Radiation resistance and ohmic losses

[matteop]

Hi there,
I am trying to simulate with Sonnet Lite (that's the only sw I can use, for now) the efficiency of an antenna.
I have to admit, I am quite unpractical with antennas but I'll have to get used.

My idea was to simulate the input resistance (getting Rin=Rrad+Rohm), then switch all materials to "lossless" and resimulate again (then getting Rrad). The point is that I obtain Rin<Rrad, is there something wrong in the method I used?

Point 2, in Sonnet you can set the "terminations" at the port, and since I have a load there, I typically put that value. I found out, anyway, that changing the termination changes the Zin which Sonnet gives to me... so I am also a bit confused about what is really Zin for Sonnet...

Thank you in advance for any help!
Matteo

 

[WimRFP]

Can it be the reference impedance that is used to normalize the smith chart? If you enter 80, the center of the Smith Chart is ten 80 Ohms.

If you double the value, and the value you read halves, you can be sure it is the port's reference impedance.

 

[matteop]

Can it be the reference impedance that is used to normalize the smith chart? If you enter 80, the center of the Smith Chart is ten 80 Ohms.

If you double the value, and the value you read halves, you can be sure it is the port's reference impedance.

Mhm in the software it is called "port termination" and unfortunately the Smith chart is without labels...

Anyway I found out that the output called "Zin" has this behavior (changes with the port termination value) while the output "Z11" is instead constant wrt the load, as I expect from the impedance of an antenna. Moreover Z11 with losses is higher than Z11 without losses, does it make sense to consider the efficiency as Re(Z11|nolosses)/Re(Z11|withlosses)?

 

[WimRFP]

Depending in where you feed your antenna (that is current maximum or current minimum), the impedance without ohmic loss may increase, or decrease.

If you have a half-wave dipole, or quarter-wave monopole, increasing ohmic loss, increases the total feed point impedance. But when having a full-wave dipole or half-wave monopole, more ohmic loss, reduces the feed point impedance. This is all because of the transmission line behavior of antenna elements.

If you have doubt about impedances, just simulate a half wave dipole in free space and see whether your simulated results match your expected result. A thin half wave dipole should be around 60-70 OHms when at resonance. For a quarter-wave thin monopole (to check you port scheme) over infinite ground you may expect 30..35 Ohms when at resonance.

 

[matteop]

Thank you, indeed my antenna is not so far from a dipole fed differentially at the center, and I get values in the range you mentioned.

Finally also the efficiency starts to make sense: pretty high at the moment, around 80%, but I'll have to check the materials.

 

[WimRFP]

Don't forget to see the difference between radiation efficiency and antenna or total efficiency.
Given your first posting, your interest is in radiation efficiency ( =Pradiated/( net input power) ).

Net input power = forward power - reflected power