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Halfwave Balun

Altair Forum User

I would like to model a coaxial halfwave balun, a dipole and a coaxial line in the typical 4:1 configuration of the halfwave balun.

Can I have some guidance on how to do this please? I have modelled dipoles and Yagis with FEKO, but not sure where to start to model the coaxial line and the halfwave balun.

Then I would like to be able to calculate some balun parameters such as balance ratio, bandwidth, impedance transformation, etc.

Thanks,

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Torben Voigt

Hi @FEKOFan,

you can of course always imitate the physical real-life scenario. Means that you physically model the coaxial cables. Since this would mean some computational effort die to lots of mesh triangles, you may use a TX line in CADFEKO:

image.png.bf662b9fe6a6ff089ec6baf425a49fc1.png

This can be placed between two ports and will act as if a piece of cable would be present. Only that it's not a scattering object, it's rather a black box. I did't yet check the feasibility of this approach. Do you think this could be useful?

Best regards,

Torben

Altair Forum User

Thanks. Not sure yet, but it gives me a place to start, I'll have a look to see where it takes me.

Best regards,

Altair Forum User

I have been trying to do this using a transmission line. I put a wire port on a wire and then add a transmission line to the model, but the TX Line object doesn't give me enough flexibility to define all the interconnections of a halfwave balun. Once a figure out how to do this I would like to model more complex baluns and I can't see how I will be able to do that.

TransmissionLine.jpg.51378022f565b81139ffc7086c3fb025.jpg

So I looked at the Cable models. These allow me to define Pin1 and Pin2 at the end of a coaxial line. Started with the simple example below, a complex impedance at the end of the coax and a source at the start.

However this model throws an error when I do CEM Validation saying that the Cable Harness does not have an excitation defined.

It seems that FEKO does not recognise this source in the same way that it does for Sources attached to a Wire Port. But I cannot attach a Wire Port, because there are no wires in this model. Even if I add a wire, I can't find how to connect the ends of the wires to the Pins of the connectors.

Coax.jpg.a33cf12c62613348fb439a65044ca3c1.jpg

More suggestions would be much appreciated.

Best regards,

Torben Voigt

Hi @FEKOFan,

You have to set the CableHarness to 'Radiating' or 'Radiating (taking irradiation into account)':

image.png.7a686cca500c4d0feb21fa458b114b2a.png

If a source is applied to a cable, it must not be considered purely irradiating, that's why the error occurs.

Altair Forum User

Thanks, understood. That works fine now, after adding also a PEC to the model.

So I can calculate a source connected to a dummy load by a piece of RG58.

Now I want to replace the dummy load with a dipole. I have a connector with two pins at the end of the coax. The position of the connector is defined because I checked 'Cable path terminal' and is set to CablePath1.End. How do I connect a wire (half of the dipole) to Pin1? Even if set the Connector position to 3D Coordinate I can't see how to distinguish the position of Pin1 and Pin2.

Torben Voigt

Hi @FEKOFan,

Connecting MTL cables with MoM pots is unfortunately not supported. MTL cables interact with the surrounding only via radiation/irradiation. What may be possible is to cumpute s11 of the dipole, save as touchstone file and include this in your CableHarness.

Question: Why do you want to model the Balun? Applying a voltage source directly to your antenna would basically represent an ideal Balun. Just saw that you wrote the goal in the original post:

Then I would like to be able to calculate some balun parameters such as balance ratio, bandwidth, impedance transformation, etc.

If you want to simulate the effect of the actual cable as radiating object, you would have to actually model the feeding cable and Balun cable. This would of course be quite an effort. I will have to discuss with a colleague if there is a better way to do this.

Altair Forum User

Ok. Looking forward to reading the outcome of discussions.

For other problems I want to solve I would also have to calculate common mode currents on the exterior of the coaxial cable braid when connected to an antenna. I am not sure if this will be possible.