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Friis Transmission

 
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btb4198
Antenna Theory Regular


Joined: 19 May 2011
Posts: 20

PostPosted: Fri May 20, 2011 5:06 pm    Post subject: Friis Transmission Reply with quote

In Friis Transmission Formula, How do I determine what my Gt and Gr is ?

like what values there are

can GT be 0? like what if I do not use an amplifier to send the singal?
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Fri May 20, 2011 5:28 pm    Post subject: Reply with quote

Gt can be 0 dB, but if it is zero that means it radiates nothing.

See this page on conversion between linear and dB units for Friis Transmission Formula:
http://www.antenna-theory.com/definitions/decibelMath.php

Gain is just efficiency times directivity. The directivity must be greater than 1.0 (0 dB), by definition:
http://www.antenna-theory.com/basics/directivity.php

Efficiency is the ratio of power radiated to power delivered to the antenna. Hence, if the gain is zero, then the efficiency is zero.

The values are very antenna dependent. For instance, large dishes can have a gain of 40 dB (10,000). Cell phone antennas can have a gain as low as -3 dB (0.5). You have to know the application.

For WIFI, routers have antennas with efficiency about -1 dB, and directivity of +3 dB (approx). So the gain is +2 dB. For wifi in handheld devices, the gain is lower, maybe -2 dB (a guess).
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btb4198
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PostPosted: Fri May 20, 2011 6:36 pm    Post subject: Reply with quote

wait so Gt and Gr are not the Voltage gain from an op-amp?

gt would not be Vout/Vin ?
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bigSteve
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PostPosted: Fri May 20, 2011 6:47 pm    Post subject: Reply with quote

No, any gain from electronics would be factored into Pt: transmitted power.

The Friis Transmission equation assumes you have some power being supplied to the antennas, then calculates received power delivered tot he receiving antenna. Friis only deals with physics of propagation and factors in frequency, separation (R) and antenna gains to relate transmitted power and received power.
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btb4198
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PostPosted: Fri May 20, 2011 6:58 pm    Post subject: Reply with quote

so Friis Transmission equation does not factor in op amps?
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bigSteve
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PostPosted: Fri May 20, 2011 8:25 pm    Post subject: Reply with quote

See previous response Smile
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