Difference between measuring radiated power and S11

[kae_jolie]

how is power radiated measured compared to S11?

my understanding is:

S11 is how much of the incident power from source gets accepted by antenna to be radiated.

Power radiated is a measure of how much power gets radiated out of antenna.

If my understanding is correct, how is S11 and power radiated measured in a real setting? If I have one antenna as RX and TX, then I imagine the power radiated would be measured by this one antenna, correct?

Thanks.

 

[biff44]

In a very loose sense, if S11 magnitude is small (ideally zero) you will radiate more power than if S11 is higher. That is about all you can say. S11 is composed of many factors, only one of which is radiation resistance. You could have an antenna with S11 very good (low) that does not radiate at all!

 

[kae_jolie]

Thanks for reply. Let me ask my question in a different way:

In HFSS, there are S11 plots that will tell you the bandwidth and dB measurement which preferrably should be below -10dB.
Also, in HFSS, there are antenna parameters that tell you the power incident, accepted, radiated, gain, and directivity.

My Question is: how HFSS computes S11 and power radiated and how they are computed in real life scenario? Also, what is the relationship, if any, between S11 and power radiated?

Thanks.

 

[varuniyengar]

S12 that must be below -10dB

---------- Post added at 23:42 ---------- Previous post was at 23:40 ----------

S11 signifies return loss! or power that is reflected back to the lumped/discrete port

 

[kae_jolie]

varuniyengar,
Agree. How is S11 measured compared to power radiated from antenna? I would appreciate a direct answer to my question.

 

[FvM]

S11 can be measured very easily with a network analyzer, radiated power is not so easy and always less accurate.

But you shoul consider a simple fact, that hasn't been mentioned yet. If the antenna is lossless, all input power will be either reflected or radiated.

 

[kae_jolie]

FvM,
You mentioned: "If the antenna is lossless, all input power will be either reflected or radiated." Why is that?

Also, how would you measure radiated power? Add to it how would you measure gain?

 

[RCinFLA]

There is no specific relationship between S11 and radiated power, other then you know if power is reflected from antenna it cannot be radiated.

A resistor with a stub wire can have a good S11 but not much radiated power.

It is measured by making spherical field power measurement around an antenna. This is done to specify the radiation efficiency of a cellular phone.

 

[FvM]

There is no specific relationship between S11 and radiated power

I don't agree. An antenna, that has low losses by design, e.g. a wire antenna with good conductivity is known to have low losses. Than the relation is obvious.

Also, how would you measure radiated power? Add to it how would you measure gain?

There is no easier way than measure the field strength for both polarizations, rotating the antenna in two axis in an anechoic chamber. Near field effects and limited calibration accuracy of the measurement antenna has to be considered.

"If the antenna is lossless, all input power will be either reflected or radiated." Why is that?

It's called conservation of energy. Of course, no real antenna is exactly lossless, but it can be low loss.

 

[varuniyengar]

Its not the material that determines the reflection but the impedance mismatch!!

The radiation field is measured using far field to near field conversion using antenna dish.

 

[kae_jolie]

"Its not the material that determines the reflection but the impedance mismatch!!"

I agree with this statement. My dipole is PEC and I have S11 <-10dB.

---------- Post added at 05:21 ---------- Previous post was at 04:18 ----------

An IEEE paper I was reading stated the following: "The S11 of the antenna array shows that the maximum power is radiated at ~5GHz"

S11 at 5GHz was almost -20dB, which was max dB. Anyways, the author of this paper seems to make a relationship between S11 and power radiated, which I still don't see how. Maybe, if someone could present an equation that links the two.

I don't think you can make the assumption that all power accepted (after impedance mismatch) by antenna is radiated.

 

[FvM]

Maybe, if someone could present an equation that links the two.

It's basically simple. If you assume a lossless antenna, which is just another word for "that all power accepted (after impedance mismatch) by antenna is radiated", then you can derive the equation from elementary network theory (s11²+s21²=1). If the losses are low, the result will be essentially the same.

