FSM (for Field Strength Meter) is a software application that extends a conventional SSB receiver to allow measurement and calculation of field strength of radio signals or interference. FSM is a software implementation of a development of the technique described by Ed Hare of the ARRL in "Manual Testing of Field-Strength Levels Using Conventional Receivers" dated August 2004 .



Case studies

A selection of studies based on FSM measurements:


The technique is to use an SSB receiver to "down convert" a narrow radio frequency spectrum segment of interest to audio frequencies, and to analyse the audio output of the receiver using FSM software.

The technique depends on the fact that the audio output power of an SSB receiver is linearly related to the RF input power (including the equivalent internal noise power) up to the onset of AGC action, which is typically more than 20dB above the equivalent  input noise power. By using a known external attenuator to keep measured signals in this linear range, valid relative measurements can be made of the receiver audio output power and absolute results calculated by factoring in the attenuator, receiver equivalent internal noise power, and other configuration variables.

The figure below shows the FSM indication vs  RF power in and measurement error using an IC706IIG with a Rx Noise Floor of -135dBm.

  FSM linearity with IC706IIG.

FSM is a web enabled application that can file its measurement results online on a web server, or nearline by email.


FSM features include:

  • measurement of true RMS, quasi peak and peak power audio  power;
  • calculation of received RF power (RMS, QP, and Peak) in dBm based on known receiver noise floor;
  • calculation of field strength (RMS, QP, and Peak) in dBuV/m based on known antenna gain or antenna factor;
  • extrapolation of calculated field strengths to a normalised (1Hz) bandwidth for comparisons; and
  • flexible output options to save results to text files, email, and online / nearline web transactions.


My test setup for measurements on the 7MHz amateur band is:

  • half wave inverted V dipole, average gain -1.2dBi from EZNEC model;
  • 20.5m RG6 feedline transforms shortened dipole impedance to 50ohms (see article), 0.4dB loss;
  • Kenwood R5000 receiver, NF=14.6dB;
  • HP355D 0 - 120dB, 10dB step attenuator;
  • DI box to isolate receiver ground from PC ground;
  • Creative Vibra128 sound card;
  • FSM software.
FSM test hardware configuration.


Test setup: Kenwood R5000 receiver and HP355D step attenuator.

The "Record" audio output of the R5000 on the front panel is convenient and independent of the AF gain setting. It is a relatively high level, and will overload the line input of a sound card, so the DI box 20dB attenuator is used to not only lift the ground to avoid hum loops, but the 20dB attenuation prevents clipping in the linear stages of the sound card.

FSM screen at completion of Step 3.

You may not need the ground isolator, but if you have having trouble achieving 20dB (or 10 x voltage) increase between the Step 1 RMS Voltage and Step 2 RMS Voltage, you may have a hum loop and need a ground isolator or some other solution to the hum. In the example above, the sound card + connection internal noise is 47dB below the receiver noise.

Plot using DPLOT of the wave file from Step 3.


Portable FSM - ICR20 receiver, HP355D attenuator and notebook computer.



© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.