Antenna - Theory .com: The Antenna Site

Antenna Basics Antenna Types Smith Charts Antenna Measurements

antenna-theory.com :: View topic - convert dbi to watt
antenna-theory.com Forum Index antenna-theory.com
Antenna Theory
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

convert dbi to watt

 
Post new topic   Reply to topic    antenna-theory.com Forum Index -> Questions Related to Antenna Theory
View previous topic :: View next topic  
Author Message
btb4198
Antenna Theory Regular


Joined: 19 May 2011
Posts: 20

PostPosted: Sat Jun 25, 2011 4:59 pm    Post subject: convert dbi to watt Reply with quote

how do you convert a 9 dbi gain to watt?
and 3 dbm to db?
Back to top
View user's profile Send private message
Schubert
Antenna Wizard


Joined: 08 Apr 2009
Posts: 152

PostPosted: Sun Jun 26, 2011 5:24 am    Post subject: Reply with quote

9 dBi gain to Watts doesn't make sense. dBi is the gain of an antenna relative to a dipole.... It is a measure of how efficient and directive an antenna is, which isn't a function of how much power you input to the antenna.

3 dBm = 3 mW. Assuming "dB" means Watts in dB, then 3 dBm = 3 dBw - 30 = -27 dBW = -27 dB. Note, the 30 dB comes from:

1 W = 1000 mW
0 dB = 30 dBm
Back to top
View user's profile Send private message
R. Fry
Antenna Theory Regular


Joined: 06 Jun 2011
Posts: 30
Location: Illinois USA

PostPosted: Sun Jun 26, 2011 10:42 am    Post subject: Re: convert dbi to watt Reply with quote

btb4198 wrote:
how do you convert a 9 dbi gain to watt?

"dBi" is the unit of measure for the gain of an antenna relative to the gain of an isotropic radiator. The radiation pattern of an isotropic source is perfectly spherical. An isotropic radiator exists only in theory, but is used as a reference for the gain of real antennas.

The peak gain of a 1/2-wave, center-fed dipole in free space is 2.15 dBi, because due to its directional radiation pattern that gain is 2.15 dB (1.64X) greater than from an isotropic radiator.

The gain of an antenna sometimes is given with respect to that of a 1/2-wave, center-fed dipole in free space. In that case the unit of measure is "dBd." Gain in dBd can be converted to dBi by adding 2.15 dB to the gain expressed in dBd. So an antenna with a gain of 3 dBd has a gain of 5.15 dBi.

Some antenna manufacturers prefer to use dBi when specifying antenna gain because it results in a larger, more impressive number. But for the same antenna input power an antenna with 3 dBd gain in a given direction produces exactly the same radiated field as one with 5.15 dBi gain in that direction.

The power in watts that any antenna radiates is a function of the power flowing into its input terminals, and the gain of that antenna in the direction of interest.

Quote:
and 3 dbm to db?

The term "dB" has no specific meaning or value unless the unit of measure (watts, volts etc), and the reference value are known.

"dBm" as commonly used is the unit of measure for power expressed in decibels with respect to 1 milliwatt (mW). In this case the unit of measure (watt) is understood both by the writer and readers. More correctly the unit should be written dB(mW).

The equation for a power relationship in decibels is 10*log(P1/P2). Rearranging that equation to convert the power in 3 dBm to power to milliwatts...

10^(dBm/10) = 10^(3/10) = 1.995 mW (approx)
Back to top
View user's profile Send private message Visit poster's website
btb4198
Antenna Theory Regular


Joined: 19 May 2011
Posts: 20

PostPosted: Sun Jun 26, 2011 12:07 pm    Post subject: Reply with quote

so went you use the Friis Transmission formula and you want to get Pr in watt and you have gr and the gt in dbi what do you have to convert it to ? dbi to what watt, right?
Back to top
View user's profile Send private message
R. Fry
Antenna Theory Regular


Joined: 06 Jun 2011
Posts: 30
Location: Illinois USA

PostPosted: Sun Jun 26, 2011 3:20 pm    Post subject: Reply with quote

Convert the published gain for a free-space path in the direction of interest as given in dBi to a multiplier, using 10^(dBi/10).

As examples, a gain of zero dBi (an isotropic source) is a multiplier of 10^(0/10) = 1, and a gain of 2.15 dBi is a multiplier of 10^(2.15/10) = 1.64 (approx).

The gains of both the tx and rx antenna must be calculated for the Friis equation, and used for Gt and Gr, respectively.

Both the path distance R and the wavelength (lambda) must be entered in the same unit of measure.

Pt is the power in watts delivered to the transmit antenna, and Pr is the impedance-matched power in watts available at the terminals of the receive antenna, assuming that the polarization of both antennas is the same.

Doing the math for a 1,000 meter, 100 MHz free-space path with 100 watts applied to a tx antenna with 2.15 dBi gain, and a rx antenna with 6 dBi gain:

Pr = Pt*Gt*Gr*(lambda/(4*pi*R))^2
Pr = 100*1.64*3.98*(3/(12.566*1000))^2 = 0.000 037 watts, approx, or about 37 microwatts.

Note that this equation is not valid for path lengths less than several free-space wavelengths.
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic    antenna-theory.com Forum Index -> Questions Related to Antenna Theory All times are GMT
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group