Am I changing the impedance of this coax? - Page 1
Forum section: electronics / rf-microwave

author: iXod
author_rank: Frequent Contributor
timestamp: September 25, 2022, 02:35:20 pm
content: 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/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

author: radiolistener
author_rank: Super Contributor
timestamp: September 25, 2022, 03:13:43 pm
content: Quote from: iXod on September 25, 2022, 02:35:20 pmYes, 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.

author: paul@yahrprobert.com
author_rank: Regular Contributor
timestamp: September 25, 2022, 05:57:01 pm
content: 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.

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 25, 2022, 07:08:43 pm
content: 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.

author: TheBay
author_rank: Unknown
timestamp: September 26, 2022, 09:54:44 pm
content: 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.

author: iXod
author_rank: Frequent Contributor
timestamp: September 26, 2022, 11:53:28 pm
content: Quote from: TheBay on September 26, 2022, 09:54:44 pmThat’s it! That’s what I was missing. Thanks so much.Someone else suggested warming (not heating) the jacket prior to slipping on the connector. I think that will do the trick.Cheers!

author: vk6zgo
author_rank: Super Contributor
timestamp: September 27, 2022, 12:16:15 am
content: Quote from: iXod on September 25, 2022, 02:35:20 pmThe proper tool for compression "f" connectors applies quite a lot of force, but also holds the connector & coax in correct alignment to fit them together.Some years back, I re-did all of my broadcast TV cabling from antenna down.The ease of use was a revelation, compared to the old "TV coax" & Belling Lee connectors.Interestingly, RG6 is quite a bit lossier than the old stuff, which was "quasi air cored", but it is much more rugged, so will maintain its specs for decades, whereas the old stuff & Belling Lee connectors start to give trouble less than a decade out.Another interesting trick is that, by removing the outer sheath, & folding back the additional shields, BNC crimp connectors for RG59 can be fitted to, at least, the version of RG6 I have.After crimping, the extra shields can be pulled back over the crimp tube & the whole lot secured with self adhesive heat shrink tubing, making a fairly neat job.