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Topic 

frank.mathieu@GQ
Germany
24 Posts 
Posted  09/01/2021 : 02:08:39

Good morning,
this topic may once more not make sense to people who are familiar with Terms, Units and Formulas in Electromagnetism. Please excuse my ignorance.
I try to use EMF390 as an example from which I can conclude how Voltage, Amperage, Power and Resistance interact.
Using measured or given values, I try to conclude the other values. I hope that the formulas R=U/I and P=U*I can be used.
Please let me start: a) are the above formulas applicable?
Do technical specifications or approximations exist for EMF390 in regards to: b) Resistance(s) [of the Antenna(s)] c) Voltages d) Amperages
As an example e) is it possible to read or calculate how many Volts or Amperes are measured for a power of 1mW = power density of 10mW/cm²?
I am aware that the data I ask for is specific to EMF390 and can be very different in other devices.
My thanks in advance. 

Reply #1
Damien68
France
668 Posts 
Posted  09/01/2021 : 06:41:45

hello Frank,
a):Yes and no,
U = RI is true in DC in AC, this remains true if the load is purely resistive, that is to say that the current flowing through a load must remain in phase with the voltage at the terminal of this load.
in RF, for a load, we no longer speak of resistance but of impedance. an impedance is represented by a complex number which has:  a real part which is the resistance part of the load  and an imaginary part which is the reactance(capacitive or inductive part of the load).
we therefore denote this impedance Z which is a complex number. Z = R + jX R is the resistive part of this impedance, X is the reactive part of it.
and we have the following formula which links u(t) to i(t): u(t) = Z.i(t) u(t), and i(t) are whose complex functions with a real part and an imaginary part under the form:
b) I don't know how the EMF390 is made but often for this kind of device the input impedance of the RF detector (the chip which measures the power) is around 50 ohms. the antenna must have a different impedance, and must be matched to RF detector input impedance, there must be components (capacitor, inductor) to match the two impedances.
c,d,e) for a measured signal at 1mW, if the input impedance of the detector is indeed 50 ohms: at the input terminals of the RF detector there should be a voltage of approximately:
= 224mV
and a current of:
=4.5mA
but we could not measure these levels with a voltmeter or even with an oscilloscope, it would be necessary that the frequency of the signal measured is in the bandwidth of the measuring device, and the probes used always present parasitic capacitance who would crush the signal provided by the antenna. to measure this power the EMF390 must have dedicated circuits called an RF detector which is technologically very advanced. 
Mastery is acquired by studying, with it everything becomes simple 
Edited by  Damien68 on 09/01/2021 07:12:46 



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