[rescue] misc old Sun resources

[James Lockwood]

On Sun, 21 Jul 2002, it was written:

> Some people have reported success in placing two antennas connected
> with a cable and using that to bounce the signal around an
> obstruction.  This is good if it works because you do not need
> electricity running to the tower.

It's almost never worthwhile to do this.

Receive power = Transmit power + antenna gain - cable loss - freespace loss

Say, for example, you want to have a 0.5mi point to point link (2500' give
or take) and you're using nice commercial 24dBi parabolics with good
cabling:

 17dBm output power (50mW)
 24dBi parabolic antenna
 -3dB loss due to cabling/adapters (conservative?)
=38dBm EIRP

freespace loss = 96.6dB + 20log(dist in miles) + 20log(freq in GHz)
=96.6 - 6 + 7.6
=98.2

so receive level = 38 - 98.2 - 3 (losses) + 24 (receiver gain)
=-29.2dBm

Ok, this is not too bad.  Reliable 11Mbps 802.11b reception requires
around -80dBm, a 50dB margin.

Now, throw a repeater in the middle (using hypothetical 15dBi Yagis).
You now have _two_ freespace losses, at 92.2dB each (halve the distance,
you gain 6dB).  Factor in 15dBi of gain and you get:

 38dBm - 92.2 + 15 = -39.2dBm

at the midpoint in between your two Yagis in the repeater.

Ok, this goes out the other Yagi gaining 15dB (I'm assuming you can get
that with all losses factored in, pretty big assumption):

 -39.2dBm + 15dBi = -24.2dBm EIRP

but you lose for the second stretch of air (another quarter mile).
Assume your endpoint has another nice 24dB parabolic:

 -24.2dBm - 92.2 + 24 - 3 (losses) = -95.4dBm

You can't receive with this, at least not reliably.  Noise kills you when
your transmit signal is less than a tenth of a microwatt (-39dBm).

Going to 24dBi parabolics in the repeater gives you an extra 12dB total.
-83.4dBm.  This might barely work but is definitely on the marginal side,
and this is only 2 quarter-mile runs and does not factor in loss inside
the repeater itself.  We've lost 53dB just to the repeater with high
quality antennas. In other words, if you lose less than that going through
an obstacle, a repeater is a bad idea.  Trees hit you for about 0.35dB/m
when covered with leaves, so you could afford to punch through 500' of
dense forest before a passive repeater would be a viable workaround.

There is one case where a passive repeater can work, and that is where it
can be mounted relatively close to one end of the link.  Rather than 2
1250' links, if we have a 250' link and a 2250' link (again, assuming
24dBi parabolics everywhere):

38dBm EIRP - 77.7dB (250' loss) + 42dB (net repeater gain) - 95.2dB (2250'
loss) + 21dB (receiver gain) = -71.9dBm

You quickly see it is much better to put the repeater close to one end
than at a midpoint.  Unfortunately that is where it is the least useful.
Even here transmit power within the repeater is only 250 nanowatts
(-36dBm).  You had better have perfectly shielded low loss cable at that
signal level.

Running high transmit power with a parabolic can give you over 50dBm EIRP,
but you have to boost the signal at both ends to see a benefit. Amplifying
your received signal does little good as you boost noise at the same time
as signal.

-James