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Old 11-04-2017, 10:54 AM
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Findm-Keepm Findm-Keepm is offline
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Join Date: Jun 2007
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Quote:
Originally Posted by kf4rca View Post
78 db is 1 watt. 75 db is 1/2 watt.
dB (Decibels) express a ratio, not a value. Wattage is usually expressed in dBm or dBw, or in a 75 ohm system, dBmV.

30 dBm= 1 Watt
27 dBm= 1/2 Watt

The above are for a 50-ohm system.

For a 75 Ohm system, dBmV = dBm + 48.75 dB

Best calculator out there:

http://earmark.net/gesr/opamp/db_calc.htm

I graduated Channel Master's MATV/CATV course in the early 80s. They beat it into your head that 75ohm (TV) and 50 Ohm (everything else, at that time) had different calculations and that 78dBmV = 1 Watt = 30dBm in a 75 ohm system.

I went from that world into the military, where everything is pretty much based on 50 ohms, and you never forget that 0 dBm = 224mv(RMS) = 1mW and 30dBm was still one watt...

The pulsed power bench I worked in my first 6 years made it even harder, as duty cycle entered the picture, and peak power would be in the MegaWatts region, but RMS power was down in the Watts region.....

My Jerrold VHF-UHF Field Strength Meter is still the only instrument I own with a meter that will measure dBmV.
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