From: n2ic@longs.att.com (Steven M London +1 303 538 4763) Date: 25 Aug 94 12:25:00 +0600 To: cq-contest@tgv.com Subject: Open Wire Summary Many thanks to N3SL, VK5GN, K3NA, K4XU, W2VJN, WA6SDM, K7GM, N3RS, N6TR, WA3NNA, L6LL, KD5PJ, K0SF, N5OP, WA8YVR and K8CH for their extremely helpful comments on open wire line. ----- Yes. Years ago, I did just that--hardline was too expensive (No CATV). I had two runs one of about 200 ft, the other 300 ft. The shorter run fed a 10m monobander (later a 204BA), and the longer a pair of phased 40m verticals. I used 4:1 baluns at each end. I was very pleased with the results, and given the need, would do it again. ----- You might want to talk to W9RE about open wire feeders ... he used to feed several of his beams that way. As I recall, it was primarily for economic reasons in his younger days when he was -- ahem -- short on dough. I never asked him about it, but I get the impression that it was a maintenance headache and also presented a variety of "flakey" behavior problems that had to do with proximity to towers, etc. I will see if I can get him on here to comment about it. ----- Hiya, Steve! Justthought I'd comment on your idea: I think it sounds great. The 450 ohm ladder line stuff is quite good, with losses comparable to what can be acheived my rollinh your own. Also, with the spacing required for 450 ohm stuff, rolling your own is a problem; most home-made stuff is around 600-800 ohms and baluns at those transformations van get pretty lossy. About the only problem I think you'd have is if wet snow gets on the line; the stuff becomes lossy then and it's impedance will undoubtedly change. Fortunately, we don't have lots of that here. If it comes at a bad time, there's nothing to keep you from running a broom along the line to clean it off! ----- There was an excellent "how-to" article in Ham Radio Magazine in the early 80's (I'm guessing 1980 or 1981) where a W4 experimented with very long runs of open wire. He had some dimens and techniques that he fell in favor with over others. I would strongly recommend checking a Ham Radio Magazine bibliography of those years (maybe even '79 and '82) and read the article. Seems to me he even had 1000' lengths on 10 and 15 meters - with good results! (noticebly better than coax or hardline). If I can find the article, I will reply. As a RF engineer, I would recommend two things: Run the lines horizontally parallel, rather than vertically parallel to ground, since the line currents will tend to unbalance if they're asymmetric to ground, which I'm assuming is the nearest object. Also, tight corners are bad (discontinuity - large bend radii being much preferred), and I would use a low characteristic impedance (200 is better than 600; i.e., closely spaced lines will be much less susceptible to loading or unbalancing by external objects and/or sag). Toroids are getting so good these days that you can wind very high quality broad-band matching transformers easily, but I would sweep them vs. freq to insure no resonances or impedance deviations before stroking up power. Also, avoid running the lines very close to gutters, towers etc. Again, I believe closely spaced lines (low zo) are the way to go since it tends to make the transmission line (which can be thought of two equal, yet opposing currents changing sinusoidally in space & time; when these aren't equal, you have an antenna, in fact, a beverege or travelling wave antenna) much less vulnerable to surrounding objects. Sag may be important in 600 ohm lines (2 - 4 inches apart), whereas a 200 (~.5 inch) ohm line will not be as sensitive to the same sag. The same W4 proposed more exotic 4 and 6 wire lines which help control the characteristic zo by spacing transmission line wires next to neutral (near ground plane reference) wires. More work, but the concept works. This system is analgous to microwave stripline. By the way, I design microstrip antennas and transmission line systems for a living. (I would not have responded if I felt I had nothing technical of value to contribute). Good luck. I am interested in your results, as I have 19 acres waiting to be populated (space, but no time or money; takes all three to be loud these days). ----- A while back (last year??) there was a similiar thread on cq-contest. I seem to recall the comments of the rf-gurus (W0UN, etc.) pointing out some relatively unknown (at least to me) pitfalls of 2-wire ladder line. Something about noise picked up from wind moving the wires or moisture. Don't quote me here, I'm not 100% sure. I *do* recall, however, the same rf-gurus pointing out the advantages of *4 wire* balanced line. Sure enough, my old (1960 vintage) ARRL Antenna Handbook has a description and formula for 4 wire line. It sez: "In cross-section, the conductors of the four-wire line are at the corners of a square, the spacings being of the same order as those used in two-wire lines. The conductor at opposite corners of the square are connected together to operate in parallel. This type of line has