DISCONTINUED Potomac Instruments FIM-71 VHF Field Strength Meter

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Key Features:

  • Direct Reading – Volts or dB
  • 45 MHz to 225 MHz, Continuous Tuning
  • Peak or Averaging Detector
  • Wide or Narrow IF Bandwidth
  • 20 dB or 60 dB Meter Range
  • AM or FM Demodulator
  • Calibrated Dipole Antenna, Case Mount or Removable
  • 140 dB Measurement Range (1 uB to 10 V)
  • 4 1/2 inch, Mirrored Scale, Taut-Band Meter
  • Front Panel Speaker
  • Rugged, Portable
  • Calibrated Signal Generator


  • AC power adapter
  • Unipod
  • Carrying case
  • Antenna elements and balun
  • Headset

The Model FIM-71 is a portable, laboratory quality Field Strength Meter designed for rigorous field applications. Combining a calibrated half-wave dipole antenna and a highly accurate tuned voltmeter with a range of 140 dB, the FIM-71 is suitable for practically all types of RF emission measurements in the 45 MHz to 225 MHz spectrum. The operator can switch select wide or narrow bandwidth, peak or average value of TV or pulse modulated signals, AM or FM demodulation, and a meter dynamic range of either 20 dB or 60 dB. A dc analog output voltage, proportional to the meter indication, is provided for driving a chart recorder. A leveled output from the calibrating oscillator is available for measuring cable insertion loss, filter response, amplifier gain, and other signal ratio measurements. The 4-1/2 inch, taut band, mirrored scale meter is calibrated in volts and dB for precise measurements in field or laboratory environments.

The tuned voltmeter is a single conversion, super-heterodyne receiver with carefully tailored sensitivity, gain, and linearity characteristics. The RF input is double tuned and designed for minimum VSWR and maximum out-of-band signal rejection. Uniform gain, independent of IF bandwidth, is provided by temperature compensated RF & IF amplifiers utilizing a combination of MOSFET, J-FET, bipolar, and monolithic integrated circuit devices. The linear detector is followed by a logarithmic shaping circuit which drives the meter in the LIN (20 dB) mode. In the LOG mode the meter indication (in dB) varies linearly over a one-thousand-to-one range of input levels.


Technical Specification

Technical Specification

Frequency Range 45 MHz-225 MHz; continuous
RF Input Impedance/VSWR 50 ohms
VSWR 1.2:1, 100µV full scale and greater;
VSWR 1.5:1, 10µV full scale
Voltage Measurement 1µV to 10 V rms in seven switch selected ranges
Metering 4-1/2 inch meter, mirror-backed scale, taut band meter
Indication Modes LINear and LOGarithmic, switch selected
Meter Scales LIN mode: 1-10 (logarithmic scale) and 0-20 dB (linear scale).
LOG mode: -20 to +40 dB (60 dB range, linear scale)
Battery voltage/External supply voltage scale.
Metering Detectors Average responding and peak responding (for television sync pulse), switch selected
Receiver Bandwidths AM/FM: 190 kHz at -3 dB, and
TV: 450 kHz at -3 dB, switch selected
Absolute Accuracy Voltage: ±1.5 dB (LIN), ±2.0 dB (LOG); for voltage >1.5µV (AM/FM) or > 3µV (TV)
Field Strength: ±3.0 dB (LIN), ±3.5 dB (LOG) for field strengths >1.8µV/M (AM/FM) or >3.7µV/M (TV) at 45 MHz; >9.1 V/M (AM/FM) or >18.1µV/M (TV) at 225 MHz; using the supplied antenna.
Note: These figures apply when using the Average Detector; for the Peak Detector, noise correction factors (supplied) are required below 10 mV
Relative Accuracy ±1 dB at one frequency, for voltage or field strength, LIN mode, for voltages >10µV, with noise correction factors.
Harmonic Measurement Measures second harmonic field strength of 87.5 Mhz-108 MHz signals to -80 dB for fundamental voltage less than 100mV
Calibrating Oscillator Output switched to receiver for internal calibration, to external output (BNC connector) of OFF. Tracks receiver frequency when connected to receiver.
Frequency Dial
Six-turn spiral, continuous tuning, movable cursor.
±0.5% of indicated frequency without cursor correction
±200 kHz typical, 87.5 MHz – 108 Mhz, after setting cursor on known signal
RCVR Spurious Response Image Rejection, 55 dB typical;
IF Rejection, 100 dB typical
Local OSC Radiation 45 MHz, 2µV; 225 MHz, 35µV; typical values across 50 ohm load at RF input connector
DemodulatorsVideo Frequency Response
Output Level
AM and FM; switch selected, phone jack (0.25″) output connector
50 Hz – 100 kHz, 3 dB max. variation
4.5V p-p max. across 75-ohm load, front panel adjustable
Audio Monitoring Internal loudspeaker; headphones plug into demodulator output jack (disconnecting speaker);
AM or FM selected by DEMOD switch; level control with disabling switch
Record Output
Tip Contact
Ring Contact
Two-circuit phone jack (.25″) output
DC analog of meter indication -0.8V to -8V (open circuit), 2000 ohm source resistance
DC output from FM discriminator, @ -5 V + or -± 3 V, 10,000 ohm source resistance
(Single circuit phone plug provides tip contact output only.)
Power Supply:
Internal Batteries
Battery Life
External Supply
1.5 volt size “D” batteries, ten required
1500 readings or 18 hours continuous operation using Eveready No. 950 batteries (or equivalent) at 70°F
11.5 volts to 19.0 volts DC, positive ground, 120 mA, Switchcraft No. 760 Connector (or equivalent)
Temperature Range + 15°F to +105°F (-10°C to +40°C)
Dimensions, Inches (CM) Without Antenna: 9.5 (24) high, 12.25 (31) wide, 7.25 (18.4) deep
With Antenna attached and retracted, 9.9 (25) high, 13.5 (34.3) wide, 7.25 (18.4) deep
Weight, Pounds (KG) 20 (9.1) with batteries, antenna, cover, cables and softcase
   Note: Values without limits are typical only. Field strength data are with
ANT-71 Antenna.
Antenna Ant-71
Type: Tunable half-wave dipole with continuously adjustable telescoping elements.
Frequency Range: 45 Mhz – 225 MHz
Calibration: Antenna Factor data supplied based on NIST calibration; overall error including NIST calibration uncertainty, ±.5 dB max
Load Impedance: 50 ohms
Mounting: Mounts on case for hand-held measurements at an antenna height of approx 7 ft.; has 1/4-20 threaded hole for mounting to other masts


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