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Field strength indicator with frequency counter

This handheld field-strength indicator 1 was developed in 1983 by the Dutch Radar laboratory (NRP), for use by the Dutch Radio Monitoring Service (RCD). It has a built-in frequency counter and was intended as a law enforcement aid when locating clandestine transmissions, often used alongside the PAN-1000 intercept receiver. It was also used by the Dutch Foreign Office (BuZa).

The device is housed in a small beige-painted BIM box, and measures 11 x 6 x 3 cm (without the controls). It is intended for uncalibrated relative field-strength measurements in the 10 to 500 MHz frequency range, with the optimum sensitivity between 80 and 180 MHz. The right­most knob at the top is used for the adjustment.

At the heart of the front panel is a standard VU meter which was commonly found on the audio equipment of the era. Below the scale however, six holes have been drilled through which the lenses of an HP 6-digit LED display protrude.
NRP field strength meter with built-in frequency counter

The LED display is used for the built-in frequency counter. The device is turned ON with a small slide switch at the right top. When it is ON, the internal frequency counter can be enabled by pressing a small push-button at the left. It order to save battery power, the counter is disabled when the push-button is released. The frequency counter can be used reliably from 10-550 MHz.

Outside this range, the counter's sensitivity rapidly decreases with approx. 6dB per octave. The optimum sensitivity is at 100 MHz (3 metre band), which suggests that its main purpose was for finding clandestine radio stations (pirates) operating in the 88-108 MHz broadcast band.

The NRP-FS was introduced with the Dutch Monitoring Service (RCD) around 1983 alongside the professional PAN-1000 intercept receiver. It was commonly used to determine the exact location from which a clandestine transmission originated, and often led directly to the antenna.
Handheld operation

In addition, it helped the law enforcement officers to determine the exact frequency of a mobile radio station, whilst driving around. If they saw someone operating a mobile radio, all they had to do was press the button in order to determine the frequency. The PAN-1000 was then adjusted to the same frequency, after which the officer could establish the legality of the transmission.

The NRP-FS was also used by the Dutch Foreign Office as one of the tools to check a meeting room or an embassy for covert listening devices (bugs). At the time, most professional bugs used frequencies between 100 and 400 MHz and would often cause a frequency counter to lock-on.

  1. As the field-strength meter was custom-built in small quantities for one or two customers only, it was not given an official name or product designator. We therefore refer to this device as the NRP-FS.

NRP field strength meter with built-in frequency counter NRP field strength meter in upright position With helical antenna installed Front view (with helical antenna fitted) Front view Handheld operation Close up of the meter with integrated frequency counter Push button on the left for enabling the built-in frequency counter
The diagram below shows the various features of the NRP field-strength indicator. It is powered by a standard commercial 9V block battery that is installed behind a lid at the bottom of the right side. When the unit is turned ON, a faint red LED lights up in the top left corner of the meter.

When locating a transmitter, small increases in field-strength can be detected by observing the needle of the meter. Alternatively, a small earpiece can be connected to the 2.5 mm jack socket at the top panel, in order to obtain an acoustic indication of the field-strength. When the signal strength increases, the pitch of the indicator tone also increases. When the reading is at its maximum, the tone switches OFF and the sensitivity (SENS) must be lowered to get a tone again.

Technical specifications
  • Frequency range
    15 - 550 MHz
  • Optimum frequency
    100 MHz
  • Visual indicator
    VDU meter
  • Audible indicator
    2.5 mm jack for earphone
  • Supply voltage
    9V (internal battery)
  • Counter resolution
    10 kHz
Further information
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Crypto Museum. Created: Saturday 28 January 2017. Last changed: Sunday, 05 February 2017 - 17:57 CET.
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