Receiver sensitivity generally is rated in terms of a voltage present at the antenna terminals across whatever impedence the receiver presents.

What make and model spectrum analyzer do you have?

I remain,

The Old Soldering Gunslinger

It is actually a Cellmaster (does cable and antenna analysis, power meter ect) The model is an Anritsu MT8212A!

I think that calibrated antennas have values associated with them. If you measure a power in dBm/m and apply the "calibrated antenna factor", then you get actual field strength.
Google the antenna calibration factor or look up EMI/EMC companies and find antenna factors used for that type testing.
kchiggins.

Higgler is correct.  To elaborate more, you are asking for a comparison between apples and oranges, or more precisely between power density and power.  The field strength meter will tell you how many volts per meter the filed pressure is.  With an assumed far field free space estimate of 120 * pi Ohms (377 Ohms) you can determine the power density.  If you thing of your antenna as big scoop picking up power you can determine the power your antenna can pick up given the field strength.  The bigger the antenna (antenna aperture is given in units of area like square meters), the more power you will capture.  If you have an antenna to try this with you can take your filed strength in volts per meter and convert that into watts per square meter.  Then take your power measurement using your spec-an and antenna.  Divide the power measured with your antenna by the power density to give you your effective aperture in square meters of your antenna.

Best Regards,
John Solar

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Pardon the delay, but I just saw this thread. I sifted through some miscellaneous equations I've accumulated over the years, and found this:

    uV/m = 10^[(dBm - G + 20*logF + L +75)/20]

where

  dBm = signal level at receiver input in dB with respect to 1 mW
  G = receiving antenna gain in dBd
  L = line loss
  F = Frequency in MHz