Hi! Is there a relationhsip between power density (Watt/m^2) and power spectral density (dbm/Hz) ? Thanks Suterr

# The relationship between power density and power spectral density

Started by ●September 14, 2005

Reply by ●September 14, 20052005-09-14

<suterr@gmail.com> wrote in message news:1126678071.725462.183830@g44g2000cwa.googlegroups.com...> Hi! > > Is there a relationhsip between power density (Watt/m^2) and power > spectral density (dbm/Hz) ? >One could say "not much" and one could say "oh, yes". Both should be fairly clear from their units. Take the output of an antenna as an example: A waveform received from the antenna at some point in space has power density (Watt/m^2). There is no mention of spectral content and the assumption may be it is broadband power (entailing a spectral character) or that it is narrowband power (with little or no spectral character). Given the same situation where there is a signal in the broadband sense then not only is there a power density (Watt/m^2) but there is also an interesting spectral density (dBm/Hz) associated with the power density. I suppose one could talk about Watt/m^2/Hz just to make the point. That's because Watts and dBm relate to the same measure but in different terms. /m^2 refers to power density at a point in space. /Hz refers to the spectral distribution. Watts/m^2 is a scalar measure over space due to the beam pattern and frequency response of the antenna and the input signal - in the example it sums over all frequencies. dBm/Hz is a scalar measure over frequency - in the example above it varies over space due the beam pattern and frequency response of the antenna. Watts/m^2/Hz is a scalar measure over space and frequency - in the example above it can be expressed as a weighted spectral plot at each point in space. Fred

Reply by ●September 14, 20052005-09-14

Not without knowing something about the physical problem. For example, if you know PSD and you know the area over which the power was measured, then you could compute power density by integrating PSD over all frequency and dividing by the area. --RY

Reply by ●September 14, 20052005-09-14

suterr@gmail.com wrote:> Hi! > > Is there a relationhsip between power density (Watt/m^2) and power > spectral density (dbm/Hz) ?It is no simple connection, you need to bring in some physics. The radio antenna "drains" some electromagnetic energy from the atmosphere, and this energy can be expressed in terms of W/m^2. Through the various physical processes that happen in the amplifier, this energy is converted to some electrical signal that has a PSD that can be expressed in terms of dbm/Hz. Very general, but that's about it. Rune