hardline copper coaxial over 1/4 inch? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: August 30, 2022, 08:43:27 pm
content: Does this exist as a RG# cable?Like imagine LMR400 but made with a copper outer tube and a silver plated steel interior wire with teflon insulation. Say 3/8 inch and larger.Now I know you can get heliax, but the point is, I want strait outer body, not the 'armored' cable heliax shape.Does this exist? For physics objects. I don't know if I should call it hardline or semi rigid tubing. I don't mean a braid thats been metalized somehow, but like a plumbing pipe. And with out a jacket. And its not about higher power rating, I just want the shape. If the power handling is low I don't care.The best I managed to do is put brass tube over a stripped braidless LMR400 cable, but I still don't like it because the middle is aluminum. I suppose I can yank the middle out also and use only the dielectric.. but I feel like this has to be available ready made?And if anyone knows the procedure for making a coaxial cable of arbitrary diameter it would be interesting to know.I imagined something like1) heat teflon in oven to some temp2) insert center conductor3) cool it down4) heat pipe in oven5) insert teflon rod with center conductor into pipe?I imagine this requires something annoying like gun-drilling teflon .

author: TimFox
author_rank: Super Contributor
timestamp: August 30, 2022, 09:00:40 pm
content: An introduction from Pasternack about "semi-rigid" cables.https://www.pasternack.com/t-Semi-Rigid-Cables.aspx?gclid=EAIaIQobChMIpa-qr7nv-QIV3MLCBB04wAOaEAAYASAAEgJCCfD_BwEThe largest size (with non-corrugated outside) seems to be 0.250 inch, but the corrugated ("Heliax") that you don't like comes much larger.https://www.pasternack.com/corrugated-50-ohm-coax-rf-cables-category.aspxThe important thing about "semi-rigid" and "Heliax" is that they can be bent (carefully) into the exact shape you need (paying attention to minimum bend radius), but are not "flexible" in the sense of RG-58/U.When I had a real budget, I found sources for pre-connectorized lengths of smaller semi-rigid cable (SMA or N connectors) to connect RF components (couplers, amplifiers, etc.) into systems, and had a careful technician use a tubing bender to produce the required interconnects.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 30, 2022, 11:32:50 pm
content: Did you look at Pyrotenax?https://www.enia.gr/wp-content/uploads/2012/11/CABLES_FireRes_new-pyro-catalogue.pdfhttps://www.nvent.com/en-us/pyrotenax/products/fire-rated-cablesAvailable in single and multi core.It can be bent to a radius with special tools.Needs Pyrotenax terminations.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2022, 12:13:06 am
content: Hmm, but that has some kind of weird insulation. I was more interested in trying to do things like enlarge the holes to make different impedances by using different diameter center condcutors or trying something like shelled (multi) dielectric cables, or drilling it out and putting structures in there. If I had a 1/2 inch to start with, it might be interesting. Mainly for curiosity, but only if I could get a large diameter and connectors for it that go to standard things like N, SMA, etc. 1/4 inch is too small to play with

author: JohnG
author_rank: Frequent Contributor
timestamp: August 31, 2022, 12:27:06 am
content: How about copper Type M 1/2" water pipe?In fact, with copper pipe, you can go really large. In my previous job, we built a 4" diameter 50 ohm line with copper pipe for a 100 MHz half-wave coaxial resonator, even plated the inside with silver. I don't recall the exact number, but the Q was about 10% below theoretical. I think we got pretty close to 50 ohms with standard pipe sizes.John

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2022, 12:29:52 am
content: yes, do you think that something like expansion foam could be used to fill them to change the dielectric constant?but how on earth did you hook up pipes to connectors?

