Digital Watchdog Starlight D1363D Technické informace stáhnout pdf (Strana 88)

signals much better.

In the field, nothing beats a good pair of ears. I've seen folks install wireless systems, find the link is

weak, and decide the best thing to do is slap an amp on. While their signal improved, for the marginal

improvement, they added greatly to the ambient noise, probably interfered with a like-minded operator

who will also just slap an amp on, and no one wins. When you use an amplifier, you amplify noise as

much as you do signal. You can also cause signal distortion through signal clipping at the receiver

when it is overdriven. These radios are designed to work well with low signal levels.

Choosing your antenna correctly is nothing more than understanding how radio works and applying

calculations to ensure your link will have good signal levels before you install.

If you are unsure of how important antennas are, try running your calculations (see Calculations,

below) using different gain antennas and at different power levels. This should give you some idea.

For example, a 6 dbi antenna listening to a 4 watt signal will receive a signal of -80db at 3.2 miles

away. But change that to a 30 dbi antenna listening to the same 4 watt signal and you will receive

-80db at 50 miles. Assuming receiver sensitivity of -86 in both cases, each has an SNR of 6db. If this

paragraph made little or no sense to you, keep reading.

Calculations

Before performing your calculations, there are a few terms you should understand. These will be

explained below. They include Fresnel zone, SNR, and more.

Environment

If you are using DSSS (direct sequence spread spectrum) technology (most 2.4GHz 802.11b radios),

FHSS (frequency hopping spread spectrum), or even OFDM (orthogonal frequency division

multiplexing), you should understand a little about how they work beyond the marketers hype and

detractors FUD. Each has its strengths and weaknesses. None is a silver bullet. All work best in a true

unobstructed line-of-sight environment. While OFDM can work in some situations without line of

sight, this is not optimal, and requires an understanding of what constitutes an optimal near line of sight

or even non line of sight environment. This text will not go into that detail, but will tell you that if all

you can see between the near antenna and the far antenna is green (or for our northern neighbors where

that white will turn to green in the summer), you have a non-optimal environment. Optimal links are

over water. Very long hauls can be made over water due to refraction of the radio waves by the water.

These links should run better than your calculations.

Fresnel Zone

When looking at line of sight, you must keep a few concepts in mind. While you might be able to see

the remote antenna, your local antenna might not be able to. Radios waves in the GHz frequency range

travel in straight lines, but these lines need room. Remember, these are waves not rays. Waves require

some horizontal and vertical room while traveling. So shooting through a hole in the trees ½ a mile

away will not work. The room these waves require is called a Fresnel (pronounced frah'nel) Zone. For

our purposes, we calculate only the first Fresnel zone. The third and fifth also affect the signal, and for

1 2 ... 83 84 85 86 87 88 89 90 91 92 93 ... 101 102

Žádné komentáře

Digital Watchdog Starlight D1363D Technické informace Strana 88