Antenna Resistivity and Reactance Help

[kmmaran]

Hi,

I am having trouble with my Antenna.

When my reactance goes positive or at 0 for first time along the frequency, I am having a negative resistance, and the reactance drop back to negative, and when second time it goes to positive again, this time my resistivity goes positive.

So in order to get the first resonance which one should I take, the first time the reactance goes to positive or the second time.

And please explain why I am getting a negative resistance when my reactance is positive.

Thanks for the help.

 

[enjunear]

Typically, you see negative impedances when you are looking into a source. ADS shows negative values when you put a Term object on an oscillator, and look at it's output impedance. If you look at how S11 is defined, you'll notice that the signs are opposite, since you are supposed to be driving current into the object, and not getting more current out (which is what an oscillator does, produces power).

My guess is that your antenna is picking up a signal, so your network analyzer is seeing the same thing ADS would... put in 1 mW, and you get out 10 (makes the answer look goofy). Try shielding your antenna from external RF sources. Depending on your frequency range, that could be difficult, since the wavelengths may be long. Point it away from fluorescent light fixtures, which are noisy and can have a wide spectrum of noise. See if anyone else in your lab, or nearby, is transmitting on/near your frequency range.

Depending on your antenna design, you should have an idea where your first resonance should be... you should have designed around that point/value. It may be somewhat off in reality, compared to your design equations, but it should be close.

 

[kmmaran]

Hi Enjunear,

Thanks for your help and explanation, actually I forget to mention one thing, I am doing a simulation and I am getting this results. Please apologize for not mentioning first time.

and like to ask, so do I take the first reactance positive frequency as my antenna resonance even i get a negative resistivity.

 

[enjunear]

Hi Enjunear,

Thanks for your help and explanation, actually I forget to mention one thing, I am doing a simulation and I am getting this results. Please apologize for not mentioning first time.

and like to ask, so do I take the first reactance positive frequency as my antenna resonance even i get a negative resistivity.

It depends on if it makes sense. If the antenna design is totally passive, then I'd be very leery about getting a negative resistance... double-check your simulation setup and parameters. Now, do you use the first resonance or not, that depends on if it's close to your target design or not.

I've never encountered a negative resistance on an antenna simulation, so this is unknown territory.

 

[kmmaran]

Thanks again. This is a passive dipole antenna and the negative resistance happen when the ground plane conductivity change from 10^-1 to 10^0.

Do you any theory that I can relate to this? I just can't find anything about the negative resistance...

 

[E Kafeman]

Negative resistance in a passive antenna => It reflects back more energy then what it is feed. If this antenna can be realized, you have a infinite free energy source.
Check your model again.

 

[enjunear]

Thanks again. This is a passive dipole antenna and the negative resistance happen when the ground plane conductivity change from 10^-1 to 10^0.

Do you any theory that I can relate to this? I just can't find anything about the negative resistance...

I think your simulation is invalid at those conditions... check your model, and limit ranges on any simulation components.

 

[aniakhan]

hi
i am facing same problem of negative impedance in simulation.have you sought out this problem?if yes how did you manage to sought it out.plz help me in this regard

Hi,

I am having trouble with my Antenna.

When my reactance goes positive or at 0 for first time along the frequency, I am having a negative resistance, and the reactance drop back to negative, and when second time it goes to positive again, this time my resistivity goes positive.

So in order to get the first resonance which one should I take, the first time the reactance goes to positive or the second time.

And please explain why I am getting a negative resistance when my reactance is positive.

Thanks for the help.