question about dipole antenna measurement

[KevinCheng]

dipole impedance folded vs open

If i want to measure the impedance/S11 of a dipole antenna, we should connect the dipole to the Baluns and connect it to the network analyzer. However, do we require to calibrate the network analyzer to the baluns differential output port? (connect the network analyzer to the input port of the baluns and the differential output port is connected to the dipole), if yes, i wonder how we calibrate the network analyzer by using the standard 50ohm, short, open calibration kit?

In previous post, it seems that somebody mention about "Once the balun is ready, to design a TRL calibration kit and enter its data into the network analyzer. ", i wonder how it works and is there any reference steps to follow?
Thanks for helping.

 

[flatulent]

uwb folded dipole

The balun is to keep the current from flowing on the outside of the coax. You do the usual calibration connecting the three different loads (short, open, 50 ohms) on the output of the balun.

 

[KevinCheng]

thanks for your reply.
Do you mean that, i connect the baluns to the network analyzer and do the one port calibration but connect the two 50ohm load, two open, two short to the output ports?
Since it really sounds strange to me that we perform the 1 port calibration but we need to calibrate the two output ports and the those signal are 180 out of phase.

 

[flatulent]

Antenna measurements are usually one port so you only need to do a one port calibration of the measurement system.

The balun should have a coax connector on one side and two wires on the other side.

For open you connect nothing to the two wires.

For short, you solder the two wires to a copper plate so that the balun is in the middle of the plate and perpendicular to it.

For the 50 ohm, you can solder a chip resistor to the two wires.

All of the above is assuming that the antenna frequency is UHF or lower in frequency.

 

[KevinCheng]

actually, i have made a uwb baluns for the uwb antenna, therefore, is there any other method to do the calibration on the uwb baluns? thanks for helping. (the uwb antenna is worked around 2.5GHz to 5GHz)

 

[flatulent]

If the balun is always connected to the antenna, there is no real practical need to separate the "antenna system" parameters between the balun and the antenna.

One method of measuring baluns and transformers is to put two in tandem. Their 50 ohm windings toward the outside world and the other windings to each other. The measured results for the transmission coefficients should be halved (loss and angle) to get the paramaters of one of them.

The above method is not totally accurate because the internal terminations are not perfect.

 

[KevinCheng]

the uwb antenna is actually work for an IC that with differential output port, therefore, i need to make and calibrate the uwb baluns in order to look at the impedance and the S11 of the uwb antenna.
Do you mean that i should make two baluns, and connect their differential output ports together, and one input port is connected to the network analyzer and the other input port is calibrated by the standard load, open and 50ohm and then halved the s parameter ?

 

[flatulent]

Yes. Connect the balanced ports together and measure between the 50 ohm coax ports.

 

[KevinCheng]

your advice is very helpful, thx a lot.

i have got an other question.

can i cut the dipole into two, that mean, the orignally differential feeding become a single feeding so that i do not need to worry about the baluns, and then using a ground plane as the mirror, and then use the single port to measure its S11 and impedance and the twice that impedance should be equal to the original dipole's impedance ?

practically, can i do so ?

 

[jallem]

can i cut the dipole into two, that mean, the orignally differential feeding become a single feeding so that i do not need to worry about the baluns, and then using a ground plane as the mirror

Sure, that is called "Monopole"....BUT,..... the size of the ground plane will
influence the values you want to obtain...

 

[flatulent]

The ground plane should be many wavelengths across.

 

[KevinCheng]

That means, the halved dipole should be placed in the middle of a very large ground plane so as to get an accurate result ?

If i would like to connect the halved dipole with the SMA connector, should i connect the ground of the SMA connector to the large mirror ground plane or just let the large mirror ground plane to be a floated metal plane?

if i want to measure a folded dipole, i wonder when i halved it, should i connect the loop to the large mirror ground plane?

Thanks flatulent and jallem for helping.

 

[flatulent]

The outer conductor of the coax is connected to the ground plane. As a test of your measurement methods, use a straight wire a quarter wave long for the antenna and see if you measure 35 +j0.

For the folded dipole, one of the two free ends goes to the ground plane and the other to the center conductor of the SMA socket.

This is a very old antenna type, the folded vertical. There are variations with many parallel wires going to ground/earth and one being fed.