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!

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

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!

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!
--

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!
--

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
--

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

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
--

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

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
--