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What it is Antenna gain? If the gain of the antenna is high, does that mean it has a big coverage area?

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  • \$\begingroup\$ What you say ?! \$\endgroup\$
    – Phil Frost
    Commented Jan 13, 2015 at 17:14
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    \$\begingroup\$ You started out OK apparently asking what antenna gain is, but what the heck is delimitation? What "job"? Antennas don't "have" dB, although their gains can be expressed in dB. Closing this mess. \$\endgroup\$ Commented Jan 13, 2015 at 17:15
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    \$\begingroup\$ High antenna gain means low coverage. \$\endgroup\$
    – Andy aka
    Commented Jan 13, 2015 at 17:22
  • \$\begingroup\$ This question is going to be closed unless you can clarify what you mean by "delimitation the antenna job" - what do you mean by "delimitation" and what "antenna job" are you referring to? \$\endgroup\$ Commented Jan 13, 2015 at 17:24
  • \$\begingroup\$ sorry the question edited now ... i mean if the antenna have high gain that should have a big coverage ? \$\endgroup\$ Commented Jan 13, 2015 at 17:36

3 Answers 3

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It seems you have general confusion about what antenna gain is.

"Gain" of a antenna, unlike what you might think from just the word, doesn't tell you how much power gets radiated as a function of the power driving the antenna. We assume the efficiency of the antenna is 1 in all cases. In other words, all real power sucked up by the antenna from its electrical feed gets radiated somehow somewhere.

"Gain" applied to a antenna is how much it radiates in a particular direction, relative to either a uniform spherical radiation pattern, usually called isotropic, or the radiation pattern of a basic dipole. Both are used, so usually "gain" should be qualified as being relative to isotropic or dipole if it's not already clear from context.

Since the total power radiated by a antenna is the same, "gain" tells us how much the antenna is capable of concentrating that radiated power in a particular direction. High gain antennas are threfore directional. If you send more power one way, you have to send less power some other way.

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  • \$\begingroup\$ ok , we know the antennas have a differences in gain! so, it's a primary condition to use this antenna in router for example ? i can say for example ... Antenna 1 [ 10 dB then used in cell phone ] Antenna 2 [ 3 dB then used in Router ]. thanks \$\endgroup\$ Commented Jan 13, 2015 at 21:35
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Yes, your question could use some improvement. However, might you mean "discrimination" or "limitation"? An antenna system can discriminate against RF signals that are out of band, (not within the resonant mode of the antenna) or of different polarations. The electrical length and Q of the antenna can determine the amount in db that it might discriminate an unwanted signal, (or in a way of speaking a limitation). This can sometimes be listed as a -Xdb value. Antennas with a low Q or with multiple length pieces might be made purposely to pick up a wider frequency range or to have a better "coverage".

An Antenna can be specified in db gain over a simple isotropic antenna (theoretically thought of as a small omnidirectional antenna). The gain comes from the fact that some antennas can pick up or transmit better in certain directions, polarities, or at certain angles. The gain is not from any power changes just the fact that the same energy is limited to or picked up from those specified directions, polarizations, or angles.

These reference may help:

http://en.wikipedia.org/wiki/Isotropic_radiator

http://en.wikipedia.org/wiki/Antenna_gain

https://books.google.com/books?id=8YDOAwAAQBAJ&pg=PA773&lpg=PA773&dq=antenna+discrimination+frequency&source=bl&ots=CXGE1KKAyt&sig=MwgGPrsZzapwM5a1-znm2et7gIQ&hl=en&sa=X&ei=J1y1VKHAJMq-ggTFnITIBg&ved=0CEAQ6AEwBQ#v=onepage&q=antenna%20discrimination%20frequency&f=false

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  • \$\begingroup\$ Thanks for you dear , but the main question is [ If Antenna Gain increase the coverage increase ? sorry about language. \$\endgroup\$ Commented Jan 13, 2015 at 18:42
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An answer to your refined question, as I understand it:

If antenna gain increases, then its coverage increases in some direction (usually in front of the antenna) and decreases in other directions (usually behind the antenna)

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