a lower characteristic impedance than the simple two-wire type. Also, because of the more symmetrical construction it is better balanced, electrically, to ground and other objects that may be close to the line. The spacers for a four-wire line may be disks of insulating material, "X"-shaped members, etc." According to the chart given, a 4-wire line of 200 ohms impedance can be built from #14 wire spaced 2" apart, the distance being measured from opposite corners of the "X". Perhaps this type of line, using 4:1 baluns, may work better. ----- Almost every shortwave broadcaster in the world uses open wire feed for runs to the antennas. I just returned from a trip to the VOA sites in SV, CN, A2 and S9. Miles of 4 wire 300 ohm line going to huge curtain arrays pointed everywhere... fed with at least 250kW. When I lived in East Tennessee, a friend used ladder line to his antennas. He lived in a real hole but owned the hill behind about 800 feet away. He supported it on trees and pressure treated 12' 2x4s using cheap ceramic electric fence standoffs. Don't run it flat - twist it a few times between supports to maintain balance and prevent flapping in the wind. Only maintenance problem was occasional falling branches from ice/snow storms. The kind of "snow" you get in TN is not available in CO. Rotator cable was a major expense. Low loss 9:1 baluns are a snap: 5 to 10 trifilar turns of #18ga Teflon wire on a 3" ferrite core in a suitable enclosure. As you said, its a low swr environment. Build two at a time and test them back to back into a 50 ohm D/L. How does ladderline compare to true open wire line? Very well. It is within .1dB/100' of true 450ohm open wire at 30 MHz. And it certainly is cheaper and easier than building your own. It is also easy to set up an experiment on the ladder line - to test out the system before you commit to it.... Regardless of where you get your hard-line, by the time you put connectors on the ends, your investment is too high - unless you already have cubic dollars and no children. ----- I would be concerned about degrading the nulls of directional antennas in the receive mode by using open wire line. I don't think the achievable real world cancellation/shielding is nearly as good as coax. ----- I tried using about 150 feet of 300 ohm open line - the old stuff with bare copper wire and the little plastic insulators spaced every couple inches - on 2 meters a few years back and it worked pretty good ... until it got wet. Then the signals disappeared !! Somewhere in the fine (invisible?) print I think it says that those super low-loss figures for open line are only valid when the stuff is DRY. ----- I think you are better off to make your own. K6NA has done this. You want to get as few insulators as possible for there to be leakage. You can find nice ceramic insulators that you can mount on top of your posts. My 300 foot runs are 7/8 inch hardline which I was lucky enough to get free. ----- Steve, Unless you are talking VHF or UHF, there is no appreciable advantage to using open wire lines. Hardline, for runs up to 500-600 feet will be well within a dB of the open wire line up through 28 MHz and you will not have to worry about the stuff getting iced-up or shorted by getting twisted in high winds. ----- Many a year ago, I was one of the operators/station assembly people at K8UDJ (now K8CH at ARRL). He had long runs and we used open wire line. The only hassles were getting it out of the house and keeping the ice off it. For you and the baluns the ice bit should be the only problem - when it gets iced up it doesn't work so well. Other than that, no losses to speak of and easy to install if you are in an area you don't have to bury the stuff. ----- When I was in the navy I was stationed at the San Francisco Communications Station, located in Stockton,CA. I went the the transmitter site a couple times at Davis, CA. They have open wire transmission lines all over the place going to the various antennas. The power levels were 10 to 50 KW. The baluns there looked like power company pole pigs. Saw pictures of commercial shortwave broadcast stations, and they make big use of open wire. One even had an open wire 'coax'. Seems like it is used by the big guys, so the idea definitly is worth looking into. ----- In the early 60s I put up a vee beam which was fed with KW 300 ohm line and tuned for 80, 40 and 20. Every time it rained the loading on my PL-172 amp would change enough to kick out the screen overload relay and the presets for band changing were useless. Even after waxing the line and twisting it the same would happen. So I made up some open wire using #14 and plastic Polaroid film spreaders left over from the Space Race. We were forever documenting circuit operation with pictures of scope traces to bring to the endless NASA design reviews so I had lots of spacers. The spacing was about 3.5 inches I think. Anyway, this completely cured the problem and there was no change in loading with rain or snow. Fair Radio has some excellant spacers