POTOMAC INSTRUMENTS INC. FIM-41 User manual

POTOMAC INSTRUMENTS INC.
INSTRUCTION MANUAL
FIELD STRENGTH METER
FIM-41
7309 GROVE ROAD UNIT D
FREDERICK, MD 21704
PHONE -301-696-5550
FAX -301-696-5553
PI

FIELD STRENGTH METER
POTOMAC INSTRUMENTS inc.
PRECISION ELECTRONIC PRODUCTS

WARRANTY FOR EQUIPMENT AND ACCESSORIES
Potomac Instruments inc. warrants each new equipment to be free
of defects in material and workmanship, for a period of one (l)
year after date of original shipment, except for tubes, lamps,
fuses, transistors, and diodes which will be warranted for a
period of ninety (90) days. Any instrument which is found 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 IN SPECIFICATIONS
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
modi~ equipment previously sold.
RETURNING EQUIPMENT TO THE FACTORY
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 unit should be forwarded first,
whereupon instructions for correcting the trouble or authorization
for returning the equipment will be given. This applies to all
units, whether under warranty or not. Shipment in an improper
container may result in damaged equipment thereby voiding warranty.
ii

TABLE CONTENTS
Paragraph Page
1.1
1.2
1.3
2.1
2.2
2.3
2.4
2.5
2.6
SECTION 1. GENERAL DESCRIPTION
Scope of Manual • • . • • •
Purpose of Equipment
Equipment
Specifications
SECTION 2. OPERATION
General . . . . . . . . . . . . . .
Operating Controls and Indicators .
FIM Calibration • • • • • • • • • •
. ..
Measuring Field Strength • • • . • • • •
Measuring Harmonic Ratio in Decibels .•.••
R. F. Bridge Measurements. (Tuned Voltmeter) .
iii
1-1
1-1
1-1
2-1
2-1
2-2
2-4
2-5
2-6

Figure
2-1
Table
1-1
1-2
2-1
LIST OF ILLUSTRATIONS
Operating Controls and Indicators • • • • • • • •
Specifications:
Specifications:
LIST OF TABLES
Type FIM-21 Field Strength Meter
Type FIM-41 Field Strength Meter
Operating Controls and Indicators • . • • • • • •
iv
2-3
1-1
1-3
2-1

SECTION 1. GENERAL DESCRIPTION
1.1 Scope of Manual
This manual provides a comprehensive technical description of the
operating and physical characteristics of the FD1-2l and FD1-41 Field Strength
Meters. It provides sufficient information concerning the operation and func-
tional theory of operation to permit effective operation of these instruments.
1. 2 Purpose of Equipment
Field Strength Meters, FD1-2l and FD1-41 are light-weight portable
instruments for the measurements of a wide range of radio signal intensities
in the .54 to 5.0 MHz frequency spectrum. The FD1-21 covers the broadcast band
(540 to 1600 KHz) while the FD1-41 covers the broadcast band and harmonic fre-
quencies to 5.0 MHz. Both units are rugged test instruments that are equally
effective for interference studies at low signal strength and for close-in meas-
urements 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.
1.3 Equipment Specifications
(See specifications, FD1-21, Table 1-1, FD1-41, Table 1-2)
Table 1-1. Specifications: Type FD1-21 Field Strength Meter
Frequency Range:
Field Intensity Range:
Accuracy
of Calibration:
Accuracy of Range Attenuator:
Selectivity
Bandwidth (6 db):
IF Rejection:
Image Rejection:
Adjacent Channel Rejection:
Panel Meter:
Antenna:
1-1
535 KHz to 1605 KHz
10 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 multi-pole
hybrid filter
75 db minimum
80 db minimum
50 db minimum 10 KHz above and
below
tuned station
411 mirrored scale, logarithmic
gradUations 1 to 10, taut band
meter movement, 2% linearity
Shielded loop, integral part of
hinged cover

