The "impedance bandwidth" is just the ordinary bandwidth of the antenna.  Normally this is defined as the range of frequencies over which the return loss is acceptable.  Percentage is referring to a quantity more commonly called fractional bandwidth (FBW).  This is simply the absolute bandwidth (or impedance bandwidth) divided by the center frequency of the antenna.

FBW = BW / fc

The fractional bandwidth is a better measure for bandwidth when comparing different antennas because it is independent of scale.  For example, if you design an antenna that operates at 100 MHz and has a bandwidth of 10 MHz, the FBW is 10%.  If you make the same antenna, but with all physical dimensions halved, this new antenna will behave the same way as the old antenna, but at 200 MHz.  Its bandwidth is now 20 MHz, but its FBW is still 10%.  If you used only the absolute bandwidth, the 200 MHz antenna would seem better than the exact same antenna designed for 100 MHz.

% bandwidth =    ( fH  -  fL  )/ Central Frequency

Consider one example: (i) Someone want to design antenna at 2.5GHz and impedance BW occurs for that antenna from 2GHz (fL) to 4GHz (fH). If we calculate %BW w.r.t 2.5Ghz, one will obtain % BW = (4 - 2)/2.5*100 = 80%. (ii) if the design frequency is 3.5GHz and for the same absolute BW (fL to fH) , %BW = (4 - 2)/3.5*100 = 57.143%. Therefore, case (i) gives apparently better % BW for same absolute BW (fL to fH) which is not correct. To avoid all confusion, % BW is always calculated with respect to center frequency (fc) i.e. % BW = (fH - fL)/fc *100 where fc = (fL + fH)/2.

Hi V. Manikonda, you can do like this:

%BW=(BW/fC)*100

Where, BW= fH-fL, Centre Frequency (fC)= (fH+fL)/2

Dear Venkateswararao Manikonda,

Use this

% Bandwidth = {fH-fL/Centre Frequncy}x100

These are good explanation and clear formulas

Percentage bandwidth =(Higher frequency-Lower frequency)/Centre frequency x100

To define the bandwidth of the antenna one normally measure its S11 magnitude as a function of frequency. The antenna bandwidth is normally defined as the frequency range at which S11 magnitude is below - 10 dBs. At such S11 value the reflected wave is much smaller than the incident wave pointing out good impedance matching between the antenna and its feed line. As the this range is a band pass range having center frequency f0, the one can define a fractional bandwidth as the bandwidth divided by the center frequency.

Best wishes

The percentage of impedance bandwidth basically depends upon two things, which are: 1) Center Frequency 2) Bandwidth. The 'Bandwidth' of an antenna is considered for some given amount of return loss i.e -10dB or -15dB. While, 'Center Frequency' is the frequency of operation associated with the antenna.

Eventually, Impedance Bandwidth = (Bandwidth/Center Freq.) x 100.

Percentage Bandwidth can be calculated as %BW=[2*(FH-FL)/(FH+FL)]*100

Bandwidth for antenna can be calculated taking reference level of VSWR=2, or reflection coefficient =-10dB or return loss =10dB.

The fractional or percentage bandwidth can be calculated as :

% BW = 2(fmax - fmin)/(fmax +fmin) *100

Where, (fmax + fmin)/2 is called center frequency (fc).

So, for wideband antennas %bandwidth must be calculated with respect to center frequency. If calculated based on resonant frequency, the percentage bandwidth will differ.

For narrow band antenna, calculation with respect to resonant frequency is ok.

I fully agree with Sudipta Das and other contributors, but as an additional remark, it should be said that, in the context of the definition, impedance bandwidth it is bandwidth, inside which the impedance has values in framework given limitations.

Sudipta Maity

Raymond Rumpf

Hi dears,

I want to compare the bandwidth of two horn antennas in CST studio but I do not know how can I show the beamwidth in CST.

I have attached the S11 response to this message.

I really appreciate it if guide me to find it.

Dear Ramin Amiri,

From the Fig as given by you, we can calculate BandWidth (BW). If you want to calculate the Beam Width, just post the far-field patterns (E and H-planes, both).

Hi

Percentage bandwidth is defined with(Higher frequency-Lower frequency) divided by Centre frequency multiplied by 100.

Agree with Dr Dudipta Das and others but it should be also defined that if there are more than one frequency with minimum number of return loss ( steep deep) in between Fmax and Fmin.then which one will be considered as Fc ( central frequency)

%BW = [(Fu - Fl)/{(Fu + Fl)/2}]*100

Dear Khalid

In case of multiple band, we need to calculate the % fractional BW for each band individually using the same formula mentioned above.