twin coax feedline - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: September 01, 2020, 07:16:40 am
content: I've been reading some stuff about using two parallel runs of coax as an "open" feedlinehttps://www.m0mcx.co.uk/how-to-use-dual-coax-feeders-as-ladder-line/https://www.hamuniverse.com/kf9focfdipole.htmleither to run a balanced feeder through walls or near obstacles or all the way from the antenna to the shack, now I wonder if anyone here tried such an approach and which were the resultsAt the moment my thought is that running some ladder/window line down from the feedpoint, connecting it to twin coax near ground, running the twin to the shack/building ingress, grounding the twin there, connecting it to a balun and then entering the shack using a single coax from the balun could possibly be the better approach; am I wrong ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2020, 07:41:00 am
content: Another case of hams doing whatever works, without doing a bit of analysis.Losses for example aren't any lower, it's still just coax.The match sucks. Two coax lines acting in series is 100 ohms. That's a 3:1 mismatch to twin lead, or even worse to ladder line I suppose. (Which isn't the same as VSWR, but the minimum VSWR can be calculated from it.)The other guy puts a 4:1 balun at the bottom, getting a slightly better match, 200 ohms into 100 ohms. The dipole might be an okay match to the twin coax (depending on ground height), but it's not a dipole, it's some bastard unequal thing. So there will -- intentionally -- be feedline currents, and that ground at the balun is obligatory.Most of this doesn't matter, because these are short lines (10s of m?) at low frequencies (80, 160m bands I guess?), on narrow bandwidths (ham bands), with tuners. What's all this "VSWR"? You can work a steel fence with a wide enough tuner, it doesn't really matter. Without SNR measurements, or radiation patterns or anything, there's no metric for success. How can you tell that some particular contact has been possible thanks to the antenna's gain, or due to random atmospheric ducting? The ham community accumulates a lot of superstition; only very few practitioners take the trouble to simulate or measure their installations.Tim

author: A.Z.
author_rank: Super Contributor
timestamp: September 01, 2020, 07:45:39 am
content: Thanks for the reply Tim, that's why I asked here; there's no measurement, so I was wondering if someone here did some test and had some further infosas for the 100 Ohm, if my brain is serving me, a sky loop antenna should have a feedpoint impedance around 120 Ohm, so a twin coax feeder may probably fit well thereEDITalso, from further readings the idea seems to date back to sixties and was then revamped in eighties and at those times analyzers and simulations weren't available at large

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2020, 10:35:20 am
content: They're current tech, you'll find twin microcoax used in mobile displays -- needed for the high bitrates (~10Gbps?), differential signaling, and mechanical flexibility. Flat flex is a close runner-up, but no good for very compact hinges like on laptops, and just not as good signal quality for the highest bitrates.You would ideally want a 4-port (or more!) VNA to do analysis here, but realizing that they're simply two coax cables acting in series, greatly simplifies things -- you can cut the problem in half down the symmetry line, and now you've got ordinary coax; you can use baluns or monopoles to fill in for balanced antennas; and you can deal with common mode (shield current) same as ever.And you are making a more complicated system by peppering it with baluns and stuff, but those can also be analyzed independently; the system is linear so there's no problem measuring individual bits then putting everything together, or measuring the whole and separating the various knowns.I actually clicked on this thread in the hopes that it would be something more novel, like intentionally using shield currents, on two evenly spaced cables, to make twin lead that way. The impedance can be controlled from 100 ohms or so (just a big fat pair, same as twisted pair) to 300 or more, depending on spacing. The former case being commercial dual-drop CATV cable for example, and the latter being anything you want to construct (assuming you don't mind the tedium of constructing ladder line by hand!). The shield probably doesn't have great losses by itself (it's not a solid, smooth conductor), but it might still be, well probably not cheaper, but maybe more convenient or more on-hand than brand new cable or wire or tubing, of the same OD. Plus the coax doesn't stop being coax, you can send more signals (or power, or whatever) up the core and separate it with isolation transformers or bias tees or whatever, to get more signals up the mast without compromising on power handling.Tim

author: richard.cs
author_rank: Super Contributor
timestamp: September 01, 2020, 02:22:48 pm
content: So far as I can see it's a perfectly reasonable approach to creating a screened, balanced feed, but as others have said it is in no way a good impedance match for ladder line. Sensibly you can create a 100 or 150 Ohm balanced line which would be a reasonable match to some types or antenna, but is nowhere near 300/450/600 Ohm twin feeder / ladder line. Using commercial 75 Ohm "shotgun coax" to create a 150 Ohm screened balanced line is quite appealing in many respects and could be transformer matched to 50 Ohms (or close enough) with say a 9:25 Z ratio transformer.

