How to measure electric field with a network analyzer?

[squalho]

Hello,

This is my problem: I have a network analyzer, that is capable of measuring the S-parameters of a 2 ports network. Let's say that I connect antenna #1 (a standard half-wavelength dipole) to port #1 of the NA, and antenna #2 (same type of dipole) to port #2 of the network analyzer. Cinsidering only 1, fixed frequency, S11 tells me about the reflection coefficient of antenna #1 and S22 the same for antenna #2. S21 (or S12, they should be the same) would give me an idea of how much power goes from port 1 to port 2 (or viceversa) for that specific frequency, i.e. it should allow me to measure the electric field received by antenna #2, in both magnitude and phase.

I know that S21 is somewhat proportional to the electric field, because its magnitude decreases as I go farther, and its phase changes with the position too, showing 360 degrees changes for every wavelength of distance between the antennas.

So my question is: given S21, in both magnitude and phase, how can I find E, magnitude and phase?

Any suggestion is greatly appreciated!

 

[sophiecentaur]

If you have two antennae (transmit and receive) then S21 will only show the combined effect. It won't actually tell you what's going on with the fields in between unless you can characterise / calibrate the antennae fully, first.

 

[berkeman]

Hello,

This is my problem: I have a network analyzer, that is capable of measuring the S-parameters of a 2 ports network. Let's say that I connect antenna #1 (a standard half-wavelength dipole) to port #1 of the NA, and antenna #2 (same type of dipole) to port #2 of the network analyzer. Cinsidering only 1, fixed frequency, S11 tells me about the reflection coefficient of antenna #1 and S22 the same for antenna #2. S21 (or S12, they should be the same) would give me an idea of how much power goes from port 1 to port 2 (or viceversa) for that specific frequency, i.e. it should allow me to measure the electric field received by antenna #2, in both magnitude and phase.

I know that S21 is somewhat proportional to the electric field, because its magnitude decreases as I go farther, and its phase changes with the position too, showing 360 degrees changes for every wavelength of distance between the antennas.

So my question is: given S21, in both magnitude and phase, how can I find E, magnitude and phase?

Any suggestion is greatly appreciated!

Are your antennas far enough apart that the RX antenna is in the far field of the TX antenna?

If you can set the drive level for your TX antenna, then you should be able to read off the RX level, which can be converted into the equivalent E-field.

 

[waht]

S parameters is a ratio between two voltages, or ratio between two electric fields.

If you want to measure absolute 'electric field' you need to measure power with either a power meter or a spectrum analyzer.

 

[squalho]

Thanks everybody for your answers. Addressing them one by one:

sophiecentaur: what do you mean with caracterize/calibrate the antennas? I can calibrate the cables up to the last connection before the antenna, but I don't think that's what you mean. Can you be more specific?

what: I have a spectrum analyzer, but it only tells me the power (i.e. magnitude square) of the field, while I'm interested in magnitude and phase (complex phasor).

berkemen: The antennas are at any distance, they could be far or near field. I can set the level of TX and I'm guessing I can read the level of the RX. Do you mean reading magnitude and phase of the ports A, B, R? How do I go from that to E_rx=magnitude+phase [V/m] ? Just using the impedance of the antenna?

 

[berkeman]

berkemen: The antennas are at any distance, they could be far or near field. I can set the level of TX and I'm guessing I can read the level of the RX. Do you mean reading magnitude and phase of the ports A, B, R? How do I go from that to E_rx=magnitude+phase [V/m] ? Just using the impedance of the antenna?

It would have to be in the far field, and you would need to have the Antenna Factor (AF) graph of the antenna versus frequency (which you could get from the antenna manufacturer, or maybe calculate for yourself for standard antenna geometries). The conversion is about 1/2 way down this page:

http://www.radioing.com/eengineer/convert.html

.

 

[squalho]

I understand the antenna factor relationship, but I don't think it's what I'm looking for.

First of all, that relation involves AF (suppose I know it from the manufacturer), E (what I want to find) and V (voltage at the output of the antenna). So none of that information is something that I can get from the network analyzer, that is my goal.

Second: that relation does not work with complex quantities, because the antenna factor is in terms of magnitude only (usually expressed in dB), i.e. I don't know what the antenna does to the phase of the signal.

Third: even giving up the NA and just trying to measure the voltage with an oscilloscope let's say, I can't measure V in both magnitude and phase easily: let's say I work at 3GHz, every 10cm of cable add me a 360 degrees rotation in the phase, so I will never be able to know precisely the phase exactly at the output of the antenna.

Does that makes sense?