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#### Mahiou

##### Newbie level 3
Helloo everyone

I am a student doing Electronics, in my final year, my project is about designing an impedance matching circuit and a feed network for next gen communications systems

I am using the ADS suite, and I want to start with designing the impedance matching circuit
Now, i have a slight idea of what I am going to do:
My circuit needs to work with different frequencies. 3g, wifi, and 4g, and it has to match the impedance at these different frequencies using switches

The questions I cant find answers for are:
1. I realize the impedance of the antenna changes when the frequency changes. Is there a formula that relates the antenna's impedance to frequency ?

2. Are all the antenna modeled in the same way, i mean in terms of impedance and size, or is every antenna different?
I mean in my case, is there a standard antenna model that i will use, or do i need to speak with my supervisor and he picks a model for me ?

3. I would really appreciate it if someone showed me a link or a tutorial about how I can design a an impedance matching circuit in ADS.. I do understand the concept of the matching circuit. But i looked at ADS, and I cant find the components i am looking for
Like... for example. An antenna..
also, please, how do i simulate the antenna's impedance using ADS while changing frequency ?

I did look at google, and I looked at ads webpage. For a start I have no idea what i am loooking for because I never did Rf before
and second, the stuff i found is just not what I am looking for

#### uWave_Matt

##### Newbie level 5
I assume at those frequencies you'll be using distributed elements rather than lumped which ADS is good for.

For the matching network, I would first start with the smith chart, figure out the length of the stub needed and its distance from the load (antenna), then plug those values into ADS rather than trying to USE ADS to do it for you.

Once you've designed the circuit, you'll want to look at S11 in the simulation results. It should show a sharp dip at the operating frequency if you've designed your matching circuit well.

If you need a book for reference, check out Microwave Engineering by Pozar.

#### lamchidinh

##### Newbie level 4
1. Yes, the impedance changes with frequency. And some formulas for antenna impedance you can find only at their resonant frequencies. For example a dipole antenna: 73 + 43j at its resonant frequency. At other frequencies, the derivation is complicated and requires lots of tedious steps, therefore usually we measure the impedance with specific tools, network analyzer for ex.

2. The final impedance of every antenna includes a resistor, and a reactive component (L or C). Add a voltage source if it is a receiving antenna.

3. Yeah, I think you should use distributed elements.

#### Mahiou

##### Newbie level 3
I already ordered the book and should receive it within two days.
More questions ?
1. "Figure out the length of the stub needed and its distance from the load (antenna)"
figure it out from where ? is this something I decide ? I have nothing basically.
I have a title that says: design of adaptive matching and a feed network for next gen mobile communication systems.
and I dont understand how to figure that out

2. "therefore usually we measure the impedance with specific tools"
Is there a link or a book that shows how to do that ? I cant do it on ADS, i cant find what i am looking for
and I am not sure if I am clear on what I want

I need to design an feed network and an adaptive matching circuit. thats all i was told, I have antenna, or design.
So I dont know how to start, and thats whats slowing me down compared to my classmates.

thank you all. again

#### uWave_Matt

##### Newbie level 5
For your antenna impedance, you can use linecalc I believe. You'll have to have the antenna designed in order to do this but once you do, you just plug in details such as dielectric constant, antenna width, etc.

As for using stubs for a matching network, you're essentially asking us to condense 4-5 weeks worth of a course on microwave engineering into a post...

The idea is that you can duplicate the effects of lumped element capacitors and inductors with certain lengths and widths of microstrip transmission line. Do you know how to design a matching network for RF circuits? A smith chart would be quite useful - theres a good few pages on it in Pozar so I recommend reading up on it.

Also, what is your antenna geometry and what frequency are you working with?

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