frequency
given
on
the
calibration
certificate
for
the
instrument.
This
procedure
is
followed
when
it
is
necessary
to
make
full
use
of
the
specif
ied
accuracy
of
the
field
meter.
5.
To
de
-
energize
the
field
meter,
rotate
the
FUNCTION
switch
to
the
OFF
position.
This
disconnects
the
batteries
from
the
circuit.
Closing
the
cover
also
removes
the
battery
power.
Measuring Harmonic
Ratio
in
Decibels
Field
Strength
Ratios
may
be
measured
directly
in
decibels
by
reading
the
db
scale
of
the
meter
in
conjunction
with
the
FULL-SCALE
range
indicator.
Since
each
step
of
the
FULL--SCALE
switch
corresponds
to
one (1)
decade
or
twenty
(20)
db,
the
ratio
is
determined
by
the
following
formula:
db1 -db2 + N x
20
=
Ratio
in
db
where:
db1 = Harmonic
field
strength
db2 = Fundamental
field
strength
N =
Number
of
FULL
SCALE
steps
between f\mdamental
and
harmonic
www.SteamPoweredRadio.Com
SECTION
2.
OPERATION
(Cont.)
2.5 Measuring Harmonic
Ratio
in
Decibels
(Cont.)
Example:
Let
us
say
that
we
want
to
measure
the
field
strength
and
the
second
and
third
harmonic
suppression
of' a
transmitter
with
an
operating
frequency
of
l500
KHz.
1.
Measure
field
strength
of
fundamental
as
outlined
in
section
2.4
of
this
manual;
note
db
scale.
Example: 560
mV/M
& 5
db.
2.
Measure
field
strength
of
second
harmonic
as
above
at
3.0
MHz.
Examp
le
:
0.35
mV/M
& 9
db.
3,
Measure
field
strength
of
third
harmonic
as
above
at
4,5
MHz. Example: 79
uV/M
& 2
db.
Calculate
harmonic
suppression:
db1 -db2 +
20
x l! =
ratio
(db)
second
harmonic
suppression=
9-5 +
60
=
64
db
third
harmonic
suppression
= 2-5 + 80 = 77 db
It
is
not
the
purpose
of
this
manual
to
describe
the
techniques
of
locating
radial
measurement
points
and
plotting
the
field
data
required
by
the
FCC.
For
this
type
of
information,
and
for
a more
detailed
descrip-
tion
of
operating
techniques
,
the
user
is
referred
to
the
current
edition
of
the
NAB
Hand
book.
2.6 R.
F.
Bridge
Measurements (Tuned
Voltmeter)
The
receiver
section
of
the
FIM-21 and FIM-41
may
be
used
as
a
tuned
voltmeter
for
applications
such
as
R.
F.
Bridge
null
detection.
As
a
tuned
voltmeter,
these
instruments
exhibit
an
i~put
i.rnpedance
of
2500
ohms
shunted
by
less
than
2
picofarads.
To
use
the
FIM
as
a
relative
indicator
of
RF
voltage,
proceed
as
follows:
1.
RF
ThTPUT
switch
2.
METER
switch
3.
FUNCTION
switch
4.
FULL
SC.ALE
switch
2-6
switch
to
EXT
position
LOG
or
LIN
-
the
LOG
mode
is
helpful
for
RF
null
detection
work
set
to
FI
-CAL
TUNE
set
to
lOV
position
or
as
required
for
meter
indication
www.SteamPoweredRadio.Com
SECTION
2.
OPERATION
(Cont.)
2.6
R.
F.
Brid
ge Measurements (Tuned
Voltmeter)
(Cont.)
Connect
RF
source
to
EXT
RF
input
on
FIM
front
panel.
Adjust
RCVR
tuning
for
desired
frequency
and
highest
reading:
adjust
FUIJ,
SCALE
switch
for
an
on-
scale
reading.
To
calibrate
the
FIM
for
absolute
voltage
me
asure
m
ent
s;
set
RCVR
tuning
to
desired
frequency
with
signal
source
connected
to
EXT
RF
input
switch
settings
as
in
steps
1,
3
&
4 above.
The
LOG-LlN
switch
nrust
be
set
to
the
LThl
position
fo
r
absolute
readings.
Switch
the
RF
ThTPUT
switch
to
ANY
and
calibrate
me
ter
as
described
in
Section
2.3.
Return
RF
ThTPUT
switch
to
EXT
and
read
meter.
The
correct
voltage
is
obtained
by
correlating
the
m
eter
indication
and
the
FULL
SCALE
attenuator
setting
as
previously
described
.
IMPORTANT
NOTE
When
operating
the
FIM
as
a
tuned
voltmeter,
it
is
necessary
to
g
uard
against
severe
overload
at
the
EXT
RF
input.
For
thi
s
reason,
start
all
measurements
in
the
lOV
FULL
SCALE
switch
position.
The
maximum
input
level
which
can
be
applied
to
EXT
input
without
causing
damage
is
25
volts
RMS
.
2-7
www.SteamPoweredRadio.Com
SECTION
3.
THEORY
OF
OPERATION
3,1
General
The
theory
of
operation
of
Field
Strength
Meters
FIM-
21
and FIM-41
is
explained
in
this
section.
Included
in
this
section
is
a
general
descrip-
tion
which
is
referenced
to
the
associated
block
diagram.
3.2
Block Diagram
Description
Refer
to
Figure
3-1,
Field
Strength
Me
ter
FIM-21/41,
Functional
Block Diagram.
