S11 frequency response

What does a S11 frequency response tells us?

This is my understanding. Pls advise if I am correct.
From what I understand, S11 is a measure of the reflection from an antenna. 0dB means that all the power is reflected, hence the matching is not good. -10dB means that 0.1 * incident power is reflected, meaning 0.9*power is accepted by the antenna. Coupled with the fact that most antenna designed are with low loss, the 0.9*power is radiated out. Hence, when we see a dip (V shape) cutting the -10dB level, we normally label the apex as the resonant frequency. However, we can also apply a matching network to the antenna to make sure it is conjugate-matched to 50ohms. If so, we can design antenna with any frequency we want?

You are right in some situation. But im most cases, the power of PA output is very expensive. So from PA to antenna, the loss should be as low as possible. And in most cases, the antenna size is limited.

S11 will tell you whether the signal transmitted through the line will get reflected or fully absorb. it will tell you if the line is
50ohm or otherwise. This may be the first step to know you antenna whether it is a 50ohm impedance characteristic. However
it does not fully tells you that the antenna will radiate efficiently and will dissipate with heat and or absorbed. its just tells you
that how much of the signal will be reflected when that frequency is at -10dB region.

S parameters are not the ratios of reflected to incident power but voltage ratios. so if s11 is -10 db then the power will be 90%. where it is showing max dip it can be taken as resonating freq of the structure.

for the next que, antenna will have a fix resonance freq and bandwidth and if you are using any matching network then its resonance freq should be in range of antenna bandwidth and its bandwidth should be more than antenna bandwidth. eg. take a dipole for 1 GHz and BW of 100MHz then matching network should have resonance within the range of 950 to 1050MHz. and its BW should be more than 100MHz for efficient use of antenna. but as antenna has the fix resonance it is not possible to resonate it with any other freq just by changing matching network.

tony_lth said:

You are right in some situation. But im most cases, the power of PA output is very expensive. So from PA to antenna, the loss should be as low as possible. And in most cases, the antenna size is limited.

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What is a PA?

---------- Post added at 14:28 ---------- Previous post was at 14:15 ----------

Why and when (at what frequency) does an antenna resonate?

This is my understanding. Pls advise if I am correct.
An antenna resonate when its impedance is real, ie zero reactance.
a. This is because when its impedance is real, it could be easily conjugate-matched to 50ohms??
b. An antenna resonate has the same meaning as an antenna radiating at maximum efficiency (which is when it is conjugate-matched to 50ohm, hence a matching network that ensures the antenna is conjugate-matched will make sure the antenna resonates or radiate with maximum efficiency.)??
c. What is the meaning of resonance? If the S11 frequency curve show a small dip (eg. from 0dB to -6dB), could it be defined as resonance? If it shows a gradual dip (maybe over a long frequency range) before it reaches -10dB (the shape is no longer a V shape), could it be defined as resonance. Lastly, if the curve shows a V shaped dip (but not starting from 0dB but from -6dB) could it be defined resonance?
d. If an antenna has a long range of frequency below -10dB, does it mean that its gain is lower as compared to one with a narrower bandwidth?

S parameters are not the ratios of reflected to incident power but voltage ratios. so if s11 is -10 db then the power will be 81%.

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The calculation is incorrect. S-Parameters represent amplitudes, but for lossless networks, reflected and transmitted power add to unity. -10 dB S11 means a magnitude factor of 0.31 respectively a power factor of 0.1. The transmitted power factor will be 0.9, as correctly calculated in the original post.

yes, fvm is right.

impedance of the antenna is the factor affecting the power transfer from the feeding structure to antenna. any structure resonates because of any discontinuity in structure, change in velocity of charges like banding or terminating or by feeding it with any oscillating signal.
its depend on the application how we take the meaning of resonance.
if the antenna is having flat s11 below -10 db for long bandwidth then its gain will be same as the antenna having -10db s11 for resonating freq if the directivity of the structures is same..

kindaichinisan said:

What is a PA?

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PA is power amplifier, that is the last power amplifier of the transmitter. Usually it's the most expensive component in the transciver system. If the output power of PA increase 1dB, the cost increase more than 1dB.

If the output power of PA increase 1dB, the cost increase more than 1dB.

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How do you calculate db$? 10*log(cost) or 20*log(cost)?

FvM said:

How do you calculate db$? 10*log(cost) or 20*log(cost)?

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20*log(cost) is the correct for voltage or amplitudes.
i used other one first time.

I think this is the reason of the actual global financial crisis.
Wall Street guys use to calculate their profits in dB's, and the profits starts rising exponential.