SECTION 1. GENERAL DESCRIPTION (Cont.)
Table 1-1. Specifications: Type FIM-21 Field Strength Meter (Cont.)
Audio Outputs:
Recorder Output:
Lights:
Batteries (6 required):
Battery Life:
Environmental:
Dimensions:
Weight:
External RF Input:
RF Input Switch:
METER Switch:
Logarithmic Dynamic Range:
1-2
Front panel loudspeaker, weather
treated cone
Headphone jack, high and low Z
(disconnects speaker)
0.4 to 4 volts DC proportional to
field intensity for each attenuator
range, 2000 ohm source resistance
Frequency dial and meter; panel
switch for night use
Standard I! volt C-2,., IIDII cells or
I! volt Alkaline celfs for extended
life at very low temperatures
Greater than 1000 FI readings (re-
duced life with frequent use of lights
and/or high speaker volume)
Continuous exposure -10
0 F
to +1300 F;
lower temperature operation practical
for IIreading time
ll
exposures
8-3/411 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 11-1/2 pounds
535 KHz to 1605 KHz, 10 microvolts to
10 volts
Selects loop antenna (ANT) or Panel
Connector (EXT)
Selects normal linear (Lm) or loga-
rithmic (LOG) operation
60 db compressed input range for FI
meter and recorder output

SECTION 1. GENERAL DESCRIPTION (Cont.)
Table 1-2. Specifications: Type FIM-41 Field Strength Meter
Frequency Range:
Field Strength Range:
Accuracy
of Calibration:
Accuracy of Range Attenuator:
Selectivity
Bandwidth (6 db):
IF Rejection:
Image Rejection:
Adjacent Channel Rejection:
Harmonic Measurement:
Panel Meter:
Antenna:
Audio
Outputs:
Recorder Output:
Lights:
Batteries:
1-3
.54 MHz - 5.0 MHz in two bands
Band "A" .535 - 1.61 MHz
Band "B" 1. 58 - 5.0 MHz
10 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 multi-pole hybrid
filter
75 db minimum
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
411 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 strength for each attenuator
range, 2000 ohm source resistance
Frequency dial and meter with panel
switch for night use
Standard 1-1/2 volt C-Z
n
IIDII cells
or 1-1/2 volt. Alka1;ine cells for
extended life at very low tempera-
tures (six required)

SECTION 1. GENERAL DESCRIPTION (Cont.)
Table 1-2. Specifications: hYPe FIM-41 Field Strength Meter (Cont.)
Battery Life:
Environmental:
Dimensions:
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 high speaker volume)
Continuous e:xposure _100
to +1300 F;
lower temperature operation practical
for "reading time" e:xposures
8-3/4" high, 1l-1/2
11
wide, 5-1/8" deep
with cover closed; 3-7/8
11
deep with
cover open (gripping width)
Approximate:q 12 pounds
540 KHz to 5.0 MHz, 10 microvolts to
10 volts
Selects loop antenna (ANT) or Panel
Connector (EXT)
Selects normal linear (LJN) or loga·-
rithmic (LOG) operation
60 db compressed input range for
FI meter and recorder output
Carrying/Shipping case
Unipod

SECTION 2. OPERATION
2.1 General
The Frn-21 and Frn-41 operating controls and indicators are de-
scribed in this section. Also provided are the basic operating instructions
which include battery voltage testing, meter calibration, measuring field
strength, and use of these instruments as null detectors for RF Bridge meas-
urements.
2.2 Operating Controls and Indicators
All operating controls and indicators are located on the front
panel of the Frn-21 and Frn-41 as shown in figure 2-1. Table 2-1 lists these
controls and indicators and the associated functions.
Table 2-1. Operating Controls and Indicators
Control or Indicator Function
FUNCTION selector switch (S2)
FULL SCALE selector switch (Sl)
AUDIO control and switch (R3)
Selects the function to be displ~ed
on the front panel meter and connects
battery.
OFF - battery disconnected from circuit,
and direct connection made across meter
terminals to protect meter.
BATT - battery output voltage
FI CAL TUNE - field strength of antenna
input developed from detected output of
receiver IF amplifier.
CAL Null -compares detector outputs from
receiver and calibrating oscillator.
Selects 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

