From owner-qrp-l@Lehigh.EDU Tue Dec 17 07:25:40 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 HAA21212 for ; Tue, 17 Dec 1996 07:25:39 -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 HAA21212 for ; Tue, 17 Dec 1996 07:25:39 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35046-24991>; Tue, 17 Dec 1996 07:20:06 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <34877-29089>; Tue, 17 Dec 1996 07:17:08 -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 HAA250759 for ; Tue, 17 Dec 1996 07:16:54 -0500 Received: from localhost by utkux4.utcc.utk.edu with SMTP (SMI-8.6/2.7c-UTK) id MAA27198; Tue, 17 Dec 1996 12:16:13 GMT Message-Id: Date: Tue, 17 Dec 1996 07:16:12 -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: Perfect Antenna Matches (long, erase if not interested) In-Reply-To: <961216220602_168719693@emout09.mail.aol.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-To: PaulKB8N@aol.com 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 Wonder if we should get organized on this subject. Here is one way to look at antenna matching. First, unless feedlines are unbalanced and act as part of the antenna, the antenna itself will radiate in just the same way, whatever feed and matching system is in use. Moreover, an antenna (and here we are talking mostly of center-fed horizontal wires) will convert to radiation all the rf electrical energy at it feedpoint after a few cycles (out of the millions per second). Third, resonance in an antenna is simply the condition of having no reactance, either inductive or capacitive, and as Paul notes, is a very narrow frequency phenomena, but one which occurs repetitively as one scans up the frequency spectrum. Note that resonance does not specify any particular resistive impedance, only zero reactance. Where can we effect a match to some specified feedline characteristic impedance (which is always considered resistive, but even feedline is not perfect)? At the antenna terminals: if the resistive part of the antenna's impedance is close to that of our feedline, we can introduce for a given frequency reactances of equal magnitude but opposite type and present the feedline with a resistive impedance. One can also place a network at these terminals, not different in priciple from ATU networks, to achieve both a compensation for reactance and a transformation of resistance to the desired level. In fact, with some ingenuity, one might place the final amp of a rig at the terminals, with remote signal and power feed and eliminate any feedline losses altogether. At the station (transmitter/receiver location): this location is generally chosen for convenience, not for best efficiency. And this is where ATU networks and inductively coupled circuits come into play. They have losses, but when well designed with high-Q components and adjusted for maximum efficiency settings, losses can be quite low--a few percent for most loads presented by incoming feedlines. When ATUs use low Q (lossy) components, are set to inefficient settings (sometimes by poor designs that only permit inefficient settings), use poor physical layouts that create stray inductances and capacitiances, etc., losses can be considerable. Between the ATU and the antenna terminals, standard practice is to use feedline with the lowest loss, which usually means 300- 450- or 600-ohm parallel feedline. Losses multiply with SWR on the line; hence a low starting figure provides the most efficient power transfer. Anywhere along the line: For any frequency and antenna feedpoint antenna impedance and feedline characteristic impedance, there will usually be points where one may introduce series or parallel reactances to achieve a resistive impedance of some desired level. This technique is widely useful, but not absolute, since certain lines and antenna impedances will not together reach a resistive value matching a desired line. When the reactance component is a parallel-connected device, it is usually called stub matching, since common practice in pre-WW II days was to use a shorted or open length of feedline as the reactance rather than using a lumped component (capacitor or inductor). HAMCALC has a program for calculating stubs for any feedpoint impedance and proposed feedline. The math of series matching, using feedline lengths as series sections, was worked out by Regier and appears in the IEEE proceeding for 1970 and Electronic Engineering for 1973, as well as QST for July 1978 (and subsequent editions of the Antenna Book). As with stubs, there are antenna impedance-feedline characteristic impedance combinations that do not permit a match, but most cases will work. These techniques are--from the perspective of convenience--best suited to monoband antennas, although one might develop switching or clipping techniques for multiband use of an antenna. As Walt Maxwell extensively discusses in Reflections (out of print?), each of these techniques in fact resonates an antenna by introducing a reactance which, when transformed up the line to the antenna terminals, presents just the required reactance to compensate for any reactance at the antenna feedpoint. Transformation of the impedance's resistive component to a desired value is done in part by the feedline (since transmission lines are impedance transformers, among other things) and--if the desired value is not obtained this way--by whatever network is employed by devices like ATUs. In many high power applications, especially where radiated power from the antenna is the user's critical question, efficiency of the matching network may not be considered significant, since lost power (so long as it does not harm the components) can be made up by adding power to the system. For QRPers, there is no such luxury. Hence, it is very useful to learn all we can of ways to improve the efficiency of our matches--wherever along the line we place them. Newcomer books tend to make transmission line and antenna subjects a matter of some rules of thumb to get folks started. Unfortunately, most of us spend too many years thinking that the antenna hand has only a thumb. Every QRPer should dig down into transmission line and antenna theory and practice to learn everything possible about how to radiate tiny power most efficiently. And there is always something new to be learned. I am always amazed that some avid QRPers will modify a kit to get a few tenths of a watt more output or a tiny fraction of a microvolt more sensitivity and then believe that any old antenna that allows contacts to be made is good enough--and indeed, that all QRPers should stick only to simple antenna systems. Like circuitry, there is no magic to antennas and feed systems; just careful application of fundamental principles. The first step is getting rid of those rules of thumb, like a resonant dipole length in feet is 468 divided by the frequency in MHz, and its unspoken corollary that this is somehow "better" than other lengths, and the connecting coax is somehow "better" than other feed systems. What makes these rules of thumb so dangerous is that they are somewhere close to the ballpark, but they are actually more wrong than right for most circumstances. Learn what is right for the exact circumstances you have. It can be done. -73- LB, W4RNL