author: A.Z.
author_rank: Super Contributor
timestamp: September 01, 2020, 08:47:13 pm
content: Quote from: richard.cs on September 01, 2020, 02:22:48 pmyou got the point, let's say one is a condo dweller and to get the feedline in the shack needs to run inside or near a building, now a ladder line would be a hell, while a twin coax feeder may solve the issue, sure, a ladder line would be better, but if it isn't viable, a twin coax may help; my doubt is mainly about losses and the best way to setup such a feeder

author: richard.cs
author_rank: Super Contributor
timestamp: September 01, 2020, 09:13:52 pm
content: Quote from: A.Z. on September 01, 2020, 08:47:13 pmWell I would expect it to be lower loss than a single coax, at least at HF where Ohmic losses dominate over dielectric losses, simply because the current is halved for the same power level.this is false, see belowThat's not an inherent advantage for a balanced screened setup though, two coaxes in parallel would have the same benefit for losses just at (a rather less convenient) 25/37.5 Ohms rather than 100/150. The main advantage is in having a balanced feed, not in lower losses which are only halved and not as low as they could be with ladder line or nicer coax. A balanced feed avoids putting the balun out in the rain, and allows the use of a balanced tuner which can be lower loss, if only because it avoids sloshing power backwards and forwards through a potentially lossy balun.A random thought, if you didn't connect the screens together at all you'd have a rather higher impedance line because the two coaxes would be in series with each other and with the parallel wire structure created by the two coax shields. With close-tied cables you might add 20 Ohms or so to the total. Probably wouldn't work all that well in that the propagation velocities between the shields wouldn't match the coax so over any significant length there would be a phase error and probably some "exciting" resonances. That and you loose most of the advantage over ladder line.Edit: losses, see below

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2020, 10:03:06 am
content: Can you explain how using more of a transmission line reduces loss?Tim

author: richard.cs
author_rank: Super Contributor
timestamp: September 02, 2020, 11:03:10 am
content: Quote from: T3sl4co1l on September 02, 2020, 10:03:06 amSure. At HF the dominant loss mechanism in a coax is resistive, largely associated with the centre conductor, and not dielectric losses. These losses scale as I2. Running two coaxes, whether in antiphase as a balanced line or in-phase giving 25 Ohms unbalanced halves the...Bugger.It doesn't, it just doubles the maximum power handling. My bad.Power per line is halved, but the impedance within the line is unchanged that only takes the currents down by sqrt(2) so losses per line are only halved, so no change in overall system losses. I'll blame that on late-night posting.

author: A.Z.
author_rank: Super Contributor
timestamp: September 02, 2020, 11:36:19 am
content: First of all, thank youThen, even if losses won't be reduced but are still near (or below) the single coax feedline ones, having a balanced line all the way down the shack and being able to run it through concrete or near to metal objects makes the idea interesting, I never pretended to replace ladder/window line but instead to have a viable solution for the "apartment dwellers" and from what you posted, the twin coax seems to be a decent approach; sure it won't be true ladder line, yet it would be a "shielded, balanced line" and that alone, in my opinion, makes it an interesting solution to a lot of problems which often surface; all in all, not all people has the luxury of being able to lay out long antennas and/or ladder feedlinesthank you again !

author: A.Z.
author_rank: Super Contributor
timestamp: September 02, 2020, 01:53:14 pm
content: found some infoshttps://owenduffy.net/transmissionline/stcm/index.htmnow, at 4K the mismatch loss is pretty high when compared to real open line, yet I believe that twin coax may be a viable workaround