SECTION 2. OPERATION (Cont.)
Table 2-1. Operating Controls and Indicators (Cont.)
Control or Indicator Function
GAIN control (R2) Adjusts gain of receiver by regulating
gain of IF amplifier.
CAL OSC tuning control (Rl)
RCVR tuning control (Cl)
FIELD-STRENGTH meter (m)
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 receiver and calibrating 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.
Illuminates meter 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

LOOP ANTENNA
AND SHIELDING
RECEIVER/OSCllLA
TOR
TUNING DIAL
MI---
R1
R2
S2-_-.;._
J1
lS1
R3
J2
Figure 2-1 Operating Controls and Indicators
2-3
S7
(FIM-41 ONLY)

SECTION 2. OPERATION (Cont.)
2.3 FIM Calibration (Cont.)
2. Rotate the FUNCTION switch to the BATT position and see that
the reading 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 dial to the
signal to be measured. Use the meter indication to obtain
peak tuning, adjusting the GAIN control and FULL SCALE switch,
and rotating the instrument to obtain a reading within the
range of the meter. Use the audio signal to identify 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
falls in the vernier range.
7. Rotate the instrument to obtain reading below 10 mV. In strong
fields the meter reading may be reduced below 10 mV by holding
the FIM horizontally or by changing RCVR T- ...... "ling av.ray 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
inadequate reduction of the received signal, or interference.
8. Place the FULL-SCALE switch in the CAL position: adjust CAL OSC
Tuning for highest 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
maximum meter 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 unipod is preferred, since it
can be easily rotated, and can remain attached to the instrument. A plate
having a hole tapped for a ~ - 20 screw is fastened to the bottom of the case,
for attachment to a support.
2-4

SECTION 2. OPERATION (Cont.)
2.4 Measuring Field Strength (Cont.)
The field meter should always be operated in a vertical position
when making field strength measurements.
as follows:
2.5
After calibrating as described in the previous section, proceed
1. Set the FULL SCALE switch to the range approximating the signal
strength expected.
2. With the FUNCTION switch set at FI, rotate the unit to orient
the loop antenna and obtain maximum 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 meter 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 specified 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:
dbl -db2 + N x 20 = Ratio in db
where:
dbl = Harmonic field strength
db2 = Fundamental field strength
N = Number of FULL SCALE steps between fundamental
and harmonic
2~5

SECTION 2. OPERATION (Cont.)
2.5 Measuring Harmonic Ratio in Decibels (Cont.)
Example:
Let us s~ that we want to measure the field strength
and the second and third harmonic suppression of a
transmitter with an operating frequency of 1500 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. Example: 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:
dbl -db2 + 20 x lJ = 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 Handbook.
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:~ut impedance of 2500 ohms
shunted by less than 2 picofarads.
To use the FIM as a relative indicator of RF voltage, proceed as
follows:
l.
RF INPUT switch
switch to EXT position
2.
METER. switch
LOG or LIN - the LOG mode is helpful
for RF null detection work
3.
FUNCTION switch
set to FI-CAL TUNE
I
,+.
FULL SCALE switch
set to 10V position or as required
for meter indication
2-6

SECTION 2. OPERATION (Cont •)
2.6
R. F. Bridge Measurements (Tuned Voltmeter) (Cont.)
Cormect RF source to EXT RF input on FIM front panel. Adjust RCVR tuning for
desired frequency and highest reading: adjust FULL SCALE switch for an on-
scale reading.
To calibrate the FIM for absolute voltage measurements; set RCVR
tuning to desired frequency with signal source cormected to EXT RF input
switch settings as in steps 1, 3 &4 above. The LOG-LIN switch must be set
to the LIN position for absolute readings. Switch the RF INPUT switch to
.ANY and calibrate meter as described in Section 2.3. Return RF INPUT switch
to EXT and read meter. The correct voltage is obtained by correlating the
meter indication and the FULL SCALE attenuator setting as previously described.
IMPORTANT NOTE
When operating the FIM as a tuned voltmeter, it is necessary to
guard against severe overload at the EXT RF input. For this reason, start
all measurements in the lOV FULL SCALE switch position. The maxiJrrwn input
level which can be applied to EXT input without causing damage is 25 volts RMS.
2-7