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converting dBm to uV/m



 
 
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  #1  
Old December 28th 05, 06:53 PM posted to rec.radio.amateur.antenna
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Default converting dBm to uV/m

Hey all,

I'm still skeptical about this formula:

Field Strength (uV/m) = 10 ^ ( (107 - |dBm| ) / 20 )

Is this valid? I saw this formula somewhere and jotted it down.

Thanks!

  #2  
Old December 28th 05, 07:04 PM posted to rec.radio.amateur.antenna
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Default converting dBm to uV/m

Ron J wrote:
Hey all,

I'm still skeptical about this formula:

Field Strength (uV/m) = 10 ^ ( (107 - |dBm| ) / 20 )

Is this valid? I saw this formula somewhere and jotted it down.

Thanks!

dBm is a measure of power, and the field strength as a function of power
is going to depend on antenna characteristics.

The equation you have is a pretty direct conversion between power in dBm
to voltage, with a magic proportionality factor (the 107) buried in the
exponent.

So I suspect it's a valid function, but only for one type of antenna.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
  #3  
Old December 28th 05, 08:48 PM posted to rec.radio.amateur.antenna
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Default converting dBm to uV/m

There's not enough information to calculate field strength. There's a
direct relationship between uV/m (field strength) and W/m^2 (power
density), providing you're in the far field and you know the intrinsic
impedance of the medium (about 377 ohms for free space). But you need a
distance from the source and the antenna gain in order to know what the
power density is for a given power (dBm). So at the very least, you have
to specify the distance from the source and the antenna gain to
calculate field strength from dBm. And then it's valid only in the far
field.

Roy Lewallen, W7EL

Ron J wrote:
Hey all,

I'm still skeptical about this formula:

Field Strength (uV/m) = 10 ^ ( (107 - |dBm| ) / 20 )

Is this valid? I saw this formula somewhere and jotted it down.

Thanks!

  #4  
Old December 28th 05, 09:22 PM posted to rec.radio.amateur.antenna
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Posts: n/a
Default converting dBm to uV/m

dBm significant the logarithm relation to Power (mW). In case of field
strength or voltage (mV) you have to multiplication with 20.

Take a look at:

http://www.sengpielaudio.com/calculator-volt.htm
http://www.nobbi.com/conversion.htm

and very good:

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

Regards,

Ralf
--
Vy 73 es 55 de Ralf, DL2MRB
E-Mail:
www.hamradioboard.de
  #5  
Old December 28th 05, 09:38 PM posted to rec.radio.amateur.antenna
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Default converting dBm to uV/m

On 28 Dec 2005 10:53:34 -0800, "Ron J" wrote:

Hey all,

I'm still skeptical about this formula:

Field Strength (uV/m) = 10 ^ ( (107 - |dBm| ) / 20 )

Is this valid? I saw this formula somewhere and jotted it down.


Your expression does not apply in general, it is the unstated
assumptions that are the issue (freq, receiver R, antenna factor.
bandwidth).

The calculator at http://www.vk1od.net/sc/FS2RPCalc.htm my be of
interest.

Owen


Thanks!

--
  #6  
Old December 28th 05, 10:04 PM posted to rec.radio.amateur.antenna
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Default converting dBm to uV/m

On Wed, 28 Dec 2005 21:38:46 GMT, Owen Duffy wrote:

On 28 Dec 2005 10:53:34 -0800, "Ron J" wrote:

Hey all,

I'm still skeptical about this formula:

Field Strength (uV/m) = 10 ^ ( (107 - |dBm| ) / 20 )

Is this valid? I saw this formula somewhere and jotted it down.


Your expression does not apply in general, it is the unstated
assumptions that are the issue (freq, receiver R, antenna factor.
bandwidth).


With an Antenna Factor of 0dB/m (eg 0dBi gain at 30.83... MHz), a rx
power of 0dBm/Hz in a matched receiver, would be the result of a field
strength of ~107dBuV/m/Hz. So, the lhs of your formulat is in units of
dBuV/m, and assumes AF=0dB/m and 1Hz bandwidth. AF depends on antenna
gain and frequency, and antenna / receiver R.

Owen


The calculator at http://www.vk1od.net/sc/FS2RPCalc.htm my be of
interest.

Owen


Thanks!

--
  #7  
Old December 29th 05, 11:57 AM posted to rec.radio.amateur.antenna
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Posts: n/a
Default converting dBm to uV/m

Everything you need plus!!!!

http://www.sengpielaudio.com/Calculations03.htm


Bill



"Ron J" wrote in message
oups.com...
Hey all,

I'm still skeptical about this formula:

Field Strength (uV/m) = 10 ^ ( (107 - |dBm| ) / 20 )

Is this valid? I saw this formula somewhere and jotted it down.

Thanks!



  #8  
Old December 29th 05, 11:32 PM posted to rec.radio.amateur.antenna
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Posts: n/a
Default converting dBm to uV/m

On Thu, 29 Dec 2005 06:57:40 -0500, "Bill"
wrote:

Everything you need plus!!!!

http://www.sengpielaudio.com/Calculations03.htm


A page full of calculators Bill, can you be more specific about a URL
for one that converts field strength to received power or vice-versa?



Bill



"Ron J" wrote in message
roups.com...
Hey all,

I'm still skeptical about this formula:

Field Strength (uV/m) = 10 ^ ( (107 - |dBm| ) / 20 )

Is this valid? I saw this formula somewhere and jotted it down.

Thanks!


--
  #9  
Old December 29th 05, 11:34 PM posted to rec.radio.amateur.antenna
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Posts: n/a
Default converting dBm to uV/m

On Wed, 28 Dec 2005 22:22:04 +0100, Ralf Ballis - DL2MRB
wrote:

dBm significant the logarithm relation to Power (mW). In case of field
strength or voltage (mV) you have to multiplication with 20.

Take a look at:

http://www.sengpielaudio.com/calculator-volt.htm
http://www.nobbi.com/conversion.htm

and very good:

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


Do any of these actually deal with field strength to receive power
conversion?

Regards,

Ralf

--
  #10  
Old December 30th 05, 12:07 AM posted to rec.radio.amateur.antenna
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Posts: n/a
Default converting dBm to uV/m

Owen Duffy wrote:

Do any of these actually deal with field strength to receive power
conversion?


At starting point I would say yes for those how know to use it.

Regards,

Ralf
--
Vy 73 es 55 de Ralf, DL2MRB
E-Mail:
www.hamradioboard.de
 




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