antenna ( VSWR Vs. Gain )

Some, but it's certainly not a direct correlation. If you have a VSWR, all you know is that you'll be reflecting energy back to your signal source. Therefore you'll have less energy getting into the antenna (and potentially being radiated).

The same can be said for a generic 2-port network. If you have a VSWR that reflects 1/2 the incident power back to the source, then the other 1/2 of the power will go into the device. Now, how much of that 1/2 power comes out the other side of the device is unknown... it could all come out (for a gain of -3 dB), or it could all be dissipated inside the device (a gain of -infinity dB), or anywhere in between.

None whatsoever - at least if you follow the IEEE standard definitions.

There is a term called "Realized gain" which is defined by the IEEE. That takes into account mismatch loss, so in that case a poorly matched antenna is likely to have low realized gain. But gain is totally independant of SWR.

Dave

I think the gain is only related to the pattern itself even the VSWR is big.

if VSWR is known, you may calculate return loss.
whereas, you may find close association between gain and directivity of the antenna, which is the maximum value of directive gain.
and yes, no direct relation between gain and swr.

hope it helps.

Regards
Ashish

thanks everyone ,

my next question , what are the main things that could make the mismatch between the antenna and the cable ?

Best regards

Basically the input impedance of the antenna varies depending on the frequency and design of the antenna. So that is what causes the mis-match of impedance's. If the antenna includes a balun, then the properties of that might change with frequency - especially if the design is based on specific lengths of transmission line. Some antennas (e.g. Yagi-Uda) are inherently quite narrow band, whereas others (e.g. axial mode helix) are inherently quite broad band.

thanks drkirkby

let me brief what have been written :
the impedance of the antenna could be changed for the following parameters :
1) The center frequency of the antenna .
2) Range of frequencies ( band ) covered by the antenna .
3) Design of the antenna ( i don't' know if you meant the shape of the antenna ).
4) length of the antenna ( specific lengths of transmission line ).

Did i understand what was written well ?

in yagi, to matching impedance u can use gamma match

really difference antenna has difference manner and its too relate to your choice
but antenna a little similar to amplifier if the gain the gain increase so bandwidth will reduce but its not a total rule because antenna current has nonlinear manner
with change in length and type of antenna u able achieve difference gain and pattern and albeit difference VSWR

You should keep your VSWR under 2, if possible. Also, ensure that the resonance (lowest S11 or reflection coefficient) occurs at the frequency you are looking to operate at. Changing the length of the antennas, normally, change resonance location. Changing the width and the feed impedance adjusts the VSWR. The better your impedance is matched, the better your VSWR will be.

Gain has no effect on VSWR or resonance unless "realized gain" is used which takes into account how much energy gets into the antenna. So a high VSWR results in a lower realized gain.