author: richard.cs
author_rank: Super Contributor
timestamp: September 02, 2020, 02:49:12 pm
content: Quote from: A.Z. on September 02, 2020, 01:53:14 pmI'm not sure I agree with the conclusions of that article. Of course he is correct that common mode currents can still flow on the transmission line, and when they do they will be on the outside of the shield, but then he concludes that it makes it pointless which seems dubious. If the antenna is reasonably balanced then the common mode signal may be 10-20 dB down on the differential mode, so the field around the screened balanced feeder due to that common mode current would then be 10-20 dB smaller than the field around a ladder line (which in the near field is due to the total power flow not just the common mode component). i.e. he's concluding that as it doesn't reduce it to zero it's of no benefit.It clearly has benefits over ladder line with regards to near-field coupling, what's less clear is how much benefit there is over feeding with a single coax with a remote balun. It also has higher losses than ladder line, but then I thought the whole point was to use it in situations when ladder line wasn't appropriate because of nearby structures, in which case so what - it's lossier than some alternative that you can't use anyway.As I see it (now I have been corrected on losses - thanks Tim) its main advantage over a single coax is allowing a balanced tuner and generally moving the transition to unbalanced indoors. Loss-wise it doesn't gain (or loose) you anything over a single ended setup with half the amount of coax, so if you are budget-constrained you might be better off with a single coax - by buying half as much of it perhaps you could afford something with lower loss.One thing I certainly wouldn't do is transition between twin coaxes and some other kind of balanced feeder, be it ribbon or ladder line, without impedance matching. A mismatch of 100/150 Ohms to 300/450/600 is just too much, though I suppose it might be just about tolerable for short lengths.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2020, 05:15:04 pm
content: Quote from: richard.cs on September 02, 2020, 02:49:12 pmThis resolves because, if it's properly balanced, then it's no better than a low loss balun and a single coax (aside from the 3dB power handling for two cables, as mentioned above), but you still have all the losses so it sucks worse than twin lead.You still have to deal with common mode, either way; there is the special case where you don't need a transformer (CMCs, CT choke) at (2N+1)/4 wavelengths. But for everything else, no difference.Also the one motivating example (from OP) specifically uses an unbalanced antenna, so not only is CM an intentional part, it means the feedline is a radiating element, too. So not only are feedline currents expected from such a design, but the radiation pattern and tuning depend on where the line is hanging, and what length it is.QuoteAnd I'm not impressed by the proximity claim -- if you can't afford spacers because the space is too cramped... just find somewhere else to put it? I don't know. Proximity for a small fraction of a wavelength is fine, it's not an entry problem.Also, everything that applies to shields and common mode, still applies to anything twin lead: you can implement even better (I would dare say?) lightning protection by using a CT choke (ground the CT). Or isolation with a balbal, etc.QuoteSpeaking of tuners -- if your antenna tends to need a high tuning ratio, put the tuner close to the antenna if at all possible (wire a servo motor to it, perhaps? and a webcam?). Best way to avoid loss due to high VSWR is to not have high VSWR, second best is to have high VSWR over a minimum length.It's not even apparent to me how a 1-λ dipole is supposed to radiate. Is it entirely due to asymmetry, nonuniformity or proximity? Or being operated just slightly off resonance? By trying to tune one up, you're literally just trying to make everything worse for yourself; the whole reason the match is shite is because you're straining to find any real resistance to dump power into -- even poorly-coupled modes, and that includes common mode on the feedline.Tim

author: richard.cs
author_rank: Super Contributor
timestamp: September 02, 2020, 05:51:02 pm
content: Quote from: T3sl4co1l on September 02, 2020, 05:15:04 pmWell clearly a full-wave dipole can be resonant and support standing waves, it just has current zeros at both ends and in the middle so it's exactly like two half-wave dipoles end to end. Functionally wouldn't change anything if you cut the wire in the middle and fed it at 1/4 and 3/4 as two independent half-wave dipoles so long as you phased them correctly.* The radiation pattern is exactly the same as if you did have two separate half-wave dipoles, in some directions is cancels and in others it sums, so the radiation pattern is different is all.Or are you meaning specifically the case of a full-wave dipole fed in the centre? It's like feeding a half-wave from the end which people commonly do through fairly high ratio matching transformers, the impedance is high but not infinite, on the order of 5-10 kOhm dependent on ground proximity and wire thickness (a fatter antenna greatly reduces the peak impedance). This would be doubled if feeding the centre of a full wave dipole. I agree it doesn't sound like the easiest or most sensible place to feed a full-wave dipole**, don't most people feed them 1/4 the way along at a current maximum so it's almost like feeding a 1/2 wave?* Hmmm, I wonder what kind of pattern you'd get as you change the phase, could this be an interesting way to make an electrically-steerable antenna I wonder? Maybe it's time to get nec out.**People largely do end-fed half-wave because the feeder can be shorter with their particular setup, feeding a full wave dipole in the centre doesn't have this advantage but retains the disadvantage of high impedance so other than wanting to re-use something from another band I don't see why you'd do it.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: September 02, 2020, 06:07:54 pm
content: Multi-feeding an antenna is possible (I've seen it done for some applications like power-combining). However, you can't just willy-nilly pick your feedpoint or 'ignore' the other feeds. Your other feed changes the 'apparent' impedance of the other loads, similar to how load-modulating works in other power-combining situations. It's already a (small) problem is antenna arrays where each antenna is entirely separate from every other antenna. Just trying to feed two points of a rod with different phases will a) give you random radiation efficiencies based on the phases b) just give you unpredictable impedance that go all over the place.So I doubt it will work well, esp if you want to 'steer' a beam - just stick to plain-old arrays of radiators that you phase-shift between

author: richard.cs
author_rank: Super Contributor
timestamp: September 02, 2020, 06:55:48 pm
content: Quote from: TheUnnamedNewbie on September 02, 2020, 06:07:54 pmWell what I was pondering was the simplest case of that, just two radiators fed seperately. But physically located end to end. In case it wasn't clear I was thinking with an insulting break in the middle. With only two radiators there's probably no particularly useful pattern, it just appeals as it only needs two supports.