The
FJN
can
be
considered
as
being
made up
of
seven
major
sections:
(1)
A
loop
antenna
(2)
A
very
stable
AM
superheterodyne
receiver
(3) A 20 db
per
step
RF/IF
attenuator
within
the
receiver
(4)
A
calibration
oscillator
(5)
A
gain
comparison (nulJ.)
circuit
(6) A
panel
meter
(7) A
regulated
power
supply
A
shielded
loop
antenna
is
used
because
of
its
directiv.i..ty
char
-
acteristic
and
its
freedom from hand
capacity
effects.
Signals
picked
up
by
it
are
fed
into
the
receiver,
and
the
carrier
to
be
measured
is
selected
by
tuning
the
receiver,
using
headphones,
or
the
panel
speaker,
and
the
panel
m
eter
as
tuning
indicators.
Both
the
audio
and
the
DC
to
operate
the
meter
are
obtained
from a
diode
rectifier
at
the
output
of
the
last
IF
stage.
T
he
RF
attenuator
is
set
to
the
range
which
prov.ides
a
reading
between
the
upper
and
lower
limits
of
the
meter,
and
the
instrument
is
rotated
for
maximum
pickup
in
the
loop.
The
six
steps
in
the
attenuate~,
plus
the
10
to
1
range
of
the
meter
,
mak
e
it
possible
for
the
instrument
to
measure
signals
over
a
range
of
10
microvolts
to
10
vo
l
ts.
With
the
loop
-
receiver-meter
section
of
the
field
meter
just
described,
it
is
possible
to
make
comparative
or
relative
field
measure-
men
ts
over
a wide
range,
and
the
shape
of
the
radiated
pattern
can
be
de-
termined
with
accuracy.
However,
the
FCC
requires
the
broadcaster
to
make
absolute
m
easurements
of
the
signal
intensity
at
any
point,
in
volts
per
me
ter,
and
for
this
purpose
the
calibration
oscillator
is
includedo
3-1
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Fie
ld
St
rength
M
eter,
Functional
Block Diagram
DC
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www.SteamPoweredRadio.Com
SECTION
J.
THEORY
OF
OPERATION
(Cont.)
Block Diagram
Description
(
Cont.)
If
the
user
were t o
hold
the
field
meter
at
a
precise
point
in
a
radiated
field
!mown
to
have a
st
ren
gth
of
200
MV/M
, and
adjust
the
receiver
gain
to
read
this
value
on
the
met
er,
the
instrument
would
then
be
calibrated
at
this
frequency
and
it
would
read
field
strength
in
volts
pe
r meter
at
all
meas
uring
points,
provided
that
the
gain
does
not
change, and
that
the
re
-
ceiver
tuning
does
not
change.
To
calibrate
the
field
meter,
then,
it
is
only
necessary
to
have
some
reference
signal
in
the
receiver
to
indicate
the
proper
setting
for
the
receiver
gain
control.
This
signal
is
provided
by
the
calibration
oscillator.
When
the
FULL
SC.ALE
switch
is
positioned
to
CAL,
and
the
FUNCTION
switch
is
positioned
to
FI,
a
precisely-determined
fraction
of
the
oscillator
output
voltage
is
injected
into
the
receiver
antenna
circuit
and
is
detected
by
the
receiver.
At
the
same
time
a
detector
fed
directly
by
the
oscillator
output
voltage
provides
a
de
voltage
for
comparison
with
the
receiver
detec-
tor
output.
In
the
Fil-1
-
21
and FIM-41
the
two
detector
outputs
that
must
be
equalized
are
compared
using
a
null
circuit,
T
he
null
circuit
produces
a
meter
reading
that
drops
as
the
receiver
detector
voltage
approaches
the
oscillator
detector
voltage
from above
or
below;
the
reading
is
minimum
when
the
voltages
are
equal.
The
receiver
gain
value
for
which
the
detector
outputs
are
equal
is
determined
by
the
relation
between
the
RF
voltage
at
the
oscillator
detector
and
the
RF
volta
ge
injected
into
the
antenna
circuit.
The
c
ir
cui
try
that
determines
this
injection
voltage
thus
determines
the
calibration
accuracy.
The
circuitry
is
factory
-
set
during
final
calibration
for
correct
field
readings.
Since
the
vol
tage
induced
in
the
antenna
by a
field
varies
with
frequency,
the
RF
injection
voltage
mus
t
also
vary
with
frequency
in
exactly
the
same
way
in
order
for
the
calibration
to
be
correct
across
the
full
frequency
range.
The
desired
variation
with
frequency
is
produced
by
a
simple
reactive
network.
It
is
the
double
function
of
the
calibration
oscillator,
as
a
signal
source
for
the
receiver
and
as
a
reference
level
for
the
null
circuit,
that
provides
the
very
important
self-compensating
feature
of
the
calibration
of
this
instrument.
For
example,
if
the
output
of
the
oscillator
were
to
be
reduced
by
20%
for
any
reason,
the
reference
level
for
the
null
circuit
-would
be
20
%
lower,
but
the
signal
fed
into
the
loop
would
also
be
20%
lower,
so
that
no change
in
the
gain
control
would
be
required
to
calibrate.
3.3
Circuit
Descri
p
tion
-
Refer
to
Figure
3-1
In
t
he
recei-ver
the
RF
signal
is
induced
by
the
magnetic
component
of
the
incide
nt
field
in
a
shielded,
unbalanced
loop
antenna
. The
loop
is
part
of
the
first
tuned
circuit
of
a
double
-
tuned
bandpass
filter
that
is
tuned
over
the
RF
frequency
range.
The
input
can
also
be
supplied
from
an
external
source;
selecting
thi
s
mode
switches
the
loop
antenna
out
of
the
circuit
while
maintaining
the
same
RF
b
andpass
characteristics.
3-3
Table of contents
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