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Input impedance vs frequency

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randy orton

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Hello all

In the attachment i have attached a pdf which contains the graph of input impedance Vs frequency of a dipole antenna and ellipse shape parasitic element at a different distance between antenna and parasitic element, I basically want to know that what can be interpreted from the graph, Input impedance vs frequency graph suggest what? what to understand and conclude from that graph?


And other attachment with the title of zero impedance shows the graph of input impedance vs frequency with dipole and U shape parasitic element with a variable distance.impedance stays almost 0 for all frequency and all distance, what does it mean?for the dipole antenna i am getting 0 impedance for the whole frequency range,is that kind of design any useful or its stupid?

Thanks in advance for your thoughts
 

Attachments

  • ellipse.pdf
    191.4 KB · Views: 99
  • zero impedance.pdf
    190.6 KB · Views: 75
Last edited:

Can you please clarify which quantity is shown as "Y" in the diagrams? The impedance of a passive component, e.g. an antenna can be expected a complex value with non-negative real part. So if Y is the reactive (imaginary) impedance part, how about the real component (the so called radiation resistance)?

P.S.: I see, that the first plot is showing reactive impeddance, it looks quite common in this case. For the second plot, you need a reasonable impedance scale.
 

FvM said:
Can you please clarify which quantity is shown as "Y" in the diagrams? The impedance of a passive component, e.g. an antenna can be expected a complex value with non-negative real part. So if Y is the reactive (imaginary) impedance part, how about the real component (the so called radiation resistance)?

P.S.: I see, that the first plot is showing reactive impeddance, it looks quite common in this case. For the second plot, you need a reasonable impedance scale.
Click to expand...

Thanks FvM

In both the figure X axis shows the frequency and Y axis shows Z11 of real and imaginary. almost flat line at 0 shows Real part and other buch of lines shows imaginary part at the different distance between antenna and parasitic element.

For the other figure i don't have access to lab for now so i can't do anything. but i can send you as soon as i can.
 

randy orton said:
Thanks FvM

In both the figure X axis shows the frequency and Y axis shows Z11 of real and imaginary. almost flat line at 0 shows Real part and other buch of lines shows imaginary part at the different distance between antenna and parasitic element.

For the other figure i don't have access to lab for now so i can't do anything. but i can send you as soon as i can.
Click to expand...

In general for antennas, the point at which mag(S11) goes to near zero, you have found the resonant frequency of the antenna (S11 --> 0, or very low dB's... means almost no energy is reflected, i.e. all the energy is going into the antenna).

It might make more sense to plot S11 on a Smith chart. The point that is closest to the center of the plot, crossing the real axis would be your resonant frequency. That looks like it happens around 84 MHz? I'm not 100% sure if I'm reading the graphs correctly, since I'm unfamiliar with the program being used.
 
Thanks for your reply.

I am using HFSS 13 and yes center frequency is 88 Mhz but i have plot Z11 Vs frequency not S11. i guess Z11 will give me the input impedance not the S11, if i am wrong than please guide me.and if i am right than what should i interpret form that?
 

You can of course plot antenna impedances as real and reactive impedance R and X (Z = R + jX). But the plot scale must allow to see both values, which isn't the case in your second diagram. In the first, the important real impedance part representing the radiation resistance is missing.

You'll find R + jX plots in many antenna text books, e.g. Ballanis. I also agree with enjunear, that a Smith chart plot of s11 is most informative. Most engineers will use it to display empirical antenna measurements. It's also the usual starting point to design a matching network.
 

View attachment New Microsoft Office Word Document.pdf
FvM said:
You can of course plot antenna impedances as real and reactive impedance R and X (Z = R + jX). But the plot scale must allow to see both values, which isn't the case in your second diagram. In the first, the important real impedance part representing the radiation resistance is missing.

You'll find R + jX plots in many antenna text books, e.g. Ballanis. I also agree with enjunear, that a Smith chart plot of s11 is most informative. Most engineers will use it to display empirical antenna measurements. It's also the usual starting point to design a matching network.
Click to expand...

Here in the attachment i have kept figure with the zoom.

what does "In the first, the important real impedance part representing the radiation resistance is missing." mean?
 

In ellipse.pdf, there's no Re(Z) plot. In zero impedance.pdf, the huge scale doesn't allow to read Re(Z). That's all.
 

Actually in ellipse.pdf if you open it in HFSS than straight line at 0 indicates Re(z) for all the distance, only imaginary value of impedance is varying, I tried to attach original HFSS file but system don't allow me to attach with HFSS extension.
 

Re(z) = 0 simply means there is no antenna. A real antenna, even an electrical small one, should have at least a few ohm radiation resistance. As said, it's important to know it.
 

Hello FvM

Here I am attaching pdf which contains tabular plot for same ellipse.pdf graph. according to it Re(z) is very close to zero but now zero. but still resultant impedance is very close to 0 so is it a good thing or not?my lumped port is set for 75 ohms so does it mean that there is mismatch and hence antenna efficiency is too low?

i am really thank ful that you are responding me, as i really want to understand that what is going on and why is it so?
 

Attachments

  • ellipse.pdf
    99.4 KB · Views: 58

O.K., I agree, that the title zero impedance somehow hits the point. This definitely doesn't look like a useable antenna, highest Re(Z) number is below 1 ohm. Resonance Q seems quite high and there's possibly a real Z peak around 100 MHz which falls between the few analysis frequency points. But that won't make things much better.

Can you show a picture of the geometry to make the result understandable?
 
Attached pdf shows the basic model diagram for this thing
 

Attachments

  • diagram.pdf
    252.8 KB · Views: 53

Hello FvM

I was going on a wrong track and with the help of your guidance i realized that, now i have step down and i am just attaching a file which contains output of a dipole antenna only, nothing else.center frequency is set to 300MHz. If it is correct than only it is advisable to step forward. so if you can take a look at it than it will really help me out.
 

Attachments

  • new dipole.pdf
    142.2 KB · Views: 49

Looks more like a dipole known from literature. But I dodn't kbow waht you mean with 300 MHz center frequency. From the impedance curve, the electrical length is about 1m, λ/2 at 150 MHz.
 

randy orton said:
but i have plot Z11 Vs frequency not S11. i guess Z11 will give me the input impedance not the S11, if i am wrong than please guide me.
Click to expand...

The large/small value difficulty in reading your plots shows why antenna designers do NOT use these Z parameters.
It will be much easier when you start using S11 in Smith chart, or VSWR.
 

Hey Volker_muehlahaus

Can you just take a look and tell me that for dipole is it correct?i was hoping to get impedance somewhere nearer to 73 but i am getting nearer to 117-118, so why is it so? is it even possible? any idea or suggestion will be grateful.
 

Attachments

  • smith chart.pdf
    123.9 KB · Views: 54
  • new dipole.pdf
    142.2 KB · Views: 43

Apperently, are operating the dipole above it's characteristic frequency. Impedance is around real 70 ohm at 128 MHz.

The Smith chart isn't very meaningful, because it has too few points around the antenna's resonance frequency. You should see a smooth curve without sharp kinks.

P.S.: this is at least the third thread, where you are posting effectively the same question. Did you ever hear of forum rules about cross-posting?
 
Last edited:

FvM said:
The Smith chart isn't very meaningful, because it has too few points around the antenna's resonance frequency.
Click to expand...

Agreed, this Smith chart does not help. The problem is the frequency sweep setting. You should use a narrower frequency sweep, with more frequency points near the resonance frequency.
 

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