author: A.Z.
author_rank: Super Contributor
timestamp: September 07, 2020, 06:58:50 am
content: Quote from: richard.cs on September 02, 2020, 02:49:12 pmThat's exactly what I meant, that is, I'm not saying that people should throw away ladder/window line and replace it with twin coax, but that, in case one cannot run ladder line all the way down the shack entry, the twin coax may represent a viable approach to still have a balanced line all the way downQuoteexactly, you may have a balanced line all the way down the shack (or shack entry), plus the twin coax may possibly be a better match for some antennas (e.g. skyloop) which has a characteristic impedance near the one of the twin coax feeder (around 100 Ohm for 50 Ohm coax, around 150 Ohm if using 75 Ohm one)Quotewell, in some cases the twin coax cut at a length may act as a matching stub, but in general, I believe you're correct, I still wonder if and how much the twin coax impedance influences the line characteristics and also what impact the twin coax (with bonded shield) config has on the velocity factor

author: tkamiya
author_rank: Super Contributor
timestamp: September 19, 2020, 03:07:56 am
content: Here's my theory on antennas.There is a right way to do it. Then there is a way that has to be done. We are hams. We make do with whatever we can get. Only few of us (certainly not me!) get to choose a site, setup a perfect arrangement, and have antennas work as designed. My current antenna is a long/random wire fed with coax. SWR is sky high. But it works with a tuner.I think your whole issue is you are going to have to use coax to feed it into your shack. Then, how about setting up an balanced/unbalanced tuner outside and closer to your antenna before it goes through obstacles? It can by DIY'd cheaply if your power level is low (like 100 watts). I believe there are kits available, too. (you might ran into impedance range issue with these) Or, you can do it old fashion way. Wind coil your own coil and mark where to connect it to, for each band.Coax twin feed line is a non-sense. Twin/ladder line has field between lines. Coax does not. Field exists between center conductor and the shield. So while it may be balanced, it's not a twin line. If I were going to do that, I'd just use a thick coax and keep it very short - just enough to go through the wall and to a tuner. Loss may be high, and technique improper, it's short and it'll work as well as it could.

author: GordonZ
author_rank: Newbie
timestamp: March 31, 2021, 10:05:13 pm
content: My HF antenna is a 92 foot attic doublet shaped as a 26 foot x 12 foot nearly closed rectangle. It is fed by two 75 ohm RG6 cables of exactly the same length. The coaxes are zip tied together every 8 inches. The coax shields are connected to each other at the antenna end and grounded at the tuner end. The coaxes are about 60 feet long. I am using a MFJ-974HB balanced line tuner. My shack is on the 3rd floor of my condo and the antenna is about 10 feet above. I used this configuration because I cannot have an outside antenna and I have a HVAC unit and ductwork in the attic.I am using this antenna with an Icom IC-7300 for FT-8 on 80m - 6m transmitting 15-25 watts. The antenna can be tuned on all bands to a 1:1 VSWR. My location is Atlanta, GA. On 30m - 10m, I can reliably reach all of North America, Central America, Caribbean, South America, Europe, Northwest Africa and South Africa. I also get strong spot reports from Antarctica south of Africa on these bands. On 40m & 80m I am usually limited to about 2500 miles, although I have had a QSO with New Zealand on 80m. On 6m, I can cover the US and Canada east of the Rockies and also the Caribbean. I can tune the antenna to a 1:1 VSWR on 160m but my most distant spots are about 50 miles. The antenna is just too short for that band.I would say for a compromise antenna, it performs very well.

author: geggi1
author_rank: Frequent Contributor
timestamp: April 01, 2021, 06:31:21 pm
content: I use a 86m circumference loop fed with two paralell RG59 and a multi range ballun at the TRX. The ballun is a 1:4 / 1:9 /1:16. When tuning the antenna i have found it to be simplest to get on resonance using 1:9 or 1:16.

author: A.Z.
author_rank: Super Contributor
timestamp: March 05, 2025, 11:36:13 am
content: Quote from: geggi1 on April 01, 2021, 06:31:21 pmsomething along these lineshttps://archive.org/details/spencerpdf/page/n2/mode/1upjust using a loop in place of the doublet

author: A.Z.
author_rank: Super Contributor
timestamp: March 05, 2025, 11:39:35 am
content: Quote from: richard.cs on September 01, 2020, 02:22:48 pmthe characteristic impedance of a transmission line and the impedance measured at its ends aren't the same thing

author: geggi1
author_rank: Frequent Contributor
timestamp: March 06, 2025, 10:03:24 pm
content: I use two RG-59 in parallel as a feedline for a 83m loop.This gives a feedline of 150ohm. This is not to far from the impedance of a loop.Use a manual tuner by the TRX and you are good.The two lengths of RG-59 must have the same lenght.