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2022, 12:31:47 am
content: can you make a conical taper out of thin copper sheet to impedance match a connector to a coaxial cable? I imagined numbers ALOT worse then 10%.

author: JohnG
author_rank: Frequent Contributor
timestamp: August 31, 2022, 12:45:52 am
content: Everything was silver soldered. Each end had a flange to mount end pieces. One end could be short or open, where the open was another foot of empty tube with a plate at the end, to reduce radiation loss from the open. The other end piece tapered to an N connector.It was used to measure capacitor and inductor Q. If you look up Boonton line, it was the same idea. The Q just implied that losses were low when it was run as a pure resonator. When measuring Q, each end had a coupling loop made using a 1/4" hardline, and you could slide and rotate them to adjust coupling to the resonator.I'm sure the whole thing got trashed after I left.John

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2022, 01:07:41 am
content: Quote from: JohnG on August 31, 2022, 12:45:52 amsounds like it scared a business major

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2022, 01:12:22 am
content: set tomeasure

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2022, 04:03:15 pm
content: Well I designed another prototype test fixture for SMD parts using DIY hardline. I have not tested it yet.You get a RG8-N connector. You get a brass tube that almost fits over the connector *~8mm ID*Then you cut a segment of tube thats maybe 1 1/4 long, and deburr it nicely so it can almost fit on the RG8 connector body (it has a slight chamfuer that the tube should sit on)Now I was not sure how to join these, so I did the rational thing, I put it length wise in a vise and heated the brass tube with a small torch (like the weller hand held butane torches) for a while and broached/pressed the brass tube on the RG8N connector. I notice they don't seem to sell LMR400 connectors that can be soldered.Then I did some precision adjustment using a old style tack hammer to bend the tube strait so it looks like an official part of the connector in the vise. Kind of tricky to fixture, but it does yield a little bit.I measured the resistance of the broach joined junction to be like sub miliohm with a kelvin measurement, since the junction is a bit dodgy, but it can't be worse then braid being compressed on the stub.So you are left with a N connector that has 1.5 inches of brass tube sticking out of it, and you have another tube that fits into it.Now you take your RG8 or LMR400 (I used LMR400) cable and strip the cable cover and the braid so you are left with a strait section of dielectric with the CCA rod inside. Now you need to widdle the dielectric covering so that it fits into the tube. On the bottom tube, you only need to widdle it a little bit so it fits into the connector body (1cm). However, the nesting tube is going to have trouble fitting it, so you need to shave it down a few hairs so it fits inside of the top tube, you want a section about 3 inches long to start with.Now you remove the CCA rod from the shaved dielectric.The idea is that the top tube will get sealed off with a center pin (much like a shorting cap) and the bottom section will get a small pogo pin that is aligned correctly soldered to the connector center pin. Then the idea is that you take the part, you drop it into the top cap interior (so you put the part inside of the dielectric on top of the "shorting anvil"), and then you press fit it on the bottom piece with the pogo pin. It looks like it should work for small part that can fit into the LMR400 cable. I measured the resistance of the bare brass (non silver plated) "piston" that was formed and the resistance is very low (it is not yet silver plated till I work out the details) for the outer connection. The middle connection should be fine also. I am worried the part will rotate or something in the "well", so you might need to make some kind of micro adapter for it to sit in the dielectric properly.I am waiting on a piece of brass tube to attach to the center conductor of the RG8N cable terminator to act as a structure for the pogo pin to sit on, and I need to machine a brass 'shorting cap" to solder into the top of the piston.And of course I imagine there is some engineering to be done to better interface the two halves of the measurement piston in terms of contacts. And this also theoretically works as an arbitary terminator, you just drop whatever part you want inside and it will act as the bridge between the middle pin and the coax inner body. I imagine putting some kind of bayonet lock (for mechanical not RF reasons) would also be beneficial so it does not spring open, but I found even with cheap common brass tubing and the nature of the dielectric cable core being kind of tight, it does seem to operate reasonably.The big problem is going to be centering the part, I think the reasonable option is to get a tiny roll of like saran wrap or something (very thin) and just wrap the part up so when you stuff it into the piston, its roughly in the middle of the bore so the pogo pin interfaces with it.I will be curious to see how this turns out, but as of now its a very affordable experiment. I also thought to make it with a screw instead of a pin, so you insert the part and tighten it from the top (so the screw is the center pin of the shorting cap), but I don't like the idea of rotation/torque on the top of the part from tightening the screw, it would need a micro electrical rotational decoupler of some kind maybe (well I would like to have a ball of liquid metal on the tip of the screw so it just bearly needs to touch but you can't get what you want), but I can see this being made out of a modified BNC to BNC connector (they are long) and shorting cap. Perhaps its enough to put a cap on the screw with the top 1mm machined under size like with a concrete anchor, but I do prefer the press fit mechanisms in general (seriously, SMA vs BNC connectors... if you don't need it, SCREW sma, BNC is like 10000 times better for actually working with).

