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!
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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
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