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24th November 2008, 09:54 #1
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dbi to dbm
Hi ,
I have a doubt. Please help me.
Actually, I am testing an patch antenna using sweep oscillator as transmitter and signal generator as receiver.
In the sweep oscillator, I keyin 10dBm in the power level "display" and also keyin 10dBm of "reference level" in the signal generator. In both equipments, I have keyin the frequency range from 1.4GHz to 1.5GHz.
I'm getting a waveform. I too getting value in dBm in the signal generator when transmitting from sweep oscillator but don't know how to calculate the gain......
Could anybody guide me how to calculate......
I need the answer in dBi.
Urgent!Urgent!Urgent!
Please help me.
Waiting for your reply.
Thank you.

24th November 2008, 13:30 #2
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dbi to dbm conversion
dBi or dB(isotropic) is the forward gain of an antenna compared to the hypothetical isotropic antenna, which uniformly distributes energy in all directions.
dBm or dB(mW) is the power relative to 1 milliwatt.
Therefore, are two different quantities. You have to calculate the hypothetical power trasmitted from an isotropic antenna in your situation and then you have to compare to the power trasmitted, in the same situation, from your real antenna. This ratio (in dB), power trasmitted (real antenna)/power trasmitted (isotropic antenna), is dBi of your antenna.
Regards
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24th November 2008, 13:30

25th November 2008, 03:20 #3
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convert dbi to dbm
Hi yuyu,
I need your guidance again.
According to my situation which I explained to you earlier, when I transmit at 10dBm in sweep oscillator , I'm getting around 60dBm in signal generator. In signal generator, to see the dBm value, I press Channel Power icon to see the dBm value and it shows 60dBm when transmits.
So to find the gain, do I need to calculate as below:
10 log Pout( in dBm) / Pin(in dBm) = ? dBi
Pout = 60dBm (at signal generator)
Pin= 10dBm ( at sweep oscillator)
So , if I follow the formulae above, I'm getting about 7.7 .
Should I assume 7.7dBi?
Please guide me .
I'm really need your help.
Urgent!Urgent!Urgent!
Waiting for your reply.
Thank you.
regards,
saran

25th November 2008, 03:20

25th November 2008, 14:32 #4
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dbi dbm
Dear saran7507,
you have to consider the Friis transmission equation: Pr/Pt=(lambda/4PgrR)2GtGr
Pr=power received from the patch antenna
Pt=power transmitted from the patch antenna
R=distance
lambda=wave length
Gt=gain of transmitting antenna (the patch antenna)
Gr=gain of receiving antenna (the term include the losses of the receiving system)
Pgr=3.14
You already know the first 4 terms. You have to know the Gr term. Then is it possible to calculate Gt:
Gt= (4PgrR/lambda)2 Pr/(Pt*Gr)
Gt (dBi)= 10 log Gt
Alternatively, if you consider an hypothetical isotropic antenna then:
Priso=power received from an hypothetical isotropic antenna (you have to calculate)
Priso=Pt Gr (lambda/4PgrR)2 Gtiso = Pt Gr (lambda/4PgrR)2
Gtiso=gain of hypothetical isotropic antenna=1 (0 dBi)
since
Pr=Pt Gr (lamda/4PgrR)2 Gt
the ratio between Pr and Priso is the gain of transmitting antenna
Pr/Priso= Gt
Regards
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25th November 2008, 14:32

26th November 2008, 03:24 #5
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convert dbi to db
Hi yuyu,
Thanks for giving me the solution.
Again, I have some doubt.
As you told me about the friis transmission equation, I don't understand the difference between Pr,Pt and Gr,Gt.
According to my situation in testing the 1.45GHz(centre frequency) patch antenna, I am using the sweep oscillator as transmitter by keying 10dBm at power level icon and using signal generator as receiver and I'm getting about 60dBm.
Question 1:
Normally to find the gain, do we use the formulae below:
Gain=10 log (Pout/Pin)
Since I'm looking for dBi in the gain, which gain equation should use? The formulae that u gave me or use the formulae above.
According to practical value above which are 10dBm and 60dBm, how should I find the gain in dBi?
Question 2:
Using the friis equation, where do I need to apply 10dBm and 60dBm. Is that Gr, Pr, Pt or ...? I'm confused.
Please reply me. I really need your help. Urgent!Urgent!Urgent!
Waiting for your reply.
Thank you.
Regards,
Saran.
[/u]

26th November 2008, 09:57 #6
dbm to dbi
antenna is one port device, it is different from two ports device which gain is defined as your description, gain=10db(pout/pin).

26th November 2008, 15:18 #7
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dbm dbi
Originally Posted by saran7507
and then you substract
17.7 10 = 7.7 dB?
I think you have a long road ahead of you....
That is the reason some people are recommending you
to read Balanis....it is a long explanation...too long for
a forum....
You need to read a lot, and take some courses in Antennas.

27th November 2008, 02:16 #8
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convert dbm to dbi
Hi yuyu,
Thanks for your answer.
Based on the data's that I gave you, from your point of view, how would you calculate to find the gain in dBi. Explain me in your way, please.
Sorry to trouble you.
Lastly, How would you recommend me to find the gain?
Please list down to me.
I would be very grateful to you if you help me.
Waiting for your reply.
Thank you.
Regards,
Saran

27th November 2008, 02:16

27th November 2008, 09:43 #9
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db to dbi conversion
Dear Saran,
I attached a file regarding your problem.
Regards
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