Resonant Frequency for UWB

[talha863]

I am trying to design an antenna for UWB.
I am confused about resonant freq.
The design equations can be found here
**broken link removed**

What should be the resonant frequency?
Should it be 3.1 or 10.6GHz or should it be the mid-point of UWB?
Can anyone tell me the exact value?

 

[jiripolivka]

You apparently never read antenna basics. Please do. UWB utilizes very wide bandwidth, so resonant antennas are not suitable for it.

 

[talha863]

What I meant was I am trying to design a microstrip antenna for UWB and there has been a lot of research papers on this topic.
What I am wondering is if I try to design new antenna, how to find the dimensions of patch, substrate and feedline?
Can you help me on this regard?

 

[jiripolivka]

What I meant was I am trying to design a microstrip antenna for UWB and there has been a lot of research papers on this topic.
What I am wondering is if I try to design new antenna, how to find the dimensions of patch, substrate and feedline?
Can you help me on this regard?

I am not a specialist on UWB antennas; it looks there are none. This is why the recent publications should be studied to see how others succeed. Good knowledge of antenna basics is recommended.

The main problem seems to be is that engineers are used to design antennas mostly of resonant types, good for narrow-band use. UWB application can be better understood by rapid charge movement and time response rather than spectrum analysis. Anything can radiate but achieving a wideband matching to line impedance is the main problem.

I guess you will not find an easy answer; you should do experiments. Maybe someone brings a computer model but most are far from reality.

 

[talha863]

But I have read many papers, people have designed microstrip antennas and published their papers even in IEEE transactions
I know microstrip antennas are not good for wideband applications and they work well for narrow band applications, but still people have designed such antennas. I have also simulated an UWB microstrip antenna. But now I want to design a novel design, but I don't know what should be the initial values of patch, substrate and transmission line?

 

[WimRFP]

Hello Talha863,

Your reference on patch antennas is really interesting. It gives useful practical design info.

From the design info you can see that a patch antenna close to a ground plane isn't a wide band antenna, so it will have really bad performance as a 3 to 10 GHz UWB antenna (so I agree with Jiripolivka).

It is possible to get a VSWR<2 over the 3 to 10 GHz band with a planar structure. You need to avoid any steep Zo transition versus distance.

It is really difficult to get both good VSWR and descent radiation pattern as this requires 3D structures mostly (note that for indoor multi-path application, the pattern is less important). Maybe you know the discone antenna that is also a wide band antenna.

The trick with UWB antennas is avoiding steep impedance transitions and getting the right curvature of structures to get a descent radiation pattern. Some people may also point to radiation center problems. You should think of 3D flared structures like this: ultra.usc.edu/assets/002/36904.pdf For omni behavior you need double curved surfaces.

 

[talha863]

THnx WimRFP!
I have read the paper and I have read your post too.
The problem is I have to stick with microstrip antenna with a rectangular patch, I know it will not give good results as compared to other antennas (e.g. the one you mentioned) but still for my research purposes I have to stick with it.
So what should I do?

Hello Talha863,

Your reference on patch antennas is really interesting. It gives useful practical design info.

From the design info you can see that a patch antenna close to a ground plane isn't a wide band antenna, so it will have really bad performance as a 3 to 10 GHz UWB antenna (so I agree with Jiripolivka).

It is possible to get a VSWR<2 over the 3 to 10 GHz band with a planar structure. You need to avoid any steep Zo transition versus distance.

It is really difficult to get both good VSWR and descent radiation pattern as this requires 3D structures mostly (note that for indoor multi-path application, the pattern is less important). Maybe you know the discone antenna that is also a wide band antenna.

The trick with UWB antennas is avoiding steep impedance transitions and getting the right curvature of structures to get a descent radiation pattern. Some people may also point to radiation center problems. You should think of 3D flared structures like this: ultra.usc.edu/assets/002/36904.pdf For omni behavior you need double curved surfaces.

 

[WimRFP]

Talha863, to be honest, I have no idea how to implement a patch antenna with about 10...25% BW/Fc though over 100% BW/Fc is required. It is a choice between worse, worse, and even more worse.

As you can see in your paper, use large height above ground plane and use air dielectric. If you have the possibility, you may build a patch with no ground under it (so the patch is on the edge of the ground plane containing the electronics). You need to design a very smooth transition from your 50 Ohms line towards the transient impedance of the patch, this is far from trivial. Sure it will outperform your patch over ground plane.

Best option is to go back to the person that forces you to use a standard patch topology and tell him that the performance will be worse to hopeless.

 

[davenn]

Hello Talha863,

Your reference on patch antennas is really interesting. It gives useful practical design info.

From the design info you can see that a patch antenna close to a ground plane isn't a wide band antenna, so it will have really bad performance as a 3 to 10 GHz UWB antenna (so I agree with Jiripolivka).
It is possible to get a VSWR<2 over the 3 to 10 GHz band with a planar structure. You need to avoid any steep Zo transition versus distance.

............SNIP........

Agreed, the paper you have linked to is for a plain single freq patch antenna. Patch antennas are going to be narrow bandwidth.
You are NOT going to achieve a UWB with that style of antenna.
Elsewhere in this forum I have posted comments/pics on PCB style UWB log-periodic antenna covering ~ 3 - 11 GHz
And as listed amongst the antennas in link WimRFP has given I have also played with a PCB style Vivaldi covering ~ 2 - 11 GHz

Thanks WimRFP, nice article

Dave

 

[WimRFP]

Hello Davenn

The PCB vivaldi antenna is also a good example of an antenna with smooth impedance change with distance and therefore inherent wide band characteristics. The vivaldi is much easier the make then the 3D tongue design, or other 3D flared structures. Thanks for pointing to the vivaldi.