author: TimFox
author_rank: Super Contributor
timestamp: August 31, 2022, 04:48:06 pm
content: TNC is slightly better than BNC for precision work: BNC guts with a screw-on outer shell.The problem with BNC (as opposed to N or SMA) is that the connection wobbles.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2022, 05:06:13 pm
content: but they have a screwso you need to screw it on and then screw it down, thats too much lolI think you can use it with a N too, that might be easier to work with, for bigger parts anyway. I like my piston idea which is for N.And I am thinking, if you made a APC-7 version, the APC-7 connector already has the inner part spring loaded, so it might be even easier to make! I just don't have a spare APC-7 connector. With the APC-7, I feel like you can almost just directly grind a surface on the center pin to seat the correct SMA part size, then you just need to figure out how to make the top screw on. Like a micro size Cubic boron nitride grinding bit to make a imprint on the center pin. You could seal it off with wax or something before you start so the mechanism does not get contaminated. I think it would make a very good test fixture RF wise? I hate those god damn LCR tweezers BTW. But the nice thing about BNC, N and I assume TNC (do not have any) is that the shorting cap has the 'blades' to make a good RF contact between top and bottom, something for APC-7 would need to be jury rigged. I imagine you could make it work for a SMA or smaller connector too, but I don't know about machining things that small for 0603 parts or smaller etc.I am curious if my elegoo mars 3d printer can make the 'centering' adapter for small parts to fit inside of the bore on the 'anvil' in correct orientation.I went with N and RG8/lmr400 because I know how to get the dielectric core out of the cable, so you can stuff the tube properly.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2022, 05:17:52 pm
content: Do you know if the Heliax cable ( i never used it) contains a 'cylinder' of dielectric material that you can use? Like if you cut the outside conductor off, does the interior look like a plastic screw, or is it a cylinder? And it might be possible to just machine that to correct dimensions, but I don't own a lathe... (I use a rotary positioner for a MF70 milling machine to make round objects,but it should handle that size object OK, especially if its plastic, so long I use a sharp end mill.

author: TimFox
author_rank: Super Contributor
timestamp: August 31, 2022, 05:37:51 pm
content: All the precision coaxial connectors screw together to avoid wobble.Check the Commscope website for information about Andrew HeliaxTMcables.From a picture there, the dielectric outer diameter is like a bolt thread.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2022, 09:08:41 pm
content: wow they have more brackets then mcmaster in that cataloglast time I did something with corrugation I got mad and karate chopped a dryer hose and ripped it out of the ceiling like tarzan. you need 5000 hours experience to work with that crapthe 'industrial' decour some apartments have with corrugated ducts is disgusting

author: TimFox
author_rank: Super Contributor
timestamp: August 31, 2022, 09:30:55 pm
content: I'm not sure, but I think Andrew or Pasternack or somebody sells coaxial cable connectors that thread directly onto the dreaded corrugated outer conductor.Corrugation is not done to annoy, but to allow reasonable bending of the coax in the field, which is not required for your application.Have you considered building your fixtures on top of an off-the-shelf (female) panel-mount N connector, on the side that normally goes inside the box?They exist in many varieties, and you can use off-the-shelf 50 ohm adapters to mate the other end to your choice of coax connectors.For the "grounded" side, you have a choice of cups, washers, etc. that either mount by the large chassis-mount nut or the (2 or 4) screw holes in the corners of the mounting flange.

author: ocw
author_rank: Regular Contributor
timestamp: August 31, 2022, 11:41:29 pm
content: If you want to cut your losses down, go with some 8 3/16" MACXLine from ERI.See:https://eriinc.com/product/macxline-rigid-coaxial-transmission-line/

