From owner-qrp-l@Lehigh.EDU Fri Feb 27 09:44:04 1998 Received: from fidoii.CC.lehigh.EDU (fidoii.CC.lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.8.5/8.8.5) with ESMTP id JAA26354 for ; Fri, 27 Feb 1998 09:44:03 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <12910-28820>; Fri, 27 Feb 1998 09:43:42 -0500 Received: from nss4.cc.Lehigh.EDU ([128.180.1.13]) by fidoii.cc.Lehigh.EDU with ESMTP id <12882-28820>; Fri, 27 Feb 1998 09:42:04 -0500 Received: from mail1.unicomp.net (root@mail1.unicomp.net [209.41.64.24]) by nss4.cc.Lehigh.EDU (8.8.8/8.8.5) with ESMTP id JAA25948 for ; Fri, 27 Feb 1998 09:41:45 -0500 Received: from carpenter (cpu209-41-110-148.unicomp.net [209.41.110.148]) by mail1.unicomp.net (8.8.5/8.8.5) with SMTP id FAA18148 for ; Fri, 27 Feb 1998 05:53:06 -0600 Message-Id: <3.0.1.32.19980227084133.00687f44@mail.webwide.net> Date: Fri, 27 Feb 1998 08:41:33 -0600 Reply-To: w5usj@webwide.net Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: Chuck Carpenter To: "Low Power Amateur Radio Discussion" Subject: Zip Cord Antenna Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Sender: w5usj@mail.webwide.net X-Mailer: Windows Eudora Light Version 3.0.1 (32) X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO All, The earlier comments on portable and backpacking antennas got me thinking about zip cord antennas we used to make. About 100 feet of zip cord stripped back 33 feet would make a 40 meter dipole fed with about 65 feet of balanced feed line. An unbalanced to balanced tuner would be used between the rig and the antenna. The ZM-2 or equivalent should be excellent for this application. The rubber insulation of the late '50s zip cord was not the best insulator and I have no idea what the losses were but the antennas worked. Radio Shack has several sizes of zip cord type clear plastic-insulated speaker wire. I did a little empirical evaluation of some that I had on hand which included 2 types with 20 ga stranded wire and 1 with 24 ga solid. Empirical Results: 20 ga (.030), .187 spacing -- 190 ohms, 15 pF/ft 20 ga (.030), .094 spacing -- 139 ohms, 20 pF/ft 24 ga (.020), .062 spacing -- 139 ohms, 17 pF/ft (These values are consistent with 75 ohm twin lead shown in earlier handbooks. The loss per 100 ft at 14 MHz is between RG-58 and RG-8, or about 1 dB.) I chose a dielectric constant value (k) of 2.5 which compares to the values for most plastics in the tables. Didn't check breakdown voltages but they should be adequate for QRP levels (I've used the 18 ga stuff for lamp cords -- ~335 V PP). The capacitance was measured on a 12 inch length by using my dip meter and a 10 pF 2% capacitor/coil combination. I checked the dip frequency with and without the cable. A reactance nomogram was used to find the approximate cable capacitance. For a test of reasonableness, I substituted a capacitor of the same value for the cable. The dip frequency stayed essentially the same. Further testing and measurement is planned with more elaborate equipment. If the values above are truly representative, it looks like a simple, easily portable antenna could be made from the speaker wire. A 2 to 1 impedance matching transformer would closely match the ~139 ohm cable to the ~73 ohm antenna. The SWR using the cable directly though, would only cause about 1 dB of additional loss and may make the transformer not worth the effort. 72/73 -- Chuck, W5USJ, EM22cv Rains County, Eagle Capitol of Texas ARCI # 5422, QRP-L # 1306, FISTS # 3984 From owner-qrp-l@Lehigh.EDU Fri Feb 27 18:50:02 1998 Received: from fidoii.CC.lehigh.EDU (fidoii.CC.lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.8.5/8.8.5) with ESMTP id SAA10984 for ; Fri, 27 Feb 1998 18:50:01 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <12859-65432>; Fri, 27 Feb 1998 18:48:11 -0500 Received: from nss4.cc.Lehigh.EDU ([128.180.1.13]) by fidoii.cc.Lehigh.EDU with ESMTP id <12826-34198>; Fri, 27 Feb 1998 18:47:11 -0500 Received: from bbs.galilei.com (bbs.galilei.com [208.19.235.34]) by nss4.cc.Lehigh.EDU (8.8.8/8.8.5) with SMTP id SAA52868 for ; Fri, 27 Feb 1998 18:47:03 -0500 Received: from uymfdlvk [208.19.235.35] by bbs.galilei.com with smtp id BBDBAPEC ; Fri, 27 Feb 1998 17:49:16 -0600 Message-Id: <1.5.4.32.19980227233904.00715cf8@bbs.galilei.com> Date: Fri, 27 Feb 1998 17:39:04 -0600 Reply-To: david.gauding@bbs.galilei.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: David Gauding To: "Low Power Amateur Radio Discussion" Subject: Re: Zip Cord Antennas Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-ROUTED: Fri, 27 Feb 1998 17:50:10 -0600 X-TCP-IDENTITY: David Gauding X-Sender: david.gauding@bbs.galilei.com X-Mailer: Windows Eudora Light Version 1.5.4 (32) X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO Gang, Gotta' agree completely with what has gone before in this thread. Zip cord antennas (or K5BDZ's "Bumble Bees") have worked here for eighteen years. The late W0VM (call recently re-issued) made a believer out of me when it came to tuned feeders. Bill Stocking also took time to point out to folks that balanced antennas *and* antenna tuners did not damn users to the everlasting fires! Further, that while on earth they would probably work enough stations to keep radio interesting. My favorite ingredient is an old spool of #24 stranded speaker wire from Radio Shack. I use an electrician's knot for a center insulator and shirt buttons at the ends. The integral feedline can be any convenient length. The concept also works well as a half-size G5RV and covers 10-40m nicely with a 51' flatop! As you might expect the total weight for antenna and feedline is measured in ounces. If your not comfortable using the electrician's knot try a heavy-duty guitar pick as the center-insulator. For portable work miniature alligator clips are great for attaching zip cord or 300 ohm feedlines. One caution! I have never had good luck with traditional lamp cord. The pigment used to color the rubber brown or black does not seem to be happy around RF. For grins, try making these center-fed zepps from two conductors of flat computer cable. For this variation switch to shirt collar buttons! The grey conductor cable becomes almost invisible against the sky after it weathers for awhile. de Dave, NF0R nf0r@slacc.com From owner-qrp-l@Lehigh.EDU Sun Mar 1 12:59:53 1998 Received: from fidoii.CC.lehigh.EDU (fidoii.CC.lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.8.5/8.8.5) with ESMTP id MAA00842 for ; Sun, 1 Mar 1998 12:59:51 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <12862-38410>; Sun, 1 Mar 1998 12:59:06 -0500 Received: from nss4.cc.Lehigh.EDU ([128.180.1.13]) by fidoii.cc.Lehigh.EDU with ESMTP id <12804-38410>; Sun, 1 Mar 1998 12:56:46 -0500 Received: from maxwell.com (server1b.maxwell.com [199.120.55.3]) by nss4.cc.Lehigh.EDU (8.8.8/8.8.5) with ESMTP id MAA37570 for ; Sun, 1 Mar 1998 12:56:37 -0500 Received: from inferno.scubed.com (inferno.scubed.com [192.31.66.42]) by maxwell.com (8.8.7/8.7.2) with SMTP id JAA24284 for ; Sun, 1 Mar 1998 09:56:19 -0800 (PST) Received: from [192.31.66.158] by inferno.scubed.com (S3.4/s3-sgi-5) id RAA20136; Sun, 1 Mar 1998 17:49:12 GMT Message-Id: Date: Sun, 1 Mar 1998 10:59:24 -0700 Reply-To: ji3m@maxwell.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: ji3m@maxwell.com (James R. Duffey) To: "Low Power Amateur Radio Discussion" Subject: Why Zip Cord Feedlines Are a Poor Choice Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Sender: ji3m@192.31.66.42 X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO Zip cord antennas work, I don't think anybody said they wouldn't, but there are better ways to feed antennas than with Zip cord. Lets start with some Ham History. Back in the days before Coax was commonly (and inexpensively) available one alternate to the almost universally used balanced feedline was 72 Ohm balanced feedline. This was also in the days before plastics were common. The 72 Ohm balanced feedline looked a lot like the "lamp cord" that was used in light duty (i.e. lamps) household wiring, but was rubber insulated. The household stuff was twisted cloth covered wire. Some Hams used the lamp cord as balanced 72 Ohm feeders instead of the commercial balanced