RE: [amsat-bb] Beginner birds

[Bob Bruninga <bruninga@xxxxxxxx>]

> > ...why is the path loss lower in Mode A than in Mode B/J?

The equation for path loss is:

[ 4 * Pi * R ]2
[------------]      Where R is the Range in meters.
[ wavelength ]

So space loss goes down as the square of wavelength.  That is why 10m is
23 dB better, and 2m is 9 dB better than 70 cm and so on.  This
equation is fundamental to all satellite communications.  It is EASY to
figure out a link.  The power received at a receiver is just the power
transmitted plus antenna gains minus all losses.

Of course, that relationship assumes the SAME kind of antenna as the
baseline (lets say a dipole).  It is obvious when you think about it
because as Tony said, a 10m dipole collects the signal from 200 times
more area then a 70 cm dipole.

But conversly, it is EASIER to get high gain as you go up in frequency.
In fact you get it all back if your antennas cover the same AREA.  Thus
a 10' TV dish will have the SAME link budget to a satellite using an
OMNI antnenna no matter what frequency is used... (within reason
Usually it needs to be 10 wavelengths across ore more))...

Tony said it this way...

> The capture area increases as the frequency is lowered.  2m has 9x the
> capture area of 70cm, and 10m has over 20x the capture area of the
> equivalent 2m antenna.  This adds considerably to the link budget.

Bob


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