Impedance bandwidth and 3-dB gain bandwidth of antennas.

[KatoAn]

Hi!
I' m confused about the bandwidth of antennas. When should I consider the impedance bandwidth of antennas and when should I consider the 3-dB gain bandwidth?
Any comment is welcome.
Best wishes.
Kato An.

 

[PlanarMetamaterials]

Impedance bandwidth only tells you over which frequency range(s) the power makes it into the antenna. There is no need for this power to radiate. The 3dB gain bandwidth actually tells you over which frequency range(s) power is being radiated from the antenna.

The two are only the same in simple single-mode systems; where you know that the antenna only supports one mode in the frequency range of interest, and that this mode radiates.

Since the purpose of the antenna is to radiate, you usually want the gain bandwidth. Unfortunately, this is the less common metric.

 

[vfone]

I've said it many times. Huge percent of antenna papers published in the last decades specify only S11 (impedance) bandwidth of the antenna.
In antenna design, this could be totally meaningless, because everybody knows that you can have the best impedance match (S11) of an antenna, and its radiation performances (gain and efficiency) to be very poor.
Thus, when designing an antenna, tune primarily for the required antenna gain over a specified bandwidth, and just keep an eye to the input impedance, to don't get unacceptable return loss over frequencies of interest.

 

[PlanarMetamaterials]

I've said it many times. Huge percent of antenna papers published in the last decades specify only S11 (impedance) bandwidth of the antenna.
In antenna design, this could be totally meaningless, because everybody knows that you can have the best impedance match (S11) of an antenna, and its radiation performances (gain and efficiency) to be very poor.

Indeed.

Thus, when designing an antenna, tune primarily for the required antenna gain over a specified bandwidth, and just keep an eye to the input impedance, to don't get unacceptable return loss over frequencies of interest.

Or, as I prefer, use realized gain. This quantity incorporates gain (as defined by the IEEE) as well as S11 -- essentially relating power injected into the antenna to the power radiated by the antenna.

 

[KatoAn]

Or, as I prefer, use realized gain. This quantity incorporates gain (as defined by the IEEE) as well as S11 -- essentially relating power injected into the antenna to the power radiated by the antenna.

Thanks, PlanarMetamaterials. I learn a lot from your answers.
When I consider 3dB gain bandwidth, the gain means realized gain or gain, e.g. , in HFSS?

 

[PlanarMetamaterials]

Gain in general refers to IEEE gain (i.e., not considering S11). Realized gain should only be considered when it is specified.

For this reason, HFSS has the ability to plot both gain and realized gain.

 

[RCinFLA]

The antenna is what it is. A termination resistor has a good bandwidth but not much radiation. Although there can be some effect on radition pattern due to terminating impedance, the bandwidth is often referred to as the points where return loss rises so half the power is reflected back from antenna input.

Other often overlooked is the effect on RF amplifier output stage detuning due to antenna impedance which can be more significant in power output reduction then antenna mismatch reflection loss.