Pasted from another Technical Forum This comes up from time to time ..well, it came up again.
Check the display for the measurement unit. Fluke 189s measure in dBV (dB referenced to one Volt), not dBm (dB referenced to one milliWatt). 0dBm (.775 VAC) is a couple dB different in voltage from 1 Volt. A good clue is on the display of the 189 over to the right, it says dB and V. When the 189 is measuring a 0 dBm tone, it indicates -2.21 dB V and the small voltage display indicates .775. If you use the old 20 log (.775/1) formula, you come up with a difference of -2.21 dB. All of the DVMs are high impedance input, so they don't measure actual power. The 189s only know that 1VAC is displayed as 0dBV and the 287s know that .775 VAC is displayed as 0dBm when it is in the dBm mode. Those of you who have the new 287s get to read dBm directly and as an added bonus, get to change batteries very frequently. Those of you who have the 189s get to use your smarts more and don't have to change batteries so often. Check the specs on your individual DVM. It may be able to count CTCSS. The old wideband AC Voltmeters are failing rapidly. You will have to use your Transmission Test Set and possibly some functionality of your DVM to replace the greatness of the old wideband AC Voltmeter.
