Electronics > RF, Microwave, Ham Radio

Am I changing the impedance of this coax?

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iXod:
I posted a question re. what F-connector to use with a coax:

[url=https://www.eevblog.com/forum/rf-microwave/what-f-conns-to-use-with-belden-1694a-rg6-coax/]https://www.eevblog.com/forum/rf-microwave/what-f-conns-to-use-with-belden-1694a-rg6-coax/[/url]

A secondary question that came out of that post is this:

If it’s VERY difficult to push on a compression connector (to the point of needing pliers to hold the connector) while pressing home the stripped and trimmed coax, isn’t this potentially changing the impedance? The impedance of a coax is determined primarily by the critical distance between the center conductor and the shield. Forcing a connector with incorrect inner diameter under the jacket and shield will stretch out those components of the coax. Changing this distance will change the impedance, yes?

Thanks,
iX

radiolistener:

--- Quote from: iXod on September 25, 2022, 02:35:20 pm ---The impedance of a coax is determined primarily by the critical distance between the center conductor and the shield. Forcing a connector with incorrect inner diameter under the jacket and shield will stretch out those components of the coax. Changing this distance will change the impedance, yes?

--- End quote ---

Yes, if you bend coax cable too much you can detect impedance changes and catch reflections from that point of cable on TDR or vector network analyzer.

This is why there is limit on coax cable bending which should not be exceeded in order to keep cable specifications.

Using wrong connector also leads to impedance mismatch and signal reflections. But below 1 MHz it's not significant, so if you're using low frequency signal it should not lead to an issue. But for RF it's critical.

paul@yahrprobert.com:
if a transmission line has a section of length l which is a slightly different Z0 than the rest, then there will be reflections from the two ends of the discontinuity.  If the change in impedance isn't too big and if the length of the section is small compared to a wavelength then the two reflections will cancel.  So unless you're trying to make an F connector work into the microwave region you don't need to worry.

mag_therm:
Did you try warming, not heating,  the coax end with heat gun before pushing in the connector?

Also you might be able to find drawings of the connector showing its diameters to compare with the standard dimensions of RG6.

TheBay:
The distance doesn't get bigger as the tube that slides under the braid is conductive and becomes part of the shield.
I just read your post that you linked to and it won't make any noticeable difference in your application.
I use Cabelcon F connectors and they can be a bit difficult to push on, as long as they are stripped correctly and no braid is getting caught you should be able to ease them on by rotating the connector back and forth as you push.

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