From owner-qrp-l@Lehigh.EDU Tue Dec 17 11:37:04 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 LAA27797 for ; Tue, 17 Dec 1996 11:37:03 -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 LAA27797 for ; Tue, 17 Dec 1996 11:37:03 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35330-29089>; Tue, 17 Dec 1996 11:36:36 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <35040-28322>; Tue, 17 Dec 1996 11:35:33 -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 LAA60238 for ; Tue, 17 Dec 1996 11:35:18 -0500 Received: from fmmail.fm.intel.com by marceau.fm.intel.com (8.8.4/10.0i); Tue, 17 Dec 1996 16:08:19 GMT Received: (from ccmgate@localhost) by fmmail.fm.intel.com (8.7.4/8.7.3) id IAA11555 for qrp-l@lehigh.edu; Tue, 17 Dec 1996 08:05:42 -0800 (PST) Received: by ccm.fm.intel.com (ccmgate 3.2 #2) Tue, 17 Dec 96 08:05:42 PST Date: Tue, 17 Dec 96 08:02:00 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: Non-resonant dipoles with ladder line X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Message-Id: <96Dec17.113636est.35330-29089+74@fidoii.cc.lehigh.edu> Status: RO This posting apparently didn't get through yesterday because of a full mailbox at qrp-l. Hope it's not a duplication. The response to my posting about throwing away the antenna tuner was overwhelming. Let me state a few principles involved: When we write an antenna impedance like 90+j450, we mean 90 ohms of resistance and 450 ohms of inductive reactance. How much inductance is 450 ohms? That depends on the frequency. On 7.040 MHz, 450 ohms of inductive reactance is 10.2 microhenries. If you want to use coax on this antenna, you don't want that 50:1 SWR caused primarily by the inductive reactance so you can install 450 ohms of capacitive reactance in series at the feedpoint to neutralize the inductive reactance and drop the SWR to less than 2:1. The value of the capacitor for 7.040 MHz would be 50.2 pf. Thus we have located the antenna tuner function at the antenna. It is inconvenient to tune a multi-band antenna at the feedpoint. So let's feed this antenna with low-loss 450 ohm window line and just tolerate a 10:1 SWR on a portion of the line. We can simply locate two 100 pf capacitors (each Xc=225 ohms), in series, 1/2 wavelength from the antenna or at one wavelength (or any multiple of a half- wavelength). 0.74 WL further along the line is a point where the impedance equals 50+j1260. If we neutralize that series inductive reactance with series caps, the result is a perfect 50 ohms. Further along the line we can use one parallel inductance or one parallel cap to achieve that perfect 50 ohms. A Parallel Cap (PC) seems the most efficient point. It is tremendously easier to match the feedline impedance when one has an idea of the magnitude of the impedance. EZNEC can give a ballpark figure, close enough that adjustments can be made to bring it to perfection. An inexpensive current pick-up loop will indicate where the current maximi are on the parallel feedline. Just past the current maximum toward the transmitter is the PC-rr point where PC-50 indicates the point where locating a Parallel Capacitor will result in a purely resistive 50 ohms impedance. PC-200 would match a 4:1 balun perfectly. This stuff is not rocket science and can be learned easily and quickly. If one wants a quick and dirty fix, use an antenna tuner and a balun. If one wants perfection, invest a couple of hours in learning how to measure the antenna/feedline impedance with a very inexpensive pick- up loop and a voltmeter. For the current loop to work on the low bands, one needs to put about 100 watts down the feedline, at least temporarily, to overcome the diode drop in the pickup loop. This approach will not work on coax. If one has access to a sensitive RF milivoltmeter, these measurements can be made at qrp levels. 73, Cecil, W6RCA, OOTC