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# How do I define Bandwidth?

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#### nwittema

##### Junior Member level 2
Hi All,

Very stupid question, but usually I refer to bandwidth (in school) as say 25MHz-100MHz. When at work it's always 6:1 or 9:1 or 2.8:1 etc. How is this defined? how can you look at a graph and just see it? I attached an image can you explain its BW?

~Nick

Hi Nick,

It is necessary to define a criterium, and it is dependent of the application.
For example, the bandwidth of a filter or a spectrum is usually defined at -3 dB points, but other levels are possible.
In your case, is would be possible to define the bandwidth at -10 dB return loss (this corresponds to VSWR<1.92:1) or at VSWR<2:1 (correponding to -9.6 dB return loss). This would give a bandwidth from 5 to 6.5 GHz approx.
If you would adopt instead the criterium of WSWR<1.5:1 (another very usual, corresponding to return loss of -14 dB) the bandwidth would be narrow in this case. Nevertheless, the design could be tuned (approaching the two resonances so that te peak at 5.85 GHz decreases) in order to wide it.
Regards

Z

Hi,

Thanks for the reply, I am more concerned as to the bandwidth as in some number : some number. Where do those numbers come from?

Nick

It is evident from the graph: for example, at the ordinate -10 dB the corresponding abscissas are approx. 5.0 and 6.5 GHz.
Regards

Z

Hii

the 3:1 or 6:1 is another way of charactersing the bandwidth of wideband antennas.

it is usually given by :

highest frequency/lowest freqeuncy

where highest and lowest frequencies are taken to be at -10dB return loss.

so its 1.3:1 for the graph posted by you

i hope this helps

regards

Hi
The bandwidth as say 25MHz-100MHz is defined as lowest frequency-highest frequency, and the bandwidth 3:1 or 6:1 is defined as highest frequency/lowest frequency.
where lowest frequency and the highest frequency are the band limits at which the response just meets your specification.
As in your attached image, if your specification is return loss less than -10dB, the lowest frequency is 5GHz and the highest frequency is 6.5GHz.
So the bandwidth is 5GHz-6.5GHz or 6.5:5=1.3:1
regards

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