> > OK, Jim, I'm not an RF expert like you, but this just seems contrary to
> the
> > laws of physics to me.
> >
> > The 36.6 term is a scale factor to account for the specific units chosen.
> > I'll trust you figured this out correctly. The 20*Log(D) term says the
> > signal is attenuated a fixed multiple for each unit distance. I was
> > expecting the inverse square law here.
>
> Recall that 10 log x^2 = 20 log x
> The inverse term is effectively linearised by the log.
>
> >The 20*Log(F) term says that there
> > is an additional fixed attenuation multiple for each unit increase in the
> > frequency. That makes no sense at all.
>
> This is to do with effective aperture size of the antenna relative to the
> wavelength..
> Think of an isotropic radiator painting a sphere with a signal radiating in
> all directions. As you concentrate that signal onto a segment of the sphere
> you get "gain". The physical size of the antenna plays a part in how large
> (or small) a part of the sphere the aerial directs the signal onto. For a
> given aerial size (eg a 1 metre dish) the gain is related to how many
> wavelengths the dish is across. If you double the frequency and halve the
> wavelength you get 4 times the area and the area illuminated by the dish
> reduces accordingly. The area effect again gives you a 20 log term rather
> than 10 log which you would otherwise expect.
>
> > Did you perhaps copy the equation for something like signal loss in a
> > transmission line or resistive media? The original poster was asking
> about
> > RF communication which I took to mean propagation between antennas. I was
> > expecting equations for the propagation of electromagnetic waves in free
> > space. What am I missing here?
>
> Free space is effectively a resistive media for these purposes once you are
> beyond the near field (ie once the E & M fields have "settled down" to a
> constamnt relationship. Interestingly (and irrelevantly to this particlular
> problem) for the large satellite and deep space dishes, the near field
> exytends to beyond the earth's atmosphere, so far field measurements are
> "difficult".
>
> Hmm - 2am - bedtime - wonder if brain is still working - AVR circular buffer
> code seems to run so maybe it is - if any of the above is gobbldey gook
> please take 2 asprin and call me in the morning. G'night.
>
>
> Russell McMahon
>
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