Voltage & waveguide

Hi.
I have a question, I hope maybe it is very obvious, so excuseme.
Always we are converting power in dbm format to voltage in for example, dbv format, we must know the system impedance.
In cables, or other media we can simply say about system impedance. 50, 75 and so on.
But in waveguide, we can not say about absolute impedance of port.
Now my problem is, if I have to say a waveguide port signal amplitude in volt, and I know thats power in dbm, what formula I can use?
Thanks.

moreho said:

Hi.
I have a question, I hope maybe it is very obvious, so excuseme.
Always we are converting power in dbm format to voltage in for example, dbv format, we must know the system impedance.
In cables, or other media we can simply say about system impedance. 50, 75 and so on.
But in waveguide, we can not say about absolute impedance of port.
Now my problem is, if I have to say a waveguide port signal amplitude in volt, and I know thats power in dbm, what formula I can use?
Thanks.

Click to expand...

There are two basic problems in your question.
First, RF voltage can be reliably measured at frequencies not higher than 50 MHz. Therefore, power can be measured at any frequency, so all relations refer to Watts, mW, or dBm. Deriving voltage from such power measurements is possible if one knows line impedance but still is not too reliable.

Waveguides or hollow pipes have been found better transmission lines at microwaves. For a chosen propagation mode waveguide impedance varies with frequency but it is real and can be transformed into coaxial line impedance by transitions for a specified bandwidth. Again, measuring power can be done quite reliably in waveguides, and deriving voltage makes some sense when very high power transmission is needed.

Formulas you ask for are simply derived from Ohm¨s Law. Units "dBm" refer the power to one milliwatt, and if you know the impedance, you can derive voltage, current, etc. For waveguides the impednce can be calculated and the same formulas apply. In coaxial lines and waveguides, everything holds well if there are no inhomogeneities. Once there is anything like a capacitor, inductor or a post or an iris, the RF field is distorted, more at higher frequencies, so measuring RF voltage becomes strongly dependent on an exact position of test lines and also the test lines becme a part of the line. Measuring RF power is easier, so it is preferred.
From RF power, for instance, S-parameters are derived, to characterize any device, passive or active. Vector Network Analyzers and other instruments operate with S-parameters and power levels.

Now my problem is, if I have to say a waveguide port signal amplitude in volt, and I know thats power in dbm, what formula I can use?

Click to expand...

Counterquestion. Can you please specify what kind of waveguide port you are talking about? And how the port voltage is related to the wave propagated in the waveguide?

In some situations, a coax-to-waveguide transition is considered a waveguide port. The transition has of course a characteristic impedance (usually 50 ohms) and calculations are all the same as for a cable. Generally, you can design different kinds of couplers (or "antennas") to be placed inside a waveguide, exciting different waveguide modes. Impedance will be different, also port voltage for a specific propagated power. Because power is invariant to the properties of the used coupler, it makes sense to describe the excited wave in power units.

I put my two probes of my voltmeter across the waveguide flange, but I only read 0 volts! wonder why!

Power is measured by using a reference R and absorbed power is measured relative to 1mW for 0dBm , 1W for 0dB
And for voltage 0dBmv for 1mV

But a DMM cannot measure RF due series lead inductance and diode shunt capacitance not acting as a rectifier

SunnySkyguy said:

Power is measured by using a reference R and absorbed power is measured relative to 1mW for 0dBm , 1W for 0dB
And for voltage 0dBmv for 1mV

But a DMM cannot measure RF due series lead inductance and diode shunt capacitance not acting as a rectifier

Click to expand...

OH BOYS! PLease go and read a book on basic electromagnetism. Otherwise more discussion is a nonsense.

the point is: you can not measure voltage in a waveguide. Voltage makes sense in TEM structures, not in metallic waveguides. You CAN measure the electric field with an Efireld probe or, as mentioned, measure the power with a power detector. If you know the E vector field vs. position in the waveguide, you can integrate the field to get a "voltage", but what good is that? It is only a mathematical construct