Dumb question perhaps, but why are calculated field densities so far removed from what my $200 RF meter indicates?
I am using the ED85EXS shown on this webpage:
It supposedly reads down to a few microwatts/m^2
Not top-of-the-range, but I had assumed it would be accurate enough for informal readings.
Robert Wade
Yes.
Because the calculated field strengths mentioned in this thread make certain assumptions. Such as, free space, no reflections, no earth, no water bag in the way, etc.
The meter is to be used to measure the field strength where it is placed, not to indicate the power level of a non-perfect source.
** The main assumption being made is that the signal source is not discontinuous.
Analogue signals like broadcast AM and FM have average carrier power levels that remain steady - but they are just about the only common ones that do.
... Phil
... Phil
Correct. But, drive along the highway and you might get fading, not just from distance, but from phase cancellation. This world is not a perfect place to do his sort of measurement. A special anechoic room for the measurements is advised.
I know you know this, Phil. This is really for the guy that doesn't get it.
Of course the confusing thing for "the guy that doesn't get it" is that the meter reads in mW/cm2.
Robert Wade
** I suspect you missed my meaning.
When the power level of a data transmitter is quoted in so many watts - is that the peak level or the average power output during operation ?
For example, SSB transmissions are quoted in "peak envelope power" cos there is no carrier.
Are not most data transmissions discontinuous ( eg GSM mobile phones) with the carrier being modulated off constantly ? That would create a disconnect between the quoted power in watts and the field strength picked up by an average responding instrument.
BTW: info on the signal format and modulation method used in Wi-Fi are not things that jump up at you with a Google search and my 50Mhz scope is not much help either.
... Phil
You haven't specified the frequency in use.
Any calculations using the inverse square relation assume far field, things in the near field can change quite dramatically.
Then there is the question, where is the limit between near and far fields. some put it about one wavelength, others about 1/2pi wavelengths or something in between.
For accurate near field measurements, you would have to make both magnetic and electric field measurements, since you can't assume the free space impedance to be 120pi = 377 ohms.
I did miss your meaning. And you're right about the rest, as usual. I don't understand all those communications methods so this is where I stop. Thanks.
Where does it say that? I see mentioned: Max, Hold, and mw/m2, v/m, dBm unit display High sensitivity : ?65dBm to 5dBm (8.2mV/m to 26.2V/m) Peak power density measurement : 0.18uW/m^2 to 1.8W/m^2
Even the little picture of the display shows uW/m^2
No mention at all of cm or cm^2.
In any case, the conversion is 10,000 cm^2 per m^2.
Thanks for pointing that out. My mistake.
Robert Wade
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