feeder. This was in the depression remember, where scrouinged parts kept many a Ham on the air who could not otherwise afford it. Getting on with higher feedline losses was more important than not getting on at all. Ham lore dies hard and when vinyl covered lamp cord (Zip Cord) became commonly available after WWII, some Hams remembered the pre war lamp cord feedlines and used the Zip Cord for feedline (and antennas). However surplus Coax was widely avaiable, and the use of Zip cord never became as big after the war as it was before the war. History lesson aside, there are several problems with using Zip cord for feedline; 1) The Vinyl insulation commonly used on Zip cord is rather high loss at HF. The losses for vinyl are much higher than the losses for the polyethylene used in coax or window line. This leads to a feedline with higher loss than even lowly RG-58U. This loss in zip cord was measured by the ARRL in the late 70s and appeared in QST at that time as well as in Handbooks until a few years ago. I forget the exact numbers, but losses are excessive above 40 M. I can look up the exact numbers and post them when I get home. 2) Manufacturing tolerances are poor for zip cord so the wire to wire spacing varies considerably. This leads to a transmission line with widely varying impedance along its length. This varying impedance will lead to a complicated reflected wave along the line, and really complicates the definition of "properly terminated" transmission line. This makes calculations of exact losses difficult. They will be higher than for a transmission line with constant spacing that is properly terminated in its characteristic impedance. 3) The Vinyl insulation used on Zip cord is designed for indoors use. It will often crack, turn brittle and depolymerize when used outdoors. Then it will be subject to wide variations in its properties when wet. The significance of this point may be difficult to grasp until you have spent an entire QSO when it is raining with one hand on the key, two hands on the antenna tuner controls, one hand on the SWR meter, and one hand on the transmitter level control. You get the point then. On 80 M the loss in Zip Cord is probably mangeable for reasonable lengths of feedlines. On 40 M and higher bands the loss increases significantly and one is much better off (lower losses) with even RG58. The losses will be very great for nonresonant antennas fed with zip cord; a 80 M dipole fed at 40 M as an example. As an emergency or experimental feedline, Zip cord can probably be justified. As an everyday feedline there are better (lower loss) choices. If you are going to Radio Shack for a feedline, even their coax will have lower losses than their zip cord (even the audiophile zipcord). I would suggest that those using zip cord feedlines for moderate runs would gain a dB or more in signal strength (more if you are using it on 20 M and higher) if they replaced the feedline portion of the zip cord antenna with a good low loss feedline such as RG213 (RG8), RG8X, or even RG58U. Window line is a good alternate for antennas to be used on multiple bands. I recognize people build zip cord antennas and are happy with them. That does mean that they are the best technology available. Even a mediocre antenna is better than no antenna at all. If you are planning a new antenna installation, and you want the best performance for your efforts, avoid zip cord. The money you save isn't worth it. There are no advantages other than cost. It is not good economics to build or buy a ham station worth several hundred dollars and then try to save ten or twenty bucks on the feedline. This isn't meant to be a flame. I just hate to see only one side of an issue presented, so I posted this. You can make contacts with zip cord antennas, but with QRP we are trying to make every milliwatt count. Zip cord will rob you of your precious power. There are better ways to feed antennas. I can address the bumble bee won't fly myth as well, but that doesn't seem to be QRP related. - Duffey KK6MC/5 James R Duffey KK6MC/5 DM65 30 Casa Loma Road Cedar Crest, NM 87008