Coax cable parameters for simulation? Feeding microstrip with coax.

[Andryuha]

I am modelling a microstrip resonant antenna and need to feed it with coax cable through the substrate in the middle.

I have 2 questions:

1)Can you guess some SMA compatible realistic 50 Ohm cable parameters? a)Signal conductor (inner) diameter "ID" b)Insulator (outer) diameter "OD" c)Insulator dielectric permittivity and loss tan.
It appears easy to find a cable, but hard to find detailed datasheet... especially distributed dielectric permittivity of composite insulator is never explained.

2)When feeding the 50 Ohm microstrip line somewhere in the middle of circuit, what should be a diameter of hole in the ground plane? I've seen some textbooks telling that the hole should be smaller than OD. But I've seen no explanation why, or any formula.

 

[per_lube]

I am modelling a microstrip resonant antenna and need to feed it with coax cable through the substrate in the middle.

I have 2 questions:

1)Can you guess some SMA compatible realistic 50 Ohm cable parameters? a)Signal conductor (inner) diameter "ID" b)Insulator (outer) diameter "OD" c)Insulator dielectric permittivity and loss tan.
It appears easy to find a cable, but hard to find detailed datasheet... especially distributed dielectric permittivity of composite insulator is never explained.

2)When feeding the 50 Ohm microstrip line somewhere in the middle of circuit, what should be a diameter of hole in the ground plane? I've seen some textbooks telling that the hole should be smaller than OD. But I've seen no explanation why, or any formula.

what I understand from your question 1, you don't need to know cable or connector parameters for the simulation. Just use a 50 ohm port at the feed point. As long as the antenna is 50 ohm matched at the operating frequency, you can feed it with a proper connector in reality.

for your Q2, what I've experienced is with the diameter of the hole in the ground plane, the matching of the feed point changes.... you can run some simulations and find it out. I think the size of the hole may vary on the substrate you use.

cheers,
per_lube

 

[khaled.ayman]

Have a look at this post:

https://www.edaboard.com/threads/281995/

 

[Andryuha]

per_lube

>what I understand from your question 1, you don't need to know cable or connector parameters for the simulation. Just use a 50 ohm port at the feed point.
No. I need it. The feeding probe disturbs the field. Field perturbations lead to phase error in the structure. Resonant conditions break.
First, I've tried feeding with microstrip, it didn't work, so I am trying to feed it with coax in the middle of the line.

>As long as the antenna is 50 ohm matched...
ummm... it's not actually perfect description. It is periodic. So it is just a Bloch impedance. I am not certain about matching with a feeder placed somewhere inside. I want to try simulation first.


>with the diameter of the hole in the ground plane, the matching of the feed point changes
Exactly. So I was expecting there's some formula exists, but I couldn't find.


khaled.ayman, thank you!


Kind regards
Andrey

 

[microwave79]

When you choose CST MWS, there is an option window (or impedance calculator) while designing a proble. by using this window you can easily calculate diameter of probe. however the length of probe can be neglected. In IE3D the probe by default is of 50 ohm. for different frequency the 50ohm probes are of different types. The cost of the probe increase exponentially as the frequency increases.

 

[Andryuha]

And what should be the size of the hole?

 

[Er Swapnil Parihar]

I actually too have the same problem, while designing in IE3D the software asks for the probe diameter , at that point i didnt disturb the default diameter values ...but now i am about to physically design it and have no idea how big the feed hole is to design it in AutoCad (the PCB producer only accepts this dwg or dxf format) Help urgent !!!

Also if the diameter is frequency/power dependent is there a IEEE paper or some site where the comparison can be made ?