who can help me on filter vswr in low temperature??

I designed a coaxial filter in comuniciation frequency
in the room temperature it is good ,but when the temperature is -25℃，to my strange ,part of my products is good ,but the other part is bad .
and when the tempertaure is back to room temperature,the vswr can't back to good,who can tell me why ???

and the other kind filter,in the low temperature the VSWRis bad but when the temperature is back to room tempperature ,it's good too ?

Check the humidity inside the chamber. At low temperature higher humidity can make the VSWR of the coaxial filter to be higher.

If there are any lumped components, they can expand and contract
and this might change the characteristics of your filter. If you can try a printed filter, they are more imune to this problem. Also, if VSWR is the
only issue you can put isolators at the input and/or output.

Good luck

This reply assumes that you mean a coaxial quarterwave stub filter.

In my experience, near full rated power operation, stub filters will arc at the open circuit end in high humidity conditions. If you can get the filter open (try not to disturb the stubs or any coupling components) you should see burn or blast marks on the end of one or more stubs. You may also be able to hear a fizzing sound from the filter under failure conditions.

There are really only three cures for this problem:

1) Reduce the power.

2) Reduce the humidity or increase the temperature. Air conditioning units which blow cold air over your filter can have the effect of suddenly lowering the temperature and causing 'rain' inside your filter (I've seen this happen in the past).

3) Get a filter with a higher power rating.

I hope this helps.

there have not lumped elements,
and if the huminity is the cause ,then why some of the filter is good??
and there is't have much power

Please post a drawing of your coax filter and a close up photo here using a digital camera. Both files in JPEG-file format. Then it's more easy to try to see the cause of error.

I understand the bad VSWR remains on some of the filters after return from cold to room temperature?