# How to choose an antenna?

• posted

Hello, I?m trying to find a good antenna to extend the range of my wireless network. I don?t know how to choose one. What im trying to figure out is the maximum distance that a particular antenna can radiate signals according to its transmitting power, antenna gain, and frequency. :huh:For example im looking at an antenna with a power input of 200mW and a 7dBi gain at 2.4GHz. That is all the information that the manufacture gives.:mad:

• posted

Without any specifics to go on, I'll toss this out.

The manufacture should at least say if its an omni-directional or directional antenna. If the picture looks like a two or three foot long piece of PVC pipe, its an omni.

How far do you want to extend your range? A 7 dB gain antenna will double your range (6 dB = 4x gain over a 0 dB antenna and

4x gain = 2x the range), assuming it a clear line of sight (not counting in Fresnel zone clearances).

200 mW input (maximum allowed power input?)?? Unless the antenna is made of 1 micron gold plating on tissue paper, it should be able to handle several watts of input power.

200 mW into the antenna with a 7 dB gain antenna will give you an effective radiated power of one full watt, or double your range.
• posted

You want to map out your whole system from where you are to where you want the signal to reach. Then you add in all the known elements and see how much radio power and antenna gain you need. This is called a "link calculation or power budget".

More explanation and calculators on this site:

Read up on things there, then go to their calculators and start plugging in your numbers. You will soon see how much gain you need to get a 20 db signal to noise (minimum). Decisions about whether to get an omni or directional antenna and how narrow of beam will depend on whether you are going point to point or point to multipoint. Sort of like, do you need a laser pointer, a spotlight, a flood light or just an open light bulb.

A big issue that is often missed when just buying antennas from stores or even online is the attached cable, if there is one. I avoid attached cables, unless VERY short, as they introduce loss. The specs on the antenna never include what is lost in the often cheap, thin cable.

One thing: At the linked site above, their rule of thumb on cable loss is not right. Go here for accurate calcs on antenna cable loss: