From owner-qrp-l@Lehigh.EDU Thu Jan 9 12:03:34 1997 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id MAA18744 for ; Thu, 9 Jan 1997 12:03:33 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id MAA18744 for ; Thu, 9 Jan 1997 12:03:33 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <34985-40973>; Thu, 9 Jan 1997 12:02:46 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <34837-40973>; Thu, 9 Jan 1997 12:01:38 -0500 Received: from utkux4.utcc.utk.edu (UTKUX4.UTCC.UTK.EDU [128.169.76.11]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with SMTP id MAA38244 for ; Thu, 9 Jan 1997 12:01:30 -0500 Received: from localhost by utkux4.utcc.utk.edu with SMTP (SMI-8.6/2.7c-UTK) id RAA11493; Thu, 9 Jan 1997 17:01:20 GMT Message-Id: Date: Thu, 9 Jan 1997 12:01:19 -0500 (EST) Reply-To: cebik@utkux.utcc.utk.edu Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: "L. B. Cebik" To: "Low Power Amateur Radio Discussion" Subject: RE: 300 OHM ANT In-Reply-To: <199701091529.PAA04946@chuck.dallas.sgi.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-To: chuck adams X-Cc: Low Power Amateur Radio Discussion X-Sender: cebik@utkux4.utcc.utk.edu X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO Lots of antennas that call themselves Windoms ain't. Technically, a Windom, named after its developer way back in the 20s, uses a single wire 600 ohm feedline. Better to call the whole lot "off-center-fed" antennas. Here are a few facts gleaned from modeling these antennas with and without parallel feedline (and I mean a physical feedline that will show whatever radiation may appear off the lines--those of you with a version of NEC having transmission line capabilities should remember that those feedlines are methematical models and are not part of the radiation calculations). 1. The resonant length of the antenna varies with the amount off-center of the feedpoint. 2. The feedpoint impedance selected (100 ohms, 300 ohms, etc.) will vary with the height of the antenna above ground. 3. Even resonant lengths do not come out any near the length of the fundamental. 4. The feedpoint impedance for any given length and height above ground increases slowly from the center-fed value and then more rapidly. The so-called 300-ohm point is on a sttep part of the curve, making its location more a matter of luck than skill. 5. The feedline radiates a bit (not a lot), and makes little difference to the patterns for all bands. The little bit of feedline radiation, however, makes the feedline part of the antenna, so all of the formulas for a 300 ohm point are pretty much negated. 6. If you want to roll your own, keep it simple, like the K5FO version. Use about 1/2 wavelength ("about" means do not be fussy) and feed the wire off center whever the feedline is most convenient to the shack entry point--just do not get more than about 50% past center or thereabouts. Run to tuner (I prefer a balanced inductively coupled tuner for this kind of application, but for QRP, little harm will come from Tees, etc.). Operate. 300 ohm or 450 ohm line is fine, since resnance is no longer a concern. Save the magic baluns, isolators, and other coax gimmicks for another day. They only introduce losses. (Yes, you mght need one at the shack window to transition to coax for your QRO operations.) 7. For all band operation, expect patterns very close to those from a center-fed wire of the same length, except on bands where the ratio of the wires each side of the feedpoint approaches certain values that change the nature of the antenna. Overall, no better, no worse. (Same applies to an end-fed Zepp of the same length.) 8. If you do not know what those patterns are likely to look like, whether using center feed, off-center feed, or end feed, come to FDIM for a quick overview. Or stay tuned for the next year to the series in Low Down for a more detailed look at them as I develop some compendia of patterns for the backyard antenna builder's notebook. 9. Then why do we see so much written on the OCF? Because everyone who makes on work amid specific quantities of backyard/neighborhood clutter and area terrain features tends to think they have the universal formulas, of which there are not any, if +/- 20% is not good enough for you--otherwise, everone's numbers work at that loose figure. OCF is just one more way to feed a wire. Do not strain to make an OCF, because there is no magic in it. However, if the OCF is natural for your layout, then go for it, since it will work as well as center and end fed systems. I understand that my studies fly in the face of certain advertised claims that appear from various sources from time to time. And if I wear certain shoes, I can jump as high as Michael Jordan. -73- LB, W4RNL