Many Ultralight receivers include a display of signal strength and S/N for the received signal.  This information is useful, although it doesn't accurately show the field intensity of the signal arriving at the receive antenna.

The display of signal strength on the receiver is labeled as "dBµ," which is easily mistaken to mean dBµV/m.  The correct S.I. unit for values of field intensity is V/m, or its sub-units such as mV/m and µV/m.

The dBµ display on the receiver is showing the value of a conducted r-f voltage based on the output signal of the receive antenna, rather than the value of the received field intensity that produced it.  The "antenna factor" of the receive antenna needs to be known for each frequency, in order to accurately determine the field intensity of the signal arriving at that antenna.

The link below leads to an example of the differences between the signal strength display on one of the Tecsun PL-310 ultralight receivers and the field intensities arriving at its receive antenna, for the conditions shown there.

https://i.postimg.cc/4y4kVCxR/Tecsun-PL-310-Signal-Readings-1.gif

Very good and interesting information Richard. Though I have never considered the readouts on my ultralights to achieve metrology grade accuracy. Rather simply a relative indication of signal strength.
It is good to know, however, that there is relationship to "real" field strength. Even though this will certainly vary unit to unit. I would think that throwing one of the baby FSL antennas or any external antenna would change the delta considerably.
I still prefer a good old analog "S" meter though. Especially when calibrated properly.

RE:  I still prefer a good old analog "S" meter though. Especially when calibrated properly

However an "S" meter even perfectly calibrated to meet the prevailing standard for it also does not take into account the antenna factor or gain of the antenna used by the receiver.

As with the Ultralights, an "S" meter indication is based on the conducted r-f voltage present across the antenna input terminals of the receiver, and does not display the real field intensity of the arriving r-f signal.

Absolutely no arguments from me on this last bit. But really, I use the meter on the radio to point an antenna, or compare received signal strengths for logging. So an indication of r-f voltage across the antenna terminals is ok. In fact this information is the endgame for me for those purposes. Still it is interesting to understand that the readings we are seeing on any of these meters is simply an interpretation of the field intensity.