parameters affecting S11 values

[kae_jolie]

As far as I understand S11, it is the return loss of incident power due to power reflected back from the antenna to the power source.

My question is: what are the things that play a role in the changing of the S11 value?

This is what I think:
1. impedance mismatch between source and antenna
2. lossy objects adjacent to the antenna

Regarding #2, do lossy objects adjacent to the antenna affect S11 reading because they absorb some of the radiated power or because of increase in impedance surrounding the antenna?

Is there a relationship between S11 and power radiated? meaning if s11 is low (-10 dB), more power is radiated, or is it more power is accepted into the antenna?

 

[FvM]

Forward power to the antenna is either reflected, radiated or dissipated in the antenna structure (according to conservation of energy law) There's no chance to determine the radiation efficiency of a lossy antenna from an input port measurement.

Objects around the antenna will generally affect input impedance respectively s11, but not necessarily increase the latter. Lossy objects may also decrease both s11 and radiation efficiency.

 

[kae_jolie]

Lossy objects may also decrease both s11 and radiation efficiency.

When you say decrease s11, does that mean, for example, if S11 = -10dB, it becomes S11= -7dB or S11= -13dB?

According to my understanding, if a lossy object is present near the antenna, it will absorb some of the power radiated, which means less power is returned (reflected) to antenna, which means, for example, if S11 =-10dB (10% reflected), it becomes S=-13dB (5% reflected).

Please elaborate.

Thanks.

 

[FvM]

I meant what i said.

But it's not necessarily the case. It's likely to absorb radiated energy by lossy objects, but the effect on reflected energy can't be easily predicted. If the antenna is perfectly matched before, lossy objects will possibly increase the reflection factor.

 

[kae_jolie]

Fvm,

Thanks for reply. This topic has been very confusing to me. Could you please go thru my last post and answer the questions I had with examples. Just short answers of yes, no would do. Thanks

 

[FvM]

As I said, it depends. If an antenna is perfectly impedance matched (no energy reflected), any objects in the proximity will cause reflections, also lossy objects.

 

[kae_jolie]

Having said all that, is s11 a good measuring tool for how much power was absorbed by an adjacent lossy object? Yes or No? and Why?

My take is: It's not, because s11 primarily measures reflected power and I don't think you get accurate estimate from it to know how much power absorbed by adjacent objects.

Thanks.

 

[FvM]

is s11 a good measuring tool for how much power was absorbed by an adjacent lossy object? Yes or No? and Why?

I mean to have answered this before. Clearly, it isn't. It's a measure for antenna matching.