From owner-qrp-l@Lehigh.EDU Thu Dec 19 12:35:35 1996 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 MAA14396 for ; Thu, 19 Dec 1996 12:35:33 -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 MAA14396 for ; Thu, 19 Dec 1996 12:35:33 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35121-19582>; Thu, 19 Dec 1996 12:24:29 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <35049-19582>; Thu, 19 Dec 1996 12:23:46 -0500 Received: from marceau.fm.intel.com (marceau.fm.intel.com [132.233.247.8]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with ESMTP id MAA142850 for ; Thu, 19 Dec 1996 12:23:35 -0500 Received: from fmmail.fm.intel.com by marceau.fm.intel.com (8.8.4/10.0i); Thu, 19 Dec 1996 17:23:33 GMT Received: (from ccmgate@localhost) by fmmail.fm.intel.com (8.7.4/8.7.3) id JAA23522 for qrp-l@lehigh.edu; Thu, 19 Dec 1996 09:20:52 -0800 (PST) Received: by ccm.fm.intel.com (ccmgate 3.2 #2) Thu, 19 Dec 96 09:20:51 PST Date: Thu, 19 Dec 96 09:16:00 PST Reply-To: Cecil_A_Moore@ccm.ch.intel.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: Cecil A Moore To: "Low Power Amateur Radio Discussion" Subject: Antenna Tidbits X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Message-Id: <96Dec19.122429est.35121-19582+97@fidoii.cc.lehigh.edu> Status: RO It's hard to believe the interest this group has demonstrated in more efficient antenna systems. Here's a few tidbits about a 102 ft center-fed dipole at 60ft high (the G5RV length). These figures are approximations from EZNEC. Freq. SWR on 300ohm line SWR on 45ohm line 3.8 14:1 15:1 7.2 11:1 8:1 10.125 23:1 15:1 14.2 3:1 4:1 18.14 12:1 8:1 21.3 13:1 10:1 24.95 4:1 4:1 28.4 10:1 7:1 Note that, using 450 ohm feedline, the SWR on 30m and 80m is the same so if this is a good length for 80m, it is also a good length for 30m (assuming no coax). One can use an inexpensive pickup loop to find the current maximum point on the ladder-line. The antenna *system* is resonant at the current maximum point. Here's the purely resistive impedances predicted by EZNEC at the Imax points. Freq. Resistance using Resistance using 300 ohm line 450 ohm line 3.8 21 ohms 30 ohms 7.2 27 ohms 56 ohms 10.125 13 ohms 30 ohms 14.2 100 ohms 113 ohms 18.14 25 ohms 56 ohms 21.3 23 ohms 45 ohms 24.95 75 ohms 113 ohms 28.4 30 ohms 64 ohms Note that almost all these points result in 50 ohm SWRs of 2:1 or less, an efficient operating point without a tuner or an easy match for most antenna tuners. To take advantage of the above, we must have some way of varying the length of the transmission line. We can do it with relays, switches, or pluggable lengths of ladder-line. I have tried all of the above and presently use 16,8,4 ft lengths of line to achieve any length between 0ft and 28ft. I have a variable capacitor that slides back and forth on 4 ft parallel runs of #10 bare copper wire to achieve perfect 1:1 SWRs with minimum losses. Except for 80m and 30m, Imax points occur on all the other HF bands between 70ft and 90ft from the antenna feedpoint for 300 ohm line and 77ft and 99ft for 450 ohm line. On 40m and 20m-10m, locating the capacitor a little closer to the transmitter than the Imax point results in a perfectly resistive 200 ohms, a perfect match for my 4:1 balun, and a perfect 50 ohm SWR of 1:1. 80m and 30m present a special problem. On 80m the Imax points are at 26ft and 26ft+halfwavelength= 125ft for 300ohm line and 1.1 times that for 450ohm line. So I use two series toroidal inductors on both 80m and 30m located around 82ft for 300ohm and 92ft for 450ohm line. Toroidal inductors are more lossy than caps but I didn't want to have to use 140ft of 450 ohm ladder-line to access an Imax point on 80m. Anyone wishing to duplicate my efforts, I suggest you optimize your antenna for your favorite band and evaluate the results. 73, Cecil, W6RCA, OOTC From owner-qrp-l@Lehigh.EDU Fri Dec 20 07:47:24 1996 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 HAA04988 for ; Fri, 20 Dec 1996 07:47:23 -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 HAA04988 for ; Fri, 20 Dec 1996 07:47:23 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35255-45339>; Fri, 20 Dec 1996 07:46:49 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <35101-19485>; Fri, 20 Dec 1996 07:45:05 -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 HAA98881 for ; Fri, 20 Dec 1996 07:44:56 -0500 Received: from localhost by utkux4.utcc.utk.edu with SMTP (SMI-8.6/2.7c-UTK) id MAA02205; Fri, 20 Dec 1996 12:41:42 GMT Message-Id: Date: Fri, 20 Dec 1996 07:41:41 -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: Antenna Tidbits In-Reply-To: <96Dec19.122429est.35121-19582+97@fidoii.cc.lehigh.edu> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-To: Cecil A Moore 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 Even though I am reading Cecil's description of the principles of his G5RV matching system the next morning, I hope no one has erased it, perhaps thinking they do not have space for doing something like this. It is too important for that. Save it, read it, and understand every move he has made. If you do not fully understand a move, ask him about it. It is a first class example of applying basic principles to achieve an efficient multiband system--even to knowing where he had to accept a little loss (series inductors) in meeting certain other needs. Note the achievement of a purely resistive 200 ohms before placing a balun in the line, giving the balun what it has to have to be efficient. Notice the room he gave himself to work with variables, like the position of the capacitor across the line to cancel/compensate for the reactance at that point. Notice the room he gave himself to alter settings as conditions change. Notice that there is not a single "black box" element in his system, but a fullly controllable set of conditions. It will not fit in a TenTec box, and certanly not an MFJ box, but it is sound work backed up by good diagnostic tools. I'd like to see this system in one of the QRP journals, along with some sketches of the layout, not so much because I want to replicate it exactly, but because it makes a good item to use for breaking out of the "black-box ATU" syndrome. -73- LB, W4RNL