For an antenna with not neglectable losses, you can still expect a qualitative relation of s11 and s21 (e.g. s11² + s21² > a), what the author of the said paper apparently does. Is is possible, that an antenna achieves low s11 at a particular frequency due to frequency selective losses rather than increased s21? Theroretically yes, but for usual antenna geometries it can be excluded. So in practical RF engineering, it's quite common to use s11 as a quick check for the antenna frequency characteristic, particularly to determine the center frequency and bandwidth of resonant antennas (which most antennas are).

The directional characteristic respectively the "gain", that can show considerable frequency dependance, can't be derived from s11 of course.

 

[biff44]

It is misleading to generalize like that. At least half of the antennas I work with are electrically small, and the S11 has little importance. In fact, I often have to tune an antenna by looking at its S21transmission to a broadband test antenna, as the S11 data is useless for frequency tuning--there are s11 nulls all over the place due to parasitics, or the S11 dip when tuned might only be 2 db. Sometimes the s11 null does not even coincide with the S21 maxima!

Just on Friday an engineer was in my lab tuning up 469 MHz antenna lengths on my network analyzer by measuring vswr. They were full size whip antennas, but the ground plane was way too small. After half a day, he was still confused about if he got it right. I helped him set up a transmission test, and only after we took 2 of his "tuned" antennas and measured transmission from one to the other did he see that he got the tuning aproximately correct. They were not "efficient" radiators, but were tuned on-frequency at the end of the day.

 

[varuniyengar]

biff44 you are right!! Bang on target.

Basically they tune it by removing metal patch if its a patch antenna.

 

[FvM]

It is misleading to generalize like that.

At least half of the antennas I work with are electrically small

I feel tempted to discuss some details, e.g. what it means if s11 only dips by 2 dB. But perhaps we can agree, that my consideration shouldn't be applied to electrical small antennas.

 

[varuniyengar]

dips means its -2dB it means out of 100% power .02% of power reflects back to the lumped port. This can be further reduced using advanced design and fab techniques.

 

[biff44]

But that is the real world. Note, the original question is "how is radiated power measured", not how is it theoretically calculated. You measure an antenna, and you get all sorts of odd results.

Usually, all you care about is how much power goes from point A to point B. S11 can be a misleading answer to that question.

 

[kae_jolie]

I agree S11 can be proportional to power radiated in case of lossless antenna, but in case the antenna is near other lossy objects like human body, the power radiated would be decreased due to losses (due to power absorbed by human body), but S11 should remain the same. Is this correct? Please confirm.

Thanks a lot guys.

 

[varuniyengar]

If is a 3D antenna for higher freq range far field to near filed conversion is done and using Cylindrical or Spherical coordinate system the Radiation Patterns are measured in the principle planes like the azimuth and 2 elevations. If its a patch Network analyser is used.

Then for return loss, its just calculated using analytical and empirical formulas. Agilent Network analyser does this function too.

Odd results because mathematical design and real time design will always not match. But this can be reduced using advanced design tools.

 

[FvM]

-2dB it means out of 100% power

In the reported case, it means the opposite: a badly matched antenna is reflecting most of the supplied energy rather than radiating it.

Usually, all you care about is how much power goes from point A to point B. S11 can be a misleading answer to that question.

Yes. But in addition, it's helpful to understand, what can be concluded from S11.

---------- Post added at 10:39 ---------- Previous post was at 10:19 ----------

but in case the antenna is near other lossy objects like human body, the power radiated would be decreased due to losses (due to power absorbed by human body), but S11 should remain the same.

Measurements of an antenna embedded in an enviroment, e.g. a mobile antenna considering the user is a more complicated problem. It goes beyond clearly specified antenna properties, that are determined in an an-echoic chamber without any disturbing objects. Objects near to the antenna will surely show in the S11, and can change it in both directions, tune as well as de-tune the antenna. But generally, if lossy objects are involved, the s11 variation can't be translated into radiation efficiency change.