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Field strength / receive power converter

This little converter transforms a selected specification of field strength or received power to the other.

Title:

Measurement:

Rx:

Field strength Receiver
FS Bandwidth (Hz) Receiver Bandwidth (Hz)
Distance to source (m) Distance to source (m)
Frequency (MHz) Receiver input resistance (Ω)
   
Distance exponent Additional loss (dB)

This calculator assumes a point source and plane wave conditions. Plane wave conditions do not apply very close to the source.

The Field strength column sets the conditions for determination of field strength from the chosen scenario. The Receiver column sets conditions for calculatation at the receiver which may be at a different distance from the source than the Field strength conditions.

Power and power density is adjusted by the relative bandwidths and distances.

Example 1: calculate the noise power in a 2000Hz wide receiver at 7MHz with a -1dBi antenna at 1000m distance from the source, if the broadband source has a field strength of 30μV/m in 9000Hz bandwidth at 30m distance from the source (FCC 15.209 limit). Answer: -103dBm (S4 in S-meter terms, and well above expected lowest ambient noise).

Example 2: if the noise power in a 2500Hz wide receiver at 7MHz with a -1dBi antenna is -53dBm (~S9+20dB), what is the field strength of a broadband source in 9000Hz bandwidth in dBμV/m?  Answer: 47.7dBμV/m (hint: use the same distance to source for both Field Strength and Receiver, the value doesn't matter, so long as it is greater than zero).

Example 3: if the external noise figure is 60dB on 3.5MHz and antenna gain is -20dBi, what is the 50Ω receiver voltage in 2000Hz bandwidth in μV?  Answer: 2.001μV (hint: use the same distance to source for both Field Strength and Receiver, the value doesn't matter, so long as it is greater than zero).

Note that for non-directional noise sources (eg background sky noise) you will want to set the antenna gain to 0dBi, as the power captured is independent of antenna gain.

If you're clever, you will have worked out you can download this page to your hard disk. If you're smart, you won't, you will run it from the web site and automatically take advantage of any updates.


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