for open wire that are ceramic and 2.5 x 0.5 inches. They would make a light weight line that would be very low loss. A few years ago I visited W1EVT, now KF1Z in the Boston area and Clem had 12 towers on a hill top with driven wire arrays for all bands and all directions. They were all fed with long runs of open wire. Anyone who has ever been in pileups with Clem knows how effective his arrays were. ----- I made the measurements about ten years ago when I worked at Ten-Tec. The comparison was made between 6" open wire line with ceramic spacers every 10', Zo about 650 ohms, and 450 ohm ladder line. Frequency was 65Mhz, the upper limit of the HP-601. I used a Boonton rf voltmeter as the detector. Johnson matchboxes were used at each end to make the bal-un transition and transform back to 50 ohms. Identical parallel 250' runs of each were measured. The difference was 0.5dB favoring the open wire, which translates to 0.092dB/100' difference at 30 MHz. Poly does two things: it provides a support/insulation role and makes the spreaders easily, and as a dielectric, it slightly reduces the spacing necessary to get to 450 ohms. The higher the impedance, the lower the loss. Windowing the line lowers the weight and the loss - both the initial dielectric loss and the 'aging loss'. Polypropylene is an outstanding rf dielectric provided it is not exposed to uv radiation which causes it (and most other plastics) to become brittle which decreases strength, causes surface cracks which permit ingress of contaminants. The brown opaque fillers added to prevent uv deterioration do increase the loss somewhat. Like many other things in life, nothing is simple! It is difficult to get good quality 450 ohm open wire line. It is hard to keep it from twisting shorted on long horizontal runs. The ladderline is inexpensive, easily obtainable at good quality. And it won't short by twisting. ----- Many thanks for your open wire information. Have been an open wire fan for decades. Just to be sure, is garden-variety poly ladder line (ie, approximately 1" ribbon line with "windows" cut out) really within 0.1 dB/100 ft of "true" open wire line at 30 MHz? Seems hard to believe with all that poly still there. Is that manufactures data or did someone actually measure it? ----- I have not used the manufactured stuff that looks like twinlead with windows punched in it. I did use ladder line for an incidental application one time. It was fine for the intended purpose (feeding an extended dipole on multiple bands). The main usage that I have made in the past is for long transmission line runs. These were constructed out of 4 wire open wire line. 4-wire line exhibits better balance characteristics and therefore (a) picks up less atmospheric noise, (b) distorts antennas patterns less by picking up less signal from unwanted directions, and (c) can be used with multiple runs closer together than 2-wire line. My feeds were as follows, looking at the wire end-on: A B B A The runs were mounted on pressure treated posts up high enough to avoid tangling with people, deer, etc. The wire were copperweld (I think #14). Each post had an insulator with 4 holes that the wire passed FREELY through. At one end of the run the wires were terminated into (in order): small thimbles, short turnbuckle, and forged eyes mounted on a block of insulating material. The opposite end of the block had another forged eyebolt and a dacron braided rope over a marine block to a heavy counterweight. The counterweight kept the line very straight and took care of temperature expansion/contraction. The turnbuckles were used to initially balance the tension in all four wires, a set and forget procedure. (You can use only 3 turnbuckles and run the 4th wire direct to the eyebolt, as you are only setting the tension of the other 3 lines to equal that in the fourth. It sound complicated but it really was a snap. I put small egg insulators a foot or two before the turnbuckles to break the line and attached the balun at this point.) Because of the counterweight tension, there was no need for any spreaders other than the post-mounted insulators. The post-mount insulators can be easily made out of a sheet of insulating material with holes drilled at the right places. I also used a holesaw to cut out the material between the wires, in order to minimize any impedance bump from the insulators. Probably a nit (as long as nothing metallic lays between the lines.) Anyhow, the thing worked great. There is a reason why commercial installations used a lot of this stuff (besides power-handling ability). Oh, the wire spacing was about 2 inches on each side of the square and that gave about 600 ohms, I think! (Check the handbook for the calculations.) ----- I use 4 wire (200 ohm) line with 4:1 baluns at each end from shack to base of tower. Works fine. We don't have the luxury of ex-cable company hard line here! Good article in Oct 1980 Ham Radio on the subject. If you have trouble getting it I will photocopy and post - just let me have your address. -----