Directional coaxial cable (twisted stranded center), a myth? - Page 1
Forum section: electronics / rf-microwave

author: kj7e
author_rank: Frequent Contributor
timestamp: January 09, 2024, 06:51:26 pm
content: I heard someone on 40m comment on how he found his coax was directional, that is, he tested the loss both directions and found one direction had more loss than the other. I don't know the details, freq, power or test methodology. I am extremely skeptical on his claims, matter of fact I'm going to put a length of RG8x I have at home on the VNA just to see if there is ANY possible validity to this. The claim is, the twist of the center conductor can affect the flow of RF.Thoughts?

author: Grandchuck
author_rank: Super Contributor
timestamp: January 09, 2024, 06:58:32 pm
content: Damaged connectors perhaps.

author: ConKbot
author_rank: Super Contributor
timestamp: January 09, 2024, 07:07:25 pm
content: When your twist length, or bumps in the conductors are measured in mm, and wavelength in meters, it would be a hard sale to get me to believe the electrically small features would have an effect that isnt something you could reduce to a lumped element model of RCL.Getting differing S21 and S12 amplitude measurements indicates cable damage to me. If you have damage that is 1/4 wave from one end and 1/2 wave from the other, you will get different insertion losses. If you're measuring with just a spec-an/tracking gen, or fixed power source and power meter(scalar measurements) you won't notice the swr being weird. Sweeping a wider frequency range, or using a VNA may help identify this.

author: kj7e
author_rank: Frequent Contributor
timestamp: January 09, 2024, 07:16:17 pm
content: Quote from: Grandchuck on January 09, 2024, 06:58:32 pmThats what I suspect as well, but the idea of twist to the center conductor did get me to think about it.

author: drvtech
author_rank: Regular Contributor
timestamp: January 14, 2024, 09:30:06 pm
content: Quote from: kj7e on January 09, 2024, 07:16:17 pmIf you think about it a little more you'll see that a twist doesn't make something directional. That's why you can screw a nut onto a piece of threaded rod from either end!

author: G0HZU
author_rank: Super Contributor
timestamp: January 14, 2024, 11:08:37 pm
content: If you measure s21 and s12 with a perfect 2 port VNA and you assume the coax is a passive reciprocal device, then the forward s21 should be the same as the reverse s12 measurement. I think it's a reasonable assumption that coax is reciprocal. Some passive devices contain ferrite and this can be non-reciprocal.If it helps any further here's a recent 2 port VNA plot of some RG213 coax I borrowed from work. I've plotted s21 and s12 for you. The markers show the same attenuation in both directions. This cable doesn't have a solid core, the centre is made up of twisted wires. I've also added a plot of about 22m of RG316 cable. The centre of this cable is also twisted strands, probably copper plated steel. You can see the s21 and s12 measurements are also the same.

author: VK5RC
author_rank: Unknown
timestamp: January 14, 2024, 11:43:25 pm
content: I would punt that it is related to those 'lovely' PL259s, 😂

author: themadhippy
author_rank: Super Contributor
timestamp: January 15, 2024, 12:08:39 am
content: Maybe they picked up the wrong reel of coax and are using audiofool grade,not rf grade cable

author: radiolistener
author_rank: Super Contributor
timestamp: January 15, 2024, 02:25:35 am
content: Quote from: VK5RC on January 14, 2024, 11:43:25 pmI'm agree, I had one PL259 which had VSWR about 1.4 at 28 MHz, just for connector... 😂It's better to replace it with SMA or N connector.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 15, 2024, 03:57:58 am
content: Bad VNA calibration alongside a damaged cable or connector.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 15, 2024, 04:45:48 am
content: Quote from: drvtech on January 14, 2024, 09:30:06 pmYep, that was my first thought too. It don't matter which way the threaded rod is flipped around, the nut still tightens the same way!

author: Bud
author_rank: Super Contributor
timestamp: January 15, 2024, 05:40:42 am
content: Quote from: kj7e on January 09, 2024, 06:51:26 pmWe should start with that.

author: fourfathom
author_rank: Super Contributor
timestamp: January 15, 2024, 06:01:45 am
content: Quote from: TERRA Operative on January 15, 2024, 04:45:48 amAs for the OP's question, no, the twist on the stranded coax center conductor makes absolutely no difference here. Even if the twisted strands were insulated from each other (and they're not) it would have no directional effect.But twist direction in RF structures definitely has an effect, if the twist dimensions are a significant fraction of a wavelength. I'm thinking of a circularly-polarized helical antenna. where left-hand and right-hand polarizations can be had.

author: kj7e
author_rank: Frequent Contributor
timestamp: January 15, 2024, 03:01:51 pm
content: Just for clarification, the guy who made the claim was stating the twist of the inner conductor in a coaxial cable would have an impact of the flow of RF energy due to the right hand rotation of the EM field. Where if the rotation aligned with the EM field rotation with the direction of flow, this would alter the path loss. I agree, smells Audiofoolery, but it did get me to think about it some.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 15, 2024, 03:07:07 pm
content: Quote from: kj7e on January 15, 2024, 03:01:51 pmI guess that's reversed when used in Australia?

author: kj7e
author_rank: Frequent Contributor
timestamp: January 15, 2024, 03:19:40 pm
content: Need to market special cable for northern and southern hemispheres. I guess it was a silly thought, if there were any validity to this, it would either effect the RX or TX and I know that is not the case, at least not to any degree that can be practically measured.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 15, 2024, 05:32:46 pm
content: Quote from: kj7e on January 15, 2024, 03:01:51 pmThe EM field in a coaxial cable doesn't rotate in the right handed direction and that's also not what "right hand rule" means in electromagnetism.The right hand rule or right handedness of EM fields in ordinary matter refers to the direction of propogation (Poynting vector) being the cross product of E and B. If E&M were left handed (as in some metamaterials for a narrow frequency band at least) the propogation would be in the opposite direction, -E x B. It doesn't have anything to do with fields rotating.It is possible to have rotating fields. This is what you get with circular polarization in free space. Here both E and B rotate as the wave propagates through space. But there is no preferred direction, left and right circular polarization are equally possible and it has nothing to do with right handedness of E & M.In any case, you don't get circular polarization in coax, or really any polarization. Coaxial cable is operated below the cutoff for the waveguide modes of the outer ground, only a single mode exists.

author: Hamelec
author_rank: Regular Contributor
timestamp: January 17, 2024, 04:40:17 pm
content: Quote from: kj7e on January 15, 2024, 03:01:51 pmbut this only happens only 3 days +- 5 hours after the full moon

author: SeanB
author_rank: Super Contributor
timestamp: January 19, 2024, 08:30:53 am
content: My bet is the cable has water ingress. When at the outer end the loss there is going to appear smaller, and when at the inside the loss is going to make the TX pull power down, because of the mismatch, and thus the lower power at the other side.