Digital Watchdog Starlight D1363D Technical Information PDF herunterladen (Seite 89)

DS they affect it negatively, while for OFDM they have a more positive effect. Anyone who's set up a

long point-to-point across a hot flat expanse of desert using DSSS has probably seen this effect

demonstrated very vividly.

Your Fresnel zone is a cigar-shaped area that is widest in the center. A number of wireless calculators

exist on the Internet, including one linked from http://www.pananix.com/ that will calculate your

Fresnel zone and more. It is difficult to envision this Fresnel zone, so if you think something might be

encroaching, it probably is. If your signal strength doesn't meet your calculations (within one or two

db), move your antenna(s). Your worst Fresnel environment will be shooting down a long, narrow

steep-sided valley with lots of trees or through a gap between hills with similar characteristics (that hole

in the trees midway between your antennas also qualifies).

SNR

A number of other things may confuse you about signal levels and SNR. Radio signals are normalized

to a decibel scale. But this scale is logarithmic, not linear. SNR stands for signal to noise ratio. Ratio,

in mathematics usually means divide, but since we are using a logarithmic scale, we subtract. So the

difference between receive sensitivity (or noise in a high noise environment) and signal at the receiver,

will translate into a signal-to-noise ratio.

What is a good SNR? Well, this will depend on a number of things. And apparently a large number of

folks don't know what constitutes a good SNR. One Internet site which shall remain unnamed, has

pretty colors to depict good, marginal, and unusable (green, yellow, and red respectively) signal levels.

But according to this site, all my links are unusable. They show green for links over 25db SNR, yellow

from 15db to 25 db, and red below 15db. Wow, I wish I could run 500W using huge dishes on my 30

miles backhauls (some of which only dream of seeing 15db and by this site are considered unusable). I

consider 10 a good, solid number for calculations purposes and will run any link over 6 db without

worry. While 3db might get through, 2db is a dead link, so you'd be advised to put a bigger antenna on

the low SNR side. Most of my links run between 10-15 db SNR, although I have one long backhaul on

a frequency I'm legally restricted to 1 W output at the antenna (EIRP – Effective Isotropic Radiated

Power). This link normally sees an SNR of 7-8 during the worst part of the day and is a critical link. It

has run reliably for months.

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Digital Watchdog Starlight D1363D Technical Information Seite 89