Antenna and line transmition impedance

[julian403]

When we see the antenna parameter, there is the impedance which it's always real (75 , 50, ... Ohms)

But the impedance antenna is \[\left\vert{} Z \right\vert{} = \sqrt{{R}^{2} + {( \omega L - \frac{1}{\omega C})}^{2}}\]

or it's just the real part of Z?

I think it's the first one, Because the antenna always has and inductance and a capacitance. But maybe there must be an ideal way to have ωL=-1/(ω C) but it's difficult to me think it can happend.

 

[FvM]

Yes, you see almost real impedance of an antenna at it's center frequency, e.g. a dipole with length of lambda/2.

An antenna is a distributed device, it has no physical L or C. In so far the quoted |Z| equation (not valid for Z) describes only a possible antenna equivalent circuit in a limited frequency range, not an actual antenna. As for the equivalent circuit, the reactive impedance components are cancelled at the center frequency, jωL=-1/(jω C) respectively ωL=1/(ω C).

 

[julian403]

So, to measure the resonance (central frecuency) I can use a signal generator and with a resistor in serie with atenna measure the voltage amplitude on the antenna with an osciloscope.

1) First of all, I put a generic resistor value and with it I find the max amplitude voltage on the antenna, changing the frecuency with the signal generator. In the frecuency where the amplitude maximum ωL= - 1/(ω C)

That measure way it's correct?

2) to measure the real part of antenna's impedance I can use a square wave, and put the osciloscope tip between the resistor and the antenna. The real part of the antenna's impedance will be equal to the resistor value when the square wave is not distorted.

that's correct too?

 

[FvM]

1. No. The impedance magnitude according to your formula is minimal not maximal in resonance. That's also the case for the fundamental resonance of most antennas, but necessarily for all. Other than the RLC equivalent circuit have antennas multiple resonances.

2. No. The square would will be always distorted because the impedance at harmonics is different from fundamental. You'll use a sine generator and adjust frequency for zero phase shift.

Also search for "impedance bridge" **broken link removed**

 

[julian403]

I put a resistor of 0.1 Ohm in serie with the antenna. With the signal generator I was changing the sin's frecuency, measuring the 0.1Ohms resistor's voltage drop and signal generator's voltage and seeing the lissajous pattern in the XY mode.

So When I had the line which goes from left to right, there is the resonance. What do you think?

Whit a dipole of 1.45 meter I got that the resonance frecuency is ≈96MHz when the line goest from left to right

And I got a line (lissajouse's figure) which goes from rigth to left on a frecuency of ≈103MHz

 

[FvM]

Measuring current phase correctly with low-ohmic resistors gonna be a problem in the 100 MHz range. The setup surely needs calibration with a known real impedance.

 

[julian403]

So, how can I measure the current at 100 MHz?

 

[FvM]

A small current transformer (a few mm toroid core, e.g. 1:5 winding ratio, 50 ohm termination) can work at 100 MHz.

 

[julian403]

if the antenna an line transmition are 300 Ohms at the secundary must be a 300 Ohms resistor?

I still do not know why I can't use a 0.1 Ohms. Because using a current transform with a resistor in the secundary there is a resistance reflected to the primary. In the case wit a 1:5 winding ratio, there is a resistor value of R/5 in the primary.

 

[FvM]

I still do not know why I can't use a 0.1 Ohms.

Theoretically you can use it. In practice, a parasitic resistor series inductance of only 1 nH creates 0.6 ohm reactive impedance, six times the resistance.

Because using a current transform with a resistor in the secundary there is a resistance reflected to the primary. In the case wit a 1:5 winding ratio, there is a resistor value of R/5 in the primary.

No it's R/5².

 

[julian403]

thanks! I will do the transformer but the last one, Why it's necesary a current transformer? why I can't put a resistor of 2 Ohms (5/25)?