On Sep 21, 1:47 am, Jeff Liebermann <je...@cruzio.com> wrote:
> nevtxjus...@gmail.com hath wroth:
>
> >On Sep 21, 12:47 am, Jeff Liebermann <je...@cruzio.com> wrote:
> >> kaon <kaon.2x8...@no-mx.wirelessforums.org> hath wroth:
>
> >> >"Much higher free space loss"
> >> >you mean compared to lower frequencies right?
>
> >> Correct. As the frequency goes up, so does the free space losses.
> >> <http://www.terabeam.com/support/calculations/free-space-loss.php>
> >> Double the frequency, and the free space loss goes up 6dB, which
> >> cancels the the 6dB gain increase in the antenna gain.
> >Not so.
>
> >The total path loss does indeed increase (as shown by on-line
> >calculator above), but not due to free space loss; rather less signal
> >is captured by the antenna.
>
> Try again.

I'll rephrase it this way.... The effective free space loss does
indeed increase (as shown by the on-line calculator above), but not
due to attenuation traveling through free space; rather less signal is
captured by the antenna.

>The free space loss calculation does not in any way
> involve the antenna. It is the loss between two isotropic antennas at
> a given frequency and a given distance only. If there were anything
> involving the antenna (capture area, aperature size, gain, beamwidth,
> etc), then it would appear in the free space loss formulas and
> calculations. They don't. There's no antenna gain in any of the
> formulas on this page:
> <http://en.wikipedia.org/wiki/Free-space_loss>

Ok, slow down...I don't think were on the same page yet.

Its not obvious in the formula for the on-line calculator, but if you
examine how that formula was derived, you can see how antenna capture
area was used.

On http://en.wikipedia.org/wiki/Free-space_loss, read down to the
Physical explanation section and you'll see "The second effect is that
of the receiving antenna's aperture, which describes how well an
antenna can pick up power from an incoming electromagnetic wave...Note
that this is entirely dependent on wavelength, which is how the
frequency-dependent behaviour arises."


> What do you mean by "total path loss"? .... .... If you
> want to measure your path loss between the antenna ports, that's fine,
> but it's NOT the free space loss.

See above. Since just about all of my equipment is similar, I don't
need to input any variables such as different antennas, I'm just so
used to saying "total path loss" to describe the free space
attenuation. Sorry for the confusion.


> Quiz: You have a broadband dish antenna at both ends of a fixed
> 2.4GHz link. Shove 0dBm into one antenna and you get -60dBm at the
> other end of the link. Now, double the frequency to 4.8GHz. What's
> the signal level at the other end?

Unwittingly, that could be a trick question.

If you had a unity gain quarter-wave antennas at the receiving end and
doubled the frequency,
the answer is -66 dBm as the 4.8 GHz antenna has one fourth the
capture area.

If you had increased the gain of the receiving antenna by making it
four times longer (as in a four element co-linear array), you increase
the capture area by four times, so the answer would be -60 dBm.

Since you didn't specify that you used a smaller antenna when you
doubled the frequency, rather retained the same square area and thus
increased the gain by four times, the answer is -60 dBm.