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2022, 01:37:16 am
content: I wonder how well a DIY tapered coaxial hardline would work like to bring a N connector to like 2 inch diameterI think its like a waveguide taper right? I wonder what angle you need. For cheap experiments I think you can use copper tape and cardboard tubes and expansion foam, maybe. Loctite tite foam might be more dimensionally uniform then normal expansion foam but I am not sure. Large diameter copper pipe is expensive.

author: Zoli
author_rank: Frequent Contributor
timestamp: September 01, 2022, 03:03:17 am
content: Semi-rigid:https://www.rfsworld.com/pim/product/html/LCF14-50JAvailable in different sizes; NAWTS.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 01, 2022, 11:45:32 am
content: I found out the hard way:Tube stock is measured ODPipe is measured ID.I do have a short piece of Hard Brass Tube COAX (about six Inches) on a 50 ohm load that measures one inch OD and has another maybe 1/2 inch OD tube inside it, air dielectric.Expensive connectors TC 600 LCM PL, I do have the N adaptor.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 05, 2022, 05:13:57 pm
content: Some place on the net id an excel spreadsheet of K&S tubing combinations for 50 Ohms.Steve

author: TimFox
author_rank: Super Contributor
timestamp: September 05, 2022, 05:34:26 pm
content: If you still need rigid "hard line" coaxial construction with air dielectric, you might look for accessories made with (now archaic) GR-874 14-mm connectors that are available on eBay.The GR-874 does not have a threaded coupling, and is "hermaphroditic" so you need not ask its pronouns.Any two connectors will mate with each other, with a relative rotation of 90 degrees about their common axis.Fixed lengths of straight coaxial construction are available, along with adapters to all the more popular connectors, and mating connectors.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 06, 2022, 03:12:39 am
content: The search is "airline coaxial k&s tubing"Hp's wonderful Appcad software will do the coax calculations for you.Steve

author: coppercone2
author_rank: Super Contributor
timestamp: September 06, 2022, 03:36:19 am
content: can you pop the dielectric out of those so you can heat it good to make a seal without it melting the plastic?The one I have looks like there is epoxy (polyester?) poured in there. Is it just 'q-dope'? The problem is if I want a good seal on this (i.e. cut off fingers and solder tube over it), its gonna char that. It looks like a very clear resin. It looks like you can slice that connector 4mm from the ridge and then have a good flange for soldering to.I love that connector too since you just slide it on lol, BNC seems like one step back, and threads seem like 4 steps back.What do you think about just fitting a tube in there and using some silver epoxy to keep it on? I mean maybe you can just stick a tube in there and the connector is good enough..

author: coppercone2
author_rank: Super Contributor
timestamp: September 06, 2022, 03:59:29 am
content: it looks like its meant for 14mm air line. What is the specification on that tube? I guess that means its intended for direct fit on a 14mm air coaxial line tube?is that the same tube in my TRL VNA kit for the airline load?and how do those airline adapters work? Does that connector end up swaging the tube, or can you disassemble it to put another tube? The finished tubes are way cheaper then the adapter by itself.And if I wanted to fill this coaxial line, to retain 50 ohm impedance, I would just need to thin the center conductor I think. That would be a requirement I think if I wanted to experiment with coaxial flares and filling, since any filler other then something with a dielectric constant less then 1 would raise the impedance unless the center conductor was thinned. That makes me wonder about a expanding airline with a small cone of dielectric inside that widens to finally fill the structure at maximum dimensions.

author: TimFox
author_rank: Super Contributor
timestamp: September 06, 2022, 01:52:13 pm
content: Physics tells us that the real part of the dielectric constant must be at least 1.

author: coppercone2
author_rank: Super Contributor
timestamp: September 07, 2022, 04:25:23 pm
content: How disappointing

author: coppercone2
author_rank: Super Contributor
timestamp: September 13, 2022, 02:32:30 am
content: just a FYI, if you are playing with brass tube, to get rid of the pressure bend from cutting, stick the tube in a drill press and press the pipe deburring tool with a drop of oil on it into the end of it to get rid of the constriction formed by the tube cutter, if you don't have a lathe, it works OK, just be careful to rub it into some sand paper on top of a sanding sponge after because it makes a sharp edge.