I don't have a 189 but have other fluke meters. Measuring in dBm, to measure an absolute value, you must first know what load you are measuring across. Then the meter must be set up to read zero dBm across that load impedance. One of the fluke meters that I have has several different settings of load resistance references so that it will read absolute dBm correctly.
If you just want to read relative dB between two different levels then the load impedance is not important. Just stick the meter on the load with the signal applied and read the reference level in dBm (or offset the zero at that level) then change the level of the signal and read the dB difference between the two. 73 Gary K4FMX > -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > buil...@yahoogroups.com] On Behalf Of skipp025 > Sent: Friday, November 20, 2009 2:38 PM > To: Repeater-Builder@yahoogroups.com > Subject: [Repeater-Builder] Re: 2 dB "Error" in DVM level readings > > > So we all (here) pulled out our meters for a comparison. > First off... our one Fluke 189 does measure in dBm as did > every meter and transmission test set in the shop. So > all is not glitters that is written in technical forum > stone (gold). > > s. > > > "skipp025" <skipp...@...> wrote: > > > 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. > > > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > >
