Feeding of monopoles antennas.

Student_21133

Hello,

I have a problem with the feeding of monopoles antenna.

I first simulated a LPD antenna (cf. "LPD Antenna.jpg") made of 7 dipoles, i.e. 7 wires with a port set on their middle. Only the first (littlest) dipole is fed with a voltage source on is port. The other dipoles are fed with a non-radiating transmission line like in the principle scheme of Fig. "LPDA principle.jpg", which is achieved with Fig. "Feeding system.jpg".

It provides good results, accorded to the afferent litterature.

But what I would like to model now is a real LPDA (cf. "Real LPDA.jpg"), in which the radiative éléments are no more dipoles but non-aligned monopoles.

The problem that I have now is to find a non-radiative feeding system like the transmission line of Fig. "Feeding system.jpg", which is not obvious anymore for me.

The Feko philosophia is to have ports which are necessary both sides surrounded by a conductor : in the Feko examples documentation, a monopole is always electrically unioned with a ground plan for instance, so its "start port" is anyway connected both sides to a conductor).

I'm searching for a way to feed a port which the "+" pole would be connected to a monopole antenna and the "-" pole would be connected to another monople antenna a bit farther (not sticked to the first one), with a non-radiating line.

 

Would anyone have any thought about this ?

Thank you in advance.

 

Torben Voigt

Student_21133 said:

Hi Mr. Voigt,
Please find my attached model (cf. LPDA.cfx).
There are 2 edge ports at the front and the rear of the antenna. The port 1 is a feed (source), the port 2 is a load. When I launch the feko solver, I get the error 21 about the port 2.
Thank you if you can understand why. This is probably due to a bad edge port construction, but the Feko manual is not very clear about that.

Hi @Student ,

It seems like the ports' faces are not connected to the antenna:

My graphics card is broken and it's very hard to work with the new CADFEKO for me right now... So I exported the geometry and imported it in legacy CADFEKO. I then set all the internal regions to PEC and used "Simplify". This deletes all the redundant internal faces.

Model is attached.

Best regards,
Torben

Torben Voigt

Hi @Student ,

Why don't you just model the antenna as is (monopoles connected through the two beams), like in the picture?

This picture is from an actual Feko model.

Best regards,
Torben

Student_21133

Well, thank you very much.

Sorry to sound ignorant, but where to find this model ?

There is a lpda model in the CADFEKO component library, but it's not so complex.

PS : I have Feko 2021.2.

Torben Voigt

Hi @Student ,

This is a model I made for a customer and which I can't share. I just wanted to say that you don't necessarily have to connect the monopoles of the LPDA with transmission lines, but you can simply design the antenna as it really is.

Best regards,
Torben

Student_21133

Ok, I understand. 

I will try to design the real antenna.

So may I ask you which kind of feed port did you set on the right in your picture (edge port ? But it seems to feed  two disjointed parts), and how did you evaluated this port impedance ?

Thank you for your help.

Torben Voigt

Hi @Student ,

In this case I used an edge port, but a wire port would be feasible as well.

I am not sure what you mean by your question about port impedance. This is calculated during the simulation.

Best regards,
Torben

Student_21133

Hi Mr. Voigt,

Thank you for your response.

It seems that you used as a feed an edge port put on a rectangle.

I designed my real antenna (cf. "Antenna.jpg") fed by an edge port. It works correctly.

By "port impedance", I was meaning the "Reference impedance" of the voltage source (cf. "Reference impedance.jpg"). I tried several values of this reference impedance, which strangely doesn't modifiy the result.

I also put a end load at the other edge of the antenna, opposite to the feed : I have been annoyed because it seems that Feko doesn't allow to put a load on an edge port, so I finally put this load on a simple wire (cf. "Wire load.jpg"), which works.

The best radiating result is supposed to be obtained with a load matched to the transmission line feeding the 7 dipoles of this LPDA.

By the way, I observe that the radiation pattern of my real antenna is much less good (i.e. more radiating back and sides) than the radiation pattern of the same antenna when a bit more idealized. My ideal antenna (cf. "Idealized antenna.jpg") consists in 7 dipoles fed by an ideal non-radiating transmission line (cf. "Transmission line.jpg").

You seem to be very good in this field : do you find normal the differences that I see between the 2 radiating diagram ? Cf. "Ideal antenna.jpg" vs. "Real antenna.jpg".

Thank you very much for your help.

Torben Voigt

Student_21133 said:

Hi Mr. Voigt,

Thank you for your response.

It seems that you used as a feed an edge port put on a rectangle.

I designed my real antenna (cf. "Antenna.jpg") fed by an edge port. It works correctly.

By "port impedance", I was meaning the "Reference impedance" of the voltage source (cf. "Reference impedance.jpg"). I tried several values of this reference impedance, which strangely doesn't modifiy the result.

I also put a end load at the other edge of the antenna, opposite to the feed : I have been annoyed because it seems that Feko doesn't allow to put a load on an edge port, so I finally put this load on a simple wire (cf. "Wire load.jpg"), which works.

The best radiating result is supposed to be obtained with a load matched to the transmission line feeding the 7 dipoles of this LPDA.

By the way, I observe that the radiation pattern of my real antenna is much less good (i.e. more radiating back and sides) than the radiation pattern of the same antenna when a bit more idealized. My ideal antenna (cf. "Idealized antenna.jpg") consists in 7 dipoles fed by an ideal non-radiating transmission line (cf. "Transmission line.jpg").

You seem to be very good in this field : do you find normal the differences that I see between the 2 radiating diagram ? Cf. "Ideal antenna.jpg" vs. "Real antenna.jpg".

Thank you very much for your help.

Hi @Student ,

Regarding the "port impedance" please have a look here: https://community.altair.com/community?id=community_question&sys_id=34b680ba1b2bd0908017dc61ec4bcb14&anchor=answer_c35114361bebd0908017dc61ec4bcb17&view_source=searchResult

Feko of course allows loads on edge ports. I wonder what is going wrong with this in your case:

Of course, there are always deviations between simulation and measurement, which is mainly due to the non-ideal conditions of the measurement. The simulation takes place under ideal conditions, whereas many factors can influence the result of the measurement. By the way, for better comparability of the results you should use the same scaling of the graphs (e.g. -50 dB to +10 dB). 

Maybe your model needs some more adjustments to the real antenna? Like lossy metal, ground etc.?

Best regards,
Torben

Student_21133

Hi Mr. Voigt,

Thank you for the link about the port impedance : it was what I thought, i.e. the source resistance R_S :

But what I didn't realize is that Feko computes a gain normalized to the power that actually enters the antenna, and one has to display the "realised gain" if the source resistance has to be taken into account.

Concerning adding a load to an edge port, I of course put one as you showed it on my edge port of a metallic rectangle :

but I had the message :

" No metallic edge found to define an edge port between regions with multiple labels 

Union1.Union2.Face1_2  Union1.Union2.Face1_1

ERROR      21: The metallic edge for the edge port does not exist"

Feko accepts to put a source on my edge port, but not a load. It only works when I replace the rectangle by a wire. I don't understand why.

Concerning the gap between the 2 antenna models, I was surprised because my ideal antenna has the same performance than the official real one (a Normarc ILS antenna), and its performance deteriorates when I modelize something more realistic. So maybe this company is actually a bit optimistic in his advertising.

I thank you a lot for the time you spent helping me, …and my best wishes for this new year.

Torben Voigt

Student_21133 said:

Hi Mr. Voigt,

Thank you for the link about the port impedance : it was what I thought, i.e. the source resistance R_S :

But what I didn't realize is that Feko computes a gain normalized to the power that actually enters the antenna, and one has to display the "realised gain" if the source resistance has to be taken into account.

Concerning adding a load to an edge port, I of course put one as you showed it on my edge port of a metallic rectangle :

but I had the message :

" No metallic edge found to define an edge port between regions with multiple labels 

Union1.Union2.Face1_2  Union1.Union2.Face1_1

ERROR      21: The metallic edge for the edge port does not exist"

Feko accepts to put a source on my edge port, but not a load. It only works when I replace the rectangle by a wire. I don't understand why.

Concerning the gap between the 2 antenna models, I was surprised because my ideal antenna has the same performance than the official real one (a Normarc ILS antenna), and its performance deteriorates when I modelize something more realistic. So maybe this company is actually a bit optimistic in his advertising.

I thank you a lot for the time you spent helping me, …and my best wishes for this new year.

Hi @Student ,

I'm surprised at the error message for your edge port. Could you maybe attach the model (or a reduced model with only the area around the edge port) here?

And all the best for the new year to you too!

Best regards,
Torben

Student_21133

Hi Mr. Voigt,
Please find my attached model (cf. LPDA.cfx).
There are 2 edge ports at the front and the rear of the antenna. The port 1 is a feed (source), the port 2 is a load. When I launch the feko solver, I get the error 21 about the port 2.
Thank you if you can understand why. This is probably due to a bad edge port construction, but the Feko manual is not very clear about that.

Torben Voigt

Student_21133 said:

Hi Mr. Voigt,
Please find my attached model (cf. LPDA.cfx).
There are 2 edge ports at the front and the rear of the antenna. The port 1 is a feed (source), the port 2 is a load. When I launch the feko solver, I get the error 21 about the port 2.
Thank you if you can understand why. This is probably due to a bad edge port construction, but the Feko manual is not very clear about that.

Hi @Student ,

It seems like the ports' faces are not connected to the antenna:

My graphics card is broken and it's very hard to work with the new CADFEKO for me right now... So I exported the geometry and imported it in legacy CADFEKO. I then set all the internal regions to PEC and used "Simplify". This deletes all the redundant internal faces.

Model is attached.

Best regards,
Torben

Student_21133

Torben Voigt_20420 said:

Hi @Student ,

It seems like the ports' faces are not connected to the antenna:

My graphics card is broken and it's very hard to work with the new CADFEKO for me right now... So I exported the geometry and imported it in legacy CADFEKO. I then set all the internal regions to PEC and used "Simplify". This deletes all the redundant internal faces.

Model is attached.

Best regards,
Torben

Hi Mr. Voigt,

I applied the tips that you described on my faulty model, which fixed it.

I'm a bit surprised, because I thought that I had correctly created these port, which in fact wasn't the case.

I find the port creation procedure not well described in the Feko manual.

Once again, I thank you very much for your help.