What is the purpose of taking -10dB bandwidth ?

when talking of bandwidth we always say of having -10dB bandwidth.... what is the purpose of taking -10dB bandwidth..? why not other..

Hi,
I'ts more useful and common to talk about -3dB bandwith related to 1/2 of power, sqrt(2)/2 voltage magnitude, and "Q" in second order linear circutis.

I have seen -10 dB bandwith criterion in the specifications of a UWB antennes.

The fact is a -10dB bandwith lies in a UWB system definition:

http://www.craf.eu/uwb.htm

For antenna antennas, you will have S11 and it is the return loss. We want to reduce it in order to improve the efficiency. The question is how big |S11| is acceptable. For this reason, we define the critiera as |S11|<-10 dB. When |S11| = -10 dB, it corresponds to about 10% of the incident power gets reflected. It is the normally accepted standard. Some special antenna may requre |S11|<-20 dB. It means the return loss is 1%. It is much harder to design and achieve.

Most of the time antennas are difficult to design for more than -10
dB of return loss. For others part of a communications
systems the parameters are very well defined and
controlled, for the antenna the impedance could change
due to external conditions. 10% (= -10dB) looks like
a good compromise point. It does not have anything to do
with -3dB or something like that. That is just plain stupid.

My take on this is that the world wants to reduce antenna performance evaluations to a single number to describe good or bad. I have not seen any universal standard that comes close to this, including bandwidth measurements.

There are a variety of bandwidth definitions in play. Your question has sparked a bit of the debate as it looks like people are responding with arguments where different bandwidth definitions have been assumed.

I suspect theat the -10 dB bandwidth you are referencing is the -10 dB (s11) bandwidth common in broadband antennas. It is based on the view that over a relatively wide frequency range one can achieve a reflection coefficient lower than -10 dB. Note that nothing is stated about the pattern shape or efficiency of the antenna. I don't see any real reason for setting the bandwidth limit to -10 dB rather than -8 dB for example. It is a convenient level. If one was designing a corrugated horn feed one might set the level to -25 dB. this kind of standard is a function of the particular situation.

My point is that too often these comparisons are incomplete. When evaluating antennas a variety of characteristics need to be evaluated. Be sensitive to the various definitions that could be applied and when debates start make sure that the definitions are at least the same.

Yes, I agree with Azulykit, there are more parameters such as radiation pattern, radiation pattern stability with frequency etc etc.

I have only one question about S11 limit:

"I don't see any real reason for setting the bandwidth limit to -10 dB rather than -8 dB for example."

Yes, It's true, but may be "De Fano's criterion" can define the theoretical limit? (looking for a lowest reflection)

but, in a more complicated sense since R(w)+jX(w) (reactance of antenna), varies with frequency and may be this impedance funcion can't be approches with an N-Lumped element networtk....


Best regards

it depends on the application S11 is related to VSWR

for communications VSWR reqd is 1.5:1
but for radars and satellite communications 2:1 is Ok

depending on this we chose -10dB or -3dB or -7dB bandwidth

Ok,
two years ago, I was into a design of UWB antennas, and in this case, the criterion was -10 dB (2:1). And in a narrow band communications I use 1.5:1.

May be In some cases (I think), for example in digital communication systems the spectral mask and time-delay performances are implied is involved (we can view a pair of antennas and free space as a filter), and in other cases the aboslute power of the system is directly related to criterion, for example, is not the same a 10mW transmission system with amount of 10% reflected power or a 10kw transmission system with same VSWR, in the first case the reflected power is 1mW in second 1kW that is disspated in heat or can cause high voltage breakdown in a cavity filter.