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!

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

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!

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

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!
--

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!
--

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!

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!


--

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
--

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

converting dBm to uV/m
 

"Ron J" wrote
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.
_______________

uV/m = 10^[(dBm - G + 20*log(F) + L +75)/20]

where

dBm = signal level at receiver input in dB with respect to 1 mW
G = receiving antenna gain in dBd
L = line loss in decibels
F = Frequency in MHz

RF

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.

Hi Ron,

To answer your question - No.

For one its units do not balance. It is not finding µV/m it is simply
and incorrectly converting to µV. There is no meters to be found on
the right hand side of the equation. The 107, as has been pointed
out, is simply the number of dB below 1mW to a 1µV signal across 50
Ohms. Even here the formula is in error as the sign would be wrong
(the dBm must be presumptively loss expressed as a positive number).
To complicate it further is the artificially forced absolute value
which invalidates any Field Strength (sic) greater than one milliwatt.

You were right to be skeptical, it is nonsense.

73's
Richard Clark, KB7QHC

converting dBm to uV/m
 

On Fri, 30 Dec 2005 01:07:46 +0100, Ralf Ballis - DL2MRB
wrote:

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.

Perhaps you know how to use it and can explain how... it isn't self
evident.


Regards,

Ralf
--

converting dBm to uV/m
 

On Wed, 28 Dec 2005 22:04:40 GMT, Owen Duffy wrote:

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.

Sorry, whilst looking into the rhs I overlooked the 10^x/20... the lhs
is in uV/m... but the formula does assume an Antenna Factor of 0db/m
(and 1Hz bandwidth if talking about power densities).

Trying to find the minimal correction to your formula to make it work:

Field Strength (uV/m) = 10 ^ ( (107 + RxPwr + AF) / 20 )

Whe
o RxPwr is in dBm
o AF (=FS/Vin) includes matching effects and is in dB (1/m) (or dB/m
for short)
o same bandwidth applies to both sides for continuous spectra


For a matched antenna in a 50 ohm system under plane wave conditions:

AF=20*log(9.73/lambda/gain^0.5)


Owen
--

converting dBm to uV/m
 

Owen Duffy wrote:

Perhaps you know how to use it and can explain how... it isn't self
evident.

Perhaps I can explain but it isn't necessary because you got the answer in
your last writing.

Regards,

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

converting dBm to uV/m
 

It is a little like asking how to convert giraffes to file cabinets. THere
is no conversion factor from dBm to microvolts/meter. One is a power. One
is a field strength.


Jim



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.

converting dBm to uV/m
 

On Sat, 31 Dec 2005 09:44:45 -0800, "RST Engineering"
wrote:

It is a little like asking how to convert giraffes to file cabinets. THere
is no conversion factor from dBm to microvolts/meter. One is a power. One
is a field strength.

Hi Jim,

I would have to disagree with that. Field Strength is naturally
assumed to be a constituent of Free Space, or at least in a path
largely defined with an impedance of 120 · pi Ohms. Volts and Current
are convertible through this. dBm is likewise related to an
impedance. It is classically 50, 75, 200, 600, or 1000 Ohms depending
upon industry usage. Without this understanding (often implicit
within those industrial applications) the comparisons of dBm citations
would become quite strained (for instance, computing gain/loss).

If posed this same query of translating dBm to Field Strength, I would
presume the underlying basis of comparison would be that same 120 · pi
Ohms. This is hardly a stretch given Field Strength can also be
described in units of power and area. The proper usage in Field
Strength context would be
dB(re 1mW/m²)
where dBm is frequently a short-hand for
dB(re 1mW into 600 Ohms)
or
dBm(600)
where, perhaps the user actually meant
dBm(50)
Hence, a bald citation of dBm carries a lot of implicit baggage.

Suffice it to say that there are a world of dB conventions for
expressing relationships.

The simple lesson learned from the original question is that when both
sides of the equation do not balance in their units; then that
equation is incorrect. There were more errors than that, but the lack
of balance was sufficient to condemn its usage.

73's
Richard Clark, KB7QHC

converting dBm to uV/m
 

Richard Clark wrote:
"---where dBm is frequently a short hand for dB (re1 mW into 600
Ohms)---."

I agree, but I`ve worked with communicatiobs systems in which the
builder specified signal (test-tone) levels throughout in dBm regardless
of the impedance at the particular point. Incinvenient, maybe, but often
the conversion factor is specified for reading with a dB meter
calibrated for 600 Ohms.

dB is a power ratio. dBm is a power level for which the 0 dBm reference
is 1 milliwatt. The impedance is not specified. 2 milliwatts is
approximately +3 dBm and 1/2 milliwatt is approximately -3 dBm.

The a-c voltmeter which reads in dB is likely calibrated at 600 Ohms
impedance. Volts vary as the square root of the power. +6 dB is 4X the
power but only double the voltage.

dBm is frequently specified as a test-tone level at various points
throughout a communications system. The dB meter will only be correct
when the impedance at the measurement point imatches the impedance for
which the meter is calibrated. When the impedance is different, The
system builder will often give a correction factor to be used with a
600-Ohm dB meter.

Knowing the power level in milliwatts, the a-c volts are easily
calculated from the square toot of PR.

Best regards, Richard Harrison, KB5WZI

-------------------------------------------------------------

Sorry for bumping up the post, but I have the same question and there are some explenations after the above post. Neither confirmed that the equation above is the right one to calculate uV/m into dBm.

I have a same question but in the other direction. I have V/m and want to convert it to dBm does anybody know the answer to this?

I have already looked at this site:
http://www.sengpielaudio.com/calculator-volt.htm

But it is only possible to convert V into dBm and not V/m into dBm.
Or is V the same as V/m? I don't that those are the same.

The reason why I ask is because the regulations of my country says that the E = 30,7 V/m may not exceed for transmitting between 2-10GHz. No where in that document I found the maximum allowed transmitted power in dBm.

grtz


Kristof
(making a study about LTE radio link budget)