Impedance when wiring to a connector - Page 1
Forum section: electronics / rf-microwave

author: jgalak
author_rank: Regular Contributor
timestamp: May 28, 2018, 04:10:16 pm
content: I am considering modifying an antenna by changing from a PL-259 connector to an N connector (this is a 2m/70cm dual band J-Pole from Arrow). The current connector is just screwed onto the stud of the driven element, with a nylon shoulder washer. So the coax part of the connector just presses against the mounting bracket (which also holds the other elements) and the center pin of the connector is bolted directly to the element.I'd like to mount a chassis-mount Type N female socket about 1" away. The body of the socket will press against the same mounting bracket, so that should be fine, but the center element will have to go to a ring terminal on the stud. Maybe 1/2" of wire.What I'm wondering is, can I predict what that will do to the impedance? How do you tell what impedance that but of wire will have?Specifically, I am trying to figure out if I'll get less losses this way or by just attaching a PL-259-to-Type N adapter onto the existing connector. My solution seems more elegant, but I don't know that it's actually better.I'm also interested not just in solving this, specific, issue, but also in learning how one calculates this in general - I'm sure it's not the last time I'll need to connect a coax connector to something else with a piece of wire.

author: CJay
author_rank: Super Contributor
timestamp: May 30, 2018, 02:05:06 pm
content: Frequency dependant, YMMV, no guarantees etc. etc. but I found a nice test report of RF plugs, adapters and related 'stuff' by a Ham with a VNA who was expecting to find all sorts of nasties but had to concede that the type of plug and the number of plug-adapter-plug connections made a measurable but considerably smaller than expected difference, certainly not the kind of difference a *lot* of Hams would tell you they 'know' it will make.Damned if I can find the tests now...*edit* Think this is the onehttp://www.iz2uuf.net/wp/index.php/2016/01/08/pl-259-vs-n-on-430-mhz/So, the answer is, it might make a measurable difference but it probably won't be noticeable at any frequency up to VHF and will probably work just fine at UHF too if it's a decent quality connector (I have UHF radios with SO-239 connectors, I don't think the effort to change them would be worth the reward).You'd be more likely to get noticeable results by changing your coax.

author: iMo
author_rank: Super Contributor
timestamp: May 30, 2018, 04:43:21 pm
content: QuoteYou may provide a calculation, but the effort related is not worth of it, imho. There is a lot of imprecise parameters or guesses you will do such your results will be fuzzy.The only way to get relevant results is to connect a good VNA or antenna analyzer and watch the data.Even at short waves a calculation seldom gives precise results.

author: KJDS
author_rank: Super Contributor
timestamp: May 30, 2018, 05:31:47 pm
content: It's possible to predict it, just needs a suitable application of Green's Dyadic functions to solve Maxwell's equations.Interestingly, Green only went to school for a year.

author: jgalak
author_rank: Regular Contributor
timestamp: May 31, 2018, 02:56:49 pm
content: Well, a VNA isn't in the cards, but I do have a good antenna analyzer (RigExpert 600). I was hoping there was a way to predict this before spending the effort on doing the modification. I guess I'll have to experiment.

author: tkamiya
author_rank: Super Contributor
timestamp: June 01, 2018, 02:23:39 pm
content: Loss of an N-UHF adapter at 70cm band is quite negligible, provided it is a quality connector. One thing about J pole is that it IS a compromised antenna. Isolation isn't perfect. Coax part also radiates some. So it does not exactly follow theory and calculations. Why don't you just try it and see? Based on experience, I'd think you see some shift in resonant frequency, VSWR, but no perceivable difference in loss of signal.

author: AG6QR
author_rank: Frequent Contributor
timestamp: June 01, 2018, 10:26:59 pm
content: If an impedance discontinuity is short enough relative to the wavelength of interest, you can generally ignore it and pretend lumped circuit theory holds true.You talked about a half inch of wire. That's less than one percent of the wavelength of 2 meter radio waves.It will be extremely hard to notice.Do make sure the connection is solid, with low resistance as measured at DC. But otherwise